Patent Publication Number: US-2015080651-A1

Title: Endoscope apparatus

Description:
This application is a continuation application based on a PCT Application No. PCT/JP2013/064827, filed May 29, 2013, whose priority is claimed on Japanese Patent Application No. 2012-125120, filed May 31, 2012. The contents of both the PCT Application and the Japanese Application are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to focal depth control in an electronic endoscope apparatus which has a high-definition solid state imaging device having fine pixels. 
     2. Description of Related Art 
     In an endoscope system, a state in which a depth of field of a photographed image is large and the whole photographed image is focused on, i.e., a so-called pan-focus state in which an object is focused on from a short distance to a long distance is ideal. For this reason, an imaging unit of the endoscope is preferably controlled in a state in which a diaphragm is narrowed as much as possible. 
     In an electronic endoscope using a solid state imaging device, in order to respond to requirements of high image quality of the photographed image, the number of pixels in the solid state imaging device is being increased. Since space in a distal end section of the endoscope is limited, the pixel structure of the solid state imaging device is miniaturized. However, according to the miniaturization of the pixel structure, problems related to a decrease in sensitivity of the solid state imaging device and darkening of the photographed image may occur. When a diameter of a permissible circle of confusion is reduced, a shallow focal depth may also occur as an optical problem. 
     For example, in an endoscope apparatus disclosed in Japanese Unexamined Patent Application, First Publication No. H07-299029, a method of reconciling both appropriate brightness and a maximum focal depth by controlling an opening of a diaphragm unit based on brightness information obtained from the solid state imaging device is disclosed. More specifically, when the brightness information obtained from the solid state imaging device is larger than predetermined brightness, the size of the opening of the diaphragm unit is controlled to be primarily reduced. Meanwhile, when the brightness information obtained from the solid state imaging device is smaller than the predetermined brightness, a light source apparatus is driven such that a quantity of light is primarily increased in a state in which the size of the opening of the diaphragm unit is minimized. After the quantity of light from the light source apparatus is maximized, the size of the opening of the diaphragm unit is controlled to be increased. According to the above-mentioned control, in the endoscope apparatus disclosed in Japanese Unexamined Patent Application, First Publication No. H07-299029, the appropriate brightness and the maximum focal depth can be reconciled. In Japanese Unexamined Patent Application, First Publication No. H07-299029, a method of controlling a focal position, based on the brightness information obtained by detecting the brightness of the imaged object image, as an optical path length between the optical system and the solid state imaging device is varied, is also disclosed. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, an endoscope apparatus includes: an illumination unit configured to radiate light from a light source toward an object; an optical system including an optical lens configured to form an object image, and a diaphragm unit configured to adjust a size of an opening in a plurality of steps; an imaging unit including a solid state imaging device configured to convert an optical image of the object captured through the optical system into an electric signal in accordance with the optical image, the imaging unit being configured to output an image signal in accordance with an image formed based on the electric signal; a region setting unit configured to set at least one region of interest in the image formed by the image signal output from the imaging unit; an estimation value calculation unit configured to calculate an estimation value showing a focus level in the region of interest and output the estimation value; an estimation value storage unit configured to store the estimation value output from the estimation value calculation unit; an estimation value comparison unit configured to read the previous estimation value stored in the estimation value storage unit as a reference value, compare the reference value and the current estimation value output from the estimation value calculation unit, and output a comparison result of the reference value and the estimation value; and a diaphragm control unit configured to output a control signal for controlling the opening of the diaphragm unit based on the comparison result output from the estimation value comparison unit. 
     According to a second aspect of the present invention, in the endoscope apparatus according to the first aspect of the present invention, the imaging unit may further include a gain adjustment unit configured to adjust brightness of the image formed based on the electric signal by electrically adjusting the electric signal of each of frames output from the solid state imaging device. The imaging unit may output the image signal in accordance with the electric signal of each of the frames adjusted such that the image has predetermined brightness. 
     According to a third aspect of the present invention, in the endoscope apparatus according to the first aspect of the present invention, the illumination unit may include a light adjustment unit configured to adjust a quantity of light radiated by the light source. The imaging unit may adjust the quantity of light radiated by the light source by the illumination unit such that brightness of the image formed based on the electric signal at each of frames output from the solid state imaging device becomes predetermined brightness. 
     According to a fourth aspect of the present invention, in the endoscope apparatus according to the first aspect of the present invention, the illumination unit may include a light adjustment unit configured to adjust a quantity of light radiated by the light source. The imaging unit may further include a gain adjustment unit configured to adjust brightness of the image formed based on the electric signal by electrically adjusting the electric signal at each of frames output from the solid state imaging device. The imaging unit may adjust, by the gain adjustment unit, the electric signal of the frame output from the solid state imaging device after the quantity of light radiated by the light source is maximized by the illumination unit. 
     According to a fifth aspect of the present invention, in the endoscope apparatus according to the first aspect of the present invention, the region setting unit may set the plurality of regions of interest to the entire image formed by the image signal output from the imaging unit with no gap. 
     According to a sixth aspect of the present invention, in the endoscope apparatus according to the first aspect of the present invention, the region setting unit may discretely set the plurality of regions of interest to the image formed by the image signal output from the imaging unit with gaps. 
     According to a seventh aspect of the present invention, in the endoscope apparatus according to the first aspect of the present invention, the region setting unit may set the regions of interest with equal sizes. 
     According to an eighth aspect of the present invention, in the endoscope apparatus according to the first aspect of the present invention, the illumination unit may include a light adjustment unit configured to adjust a quantity of light radiated by the light source. The imaging unit may further include a gain adjustment unit configured to adjust brightness of the image formed based on the electric signal by electrically adjusting the electric signal at each of frames output from the solid state imaging device. The imaging unit may adjust the quantity of light radiated by the light source by the illumination unit, and output the image signal of each of the frames obtained by adjusting, by the gain adjustment unit, the electric signal of each of the frames output from the solid state imaging device with the light radiated by the light source having the adjusted quantity of light radiated by the light source. The estimation value calculation unit may calculate the estimation value corresponding to the image signal of each of the frames, based on the image signal of each of the frames obtained by adjusting the quantity of light radiated by the light source and electrically adjusting the electric signal by the imaging unit. 
     According to a ninth aspect of the present invention, in the endoscope apparatus according to the eighth aspect of the present invention, the estimation value calculation unit may calculate the estimation value in a current frame at each of the regions of interest set by the region setting unit. The estimation value storage unit may store the estimation value in the current frame output from the estimation value calculation unit at each of the regions of interest. The estimation value comparison unit may compare the estimation value of each of the regions of interest in the current frame output from the estimation value calculation unit and the reference value corresponding to each of the regions of interest in a previous one frame read from the estimation value storage unit, and output the comparison result corresponding to each of the regions of interest obtained by comparing the estimation value and the reference value. The diaphragm control unit may control the opening of the diaphragm unit to be driven by at least one step in the same direction as a direction in which the opening of the diaphragm unit has been controlled when moved from the previous one frame to the current frame, if a number of the comparison results representing that the estimation value in the current frame is larger than the reference value in the previous one frame is equal to or larger than a preset number in the comparison results corresponding to each of the regions of interest output from the estimation value comparison unit. The diaphragm control unit may control the opening of the diaphragm unit to be driven by at least one step in an opposite direction to the direction in which the opening of the diaphragm unit has been controlled when moved from the previous one frame to the current frame, if the number of the comparison results representing that the estimation value in the current frame is larger than the reference value in the previous one frame is smaller than the preset number. 
     According to a tenth aspect of the present invention, in the endoscope apparatus according to the eighth aspect of the present invention, the estimation value calculation unit may calculate a total value obtained by adding the each estimation value corresponding to each of the regions of interest set by the region setting unit, and output the total value as the estimation value in a current frame. The estimation value storage unit may store the estimation value in the current frame output from the estimation value calculation unit. The estimation value comparison unit may compare the estimation value in the current frame output from the estimation value calculation unit and the reference value in a previous one frame read from the estimation value storage unit, and output the comparison result obtained by comparing the estimation value and the reference value. The diaphragm control unit may control the opening of the diaphragm unit to be driven by at least one step in the same direction as a direction in which the opening of the diaphragm unit has been controlled when moved from the previous one frame to the current frame, if the comparison result output from the estimation value comparison unit represents that the estimation value serving as the total value in the current frame is larger than the reference value serving as the total value in the previous one frame. The diaphragm control unit may control the opening of the diaphragm unit to be driven by at least one step in an opposite direction to the direction in which the opening of the diaphragm unit has been controlled when moved from the previous one frame to the current frame, if the comparison result represents that the estimation value serving as the total value in the current frame is equal to or smaller than the reference value serving as the total value in the previous one frame. 
     According to an eleventh aspect of the present invention, in the endoscope apparatus according to the eighth aspect of the present invention, the estimation value calculation unit may calculate a weighted average value obtained through weighted average of the each estimation value corresponding to each of the regions of interest set by the region setting unit, and output the weighted average value as the estimation value in a current frame. The estimation value storage unit may store the estimation value in the current frame output from the estimation value calculation unit. The estimation value comparison unit may compare the estimation value in the current frame output from the estimation value calculation unit and the reference value in a previous one frame read from the estimation value storage unit, and output the comparison result obtained by comparing the estimation value and the reference value. The diaphragm control unit may control the opening of the diaphragm unit to be driven by at least one step in the same direction as a direction in which the opening of the diaphragm unit has been controlled when moved from a previous one frame to the current frame, if the comparison result output from the estimation value comparison unit represents that the estimation value serving as the weighted average value in the current frame is larger than the reference value serving as the weighted average value in the previous one frame. The diaphragm control unit may control the opening of the diaphragm unit to be driven by at least one step in an opposite direction to the direction in which the opening of the diaphragm unit has been controlled when moved from the previous one frame to the current frame, if the comparison result represents that the estimation value serving as the weighted average value in the current frame is equal to or smaller than the reference value serving as the weighted average value in the previous one frame. 
     According to a twelfth aspect of the present invention, in the endoscope apparatus according to the first aspect of the present invention, the diaphragm control unit may control the opening of the diaphragm unit to move to a position at which the estimation value is maximized if a maximum estimation value is detected from the each estimation value in the current frame corresponding to each of the regions of interest set by the region setting unit, control the opening of the diaphragm unit such that a state of the opening of the diaphragm unit at the position is held for a preset time, and control the opening of the diaphragm unit to be driven by at least one step in a direction in which the opening of the diaphragm unit is reduced after the preset time elapses. 
     According to a thirteenth aspect of the present invention, in the endoscope apparatus according to the twelfth aspect of the present invention, the estimation value comparison unit may calculate an absolute value of a difference between the estimation value of the current frame corresponding to each of the regions of interest set by the region setting unit and the corresponding reference value in a previous one frame stored in the estimation value storage unit, and output the calculated result of the absolute value as a calculation result of each of the regions of interest. The diaphragm control unit may terminate holding the state of the opening of the diaphragm unit for the preset time and restart control of the opening of the diaphragm unit from a next frame, if the at least one calculation result in the calculation result corresponding to each of the regions of interest output from the estimation value comparison unit during the preset time exceeds a preset value. 
     According to a fourteenth aspect of the present invention, in the endoscope apparatus according to the eighth aspect of the present invention, the optical system may further include a lens driving unit configured to be interlocked with the opening of the diaphragm unit and to set a focal position of the optical lens. 
     According to a fifteenth aspect of the present invention, in the endoscope apparatus according to the fourteenth aspect of the present invention, the lens driving unit may set the focal position of the optical lens such that, when the opening of the diaphragm unit is largest, the object disposed at a proximal end is included in a focus range of the optical system and the object disposed at the proximal end to the object disposed at a distal end are gradually included in the focus range of the optical system as the opening of the diaphragm unit is reduced gradually, and when the opening of the diaphragm unit is smallest, a range in which the optical image of the object is converted into the electric signal by the solid state imaging device is entirely included in the focus range of the optical system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an example of a schematic configuration of an endoscope system according to a first embodiment of the present invention. 
         FIG. 2  is a timing chart showing an example of a diaphragm control operation in the endoscope system according to the first embodiment of the present invention. 
         FIG. 3  is a view schematically showing an example of a region of interest, in which an estimation value is calculated, in a solid state imaging device included in the endoscope system according to the first embodiment of the present invention. 
         FIG. 4  is a view schematically showing an example of the entire diaphragm control operation in the endoscope system according to the first embodiment of the present invention. 
         FIG. 5A  is a view schematically showing another example of the region of interest, in which an estimation value is calculated, in the solid state imaging device included in the endoscope system according to the first embodiment of the present invention. 
         FIG. 5B  is a view schematically showing still another example of the region of interest, in which an estimation value is calculated, in the solid state imaging device included in the endoscope system according to the first embodiment of the present invention. 
         FIG. 5C  is a view schematically showing still another example of the region of interest, in which an estimation value is calculated, in the solid state imaging device included in the endoscope system according to the first embodiment of the present invention. 
         FIG. 5D  is a view schematically showing still another example of the region of interest, in which an estimation value is calculated, in the solid state imaging device included in the endoscope system according to the first embodiment of the present invention. 
         FIG. 6  is a block diagram showing an example of a schematic configuration of an endoscope system according to a second embodiment of the present invention. 
         FIG. 7  is a timing chart showing an example of a diaphragm control operation in the endoscope system according to the second embodiment of the present invention. 
         FIG. 8  is a block diagram showing an example of a schematic configuration of an endoscope system according to a third embodiment of the present invention. 
         FIG. 9  is a timing chart showing an example of a diaphragm control operation in the endoscope system according to the third embodiment of the present invention. 
         FIG. 10  is a view schematically showing an example of the entire diaphragm control operation in the endoscope system according to the third embodiment of the present invention. 
         FIG. 11  is a block diagram showing an example of a schematic configuration of an endoscope system according to a fourth embodiment of the present invention. 
         FIG. 12  is a timing chart showing an example of a diaphragm control operation in the endoscope system according to the fourth embodiment of the present invention. 
         FIG. 13  is a view schematically showing an example of the entire diaphragm control operation in the endoscope system according to the fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
     Hereinafter, an endoscope apparatus according to an embodiment of the present invention will be described with reference to the accompanying drawings.  FIG. 1  is a block diagram showing an example of a schematic configuration of an endoscope system according to the first embodiment. An endoscope system  100  shown in  FIG. 1  includes an illumination unit  1 , an optical system  2 , an imaging unit  3 , a region setting unit  4 , an estimation value calculation unit  5 , an estimation value storage unit  6 , an estimation value comparison unit  7 , and a diaphragm control unit  8 . The endoscope system  100  may further include an image processing unit  9  and an image output unit  10 . 
     The illumination unit  1  includes a xenon lamp  1   a  serving as a light source apparatus. The illumination unit  1  radiates light from the xenon lamp  1   a  to an object in a body photographed by the endoscope system  100 . The optical system  2  includes an optical lens  2   a  configured to form an object image, and a diaphragm unit  2   b  configured to adjust a size of an opening according to a control signal output from the diaphragm control unit  8 . The optical system  2  delivers object light to the imaging unit  3 . 
     The imaging unit  3  includes a solid state imaging device  3   a  and a gain adjustment unit  3   b . The solid state imaging device  3   a  photoelectrically converts an optical image of the object captured through the optical system  2 , and converts the optical image into an electric signal of each frame. The gain adjustment unit  3   b  adjusts intensity of the electric signal output from the solid state imaging device  3   a  to an appropriate level according to a state of light and shade of the entire image imaged by the solid state imaging device  3   a . The imaging unit  3  outputs the electric signal of each frame having the level adjusted by the gain adjustment unit  3   b  to the estimation value calculation unit  5  and the image processing unit  9  as the image signal. 
     The region setting unit  4  sets regions of interest by dividing the entire image of one frame output by the imaging unit  3  into, for example, a plurality of regions having the same size with no gap. A method of dividing the regions of interest by the region setting unit  4  will be described below in detail. 
     The estimation value calculation unit  5  detects an amount of a high frequency element, from which a noise element is removed, from the image signal after level adjustment input from the imaging unit  3  at every region of interest set by the region setting unit  4 . The estimation value calculation unit  5  calculates an estimation value corresponding to the amount of the high frequency element of each of the detected regions of interest. The estimation value calculation unit  5  outputs the calculated estimation value to the estimation value storage unit  6  and the estimation value comparison unit  7 . The estimation value is a value showing the degree of focus in the region of interest. 
     The estimation value storage unit  6  individually stores the estimation value of each of the regions of interest input from the estimation value calculation unit  5  to one frame. The estimation value storage unit  6  outputs each of the stored estimation values to the estimation value comparison unit  7  as a reference value. 
     The estimation value comparison unit  7  compares magnitudes of the estimation value of each of the regions of interest input from the estimation value calculation unit  5  and the reference value of the corresponding region of interest stored in the estimation value storage unit  6  at each of the regions of interest set by the region setting unit  4 . The estimation value comparison unit  7  outputs the result of comparing the magnitudes (hereinafter referred to as “a comparison result”) to the diaphragm control unit  8 . 
     Comparison of the estimation value and the reference value by the estimation value comparison unit  7  is performed by reading the reference value of the corresponding region of interest from the estimation value storage unit  6 , i.e., the estimation value of the previous frame, at the timing at which the estimation value calculation unit  5  outputs the estimation value of the current frame. The estimation value comparison unit  7  outputs a signal representing whether or not the estimation value input from the estimation value calculation unit  5  is larger than the reference value read from the estimation value storage unit  6  to the diaphragm control unit  8  as a comparison result. 
     The diaphragm control unit  8  determines whether the driving control is performed in a direction of reducing the opening of the diaphragm unit  2   b  included in the optical system  2  (hereinafter referred to as “a diaphragm direction”) or a direction of increasing the opening (hereinafter referred to as “an opening direction”) based on the comparison result of the entire region of interest input from the estimation value comparison unit  7 . More specifically, the diaphragm control unit  8  counts the number of regions of interest, at which it is determined that “the estimation value&gt;the reference value” in the comparison result input from the estimation value comparison unit  7 . When the counted number is equal to or larger than a preset number, the diaphragm control unit  8  determines that the diaphragm unit  2   b  is driven by one step in the same direction in which it was previously driven (one of the diaphragm direction and the opening direction). Here, the diaphragm control unit  8  outputs a control signal for controlling the driving of the diaphragm unit  2   b  in the determined direction. When the counted number is smaller than the preset number, the diaphragm control unit  8  determines that the diaphragm unit  2   b  is driven by one step in an opposite direction to that in which it was previously driven. Here, the diaphragm control unit  8  outputs a control signal for controlling the driving of the diaphragm unit  2   b  in the determined direction. 
     The image processing unit  9  performs image processing through which the image signal after level adjustment in each of the frames input from the imaging unit  3  is converted into, for example, a format displayed in a monitor connected to the endoscope system  100 . The image processing unit  9  outputs the image signal passing through the image processing (hereinafter referred to as “image data”) to the image output unit  10 . The image output unit  10  outputs and displays the image data input from the image processing unit  9  to, for example, the monitor connected to the endoscope system  100  at each of the frames. 
     According to the above-mentioned configuration, in the endoscope system  100 , the entire image of one frame imaged by the solid state imaging device  3   a  is divided into a plurality of regions of interest. Based on the estimation values of the divided regions of interest, the opening of the diaphragm unit  2   b  when an image of the next frame is imaged is controlled. That is, in the endoscope system  100 , as the opening of the diaphragm unit  2   b  is controlled based on a distance at which the objects are distributed, focused images are obtained in all the imaged images. Accordingly, in the endoscope system  100 , when all the imaged objects are distributed in a range of a short distance in the vicinity of the focus, the opening of the diaphragm unit  2   b  can be increased, and a high resolution image can be photographed. In the endoscope system  100 , when not all the imaged objects are provided in the vicinity of the focus or when all the objects are distributed at a range of a long distance, as the opening of the diaphragm unit  2   b  is reduced, like the related art, the image in the pan-focus state in which the image is focused on as a whole (hereinafter referred to as “a focused image”) can be photographed. 
     Next, an operation of the endoscope system  100  according to the first embodiment will be described.  FIG. 2  is a timing chart showing an example of a diaphragm control operation in the endoscope system  100  according to the first embodiment. As described above, in the endoscope system  100 , the estimation value calculation unit  5  calculates the estimation value of the region of interest at each of the frames imaged by the solid state imaging device  3   a . The endoscope system  100  controls the driving of the diaphragm unit  2   b  based on the estimation values of the regions of interest. 
     In the following description, as shown in  FIG. 3 , the case in which the region setting unit  4  sets the regions of interest by dividing the entire image of one frame output from the imaging unit  3  into a plurality of regions having the same size with no gap will be described. As the regions of interest are set as shown in  FIG. 3 , the driving of the diaphragm unit  2   b  can be controlled while the region of the observation target in the endoscope system  100  is set as the entire image. 
     The imaging unit  3  outputs an image signal according to the electric signal obtained by photoelectrically converting an optical image of the object of the current frame A captured through the optical system  2  to the estimation value calculation unit  5 . In the endoscope system  100  according to the first embodiment, the imaging unit  3  includes the gain adjustment unit  3   b  configured to correct the brightness of the image, which is imaged by the solid state imaging device  3   a , varying depending on the opening and closing of the diaphragm unit  2   b  to an appropriate brightness. When the image of the frame A imaged by the solid state imaging device  3   a  is dark, the imaging unit  3  amplifies the electric signal output from the solid state imaging device  3   a  using the gain adjustment unit  3   b . Accordingly, the imaging unit  3  outputs the image signal having a level adjusted to the certain brightness, regardless of the opening and closing of the diaphragm unit  2   b , to the estimation value calculation unit  5  as the image signal of the frame A. 
     The estimation value calculation unit  5  calculates the estimation value obtained by detecting the amount of the high frequency element, from which the noise element is removed, from the image signal input from the imaging unit  3  at each of the regions of interest set by the region setting unit  4  and obtained by equally dividing the entire image of the frame A into n regions with no gap. In estimation values A1 to An shown in  FIG. 2 , “A” represents the estimation value of the frame A, and “1 to n” represent the corresponding regions of interest. In the following description, when the estimation value of the frame A is represented with no distinction of the regions of interest, it is referred to as “an estimation value A”. The estimation value calculation unit  5  sequentially outputs the calculated estimation values A1 to An corresponding to the regions of interest of the frame A to the estimation value storage unit  6  and the estimation value comparison unit  7 . 
     The calculation of the estimation values by the estimation value calculation unit  5  can be performed by a known technology using, for example, a bandpass filter or the like. Since the calculation method of the estimation value is a method generally used, for example, when a function of auto-focus of a digital camera or the like is realized, a detailed description thereof will be omitted here. 
     The estimation value storage unit  6  sequentially stores the estimation values A1 to An of the frame A input from the estimation value calculation unit  5  into the storage regions of the estimation value storage unit  6  corresponding to the regions of interest. 
     The estimation value comparison unit  7  compares the estimation value A of the frame A input from the estimation value calculation unit  5  and a reference value Z serving as the estimation value corresponding to the same region of interest of the previous frame Z stored in the estimation value storage unit  6 . In reference values Z1 to Zn shown in  FIG. 2 , “Z” represents a reference value of the frame Z, and “1 to n” represent the corresponding regions of interest. In the following description, when the reference value of the frame Z is represented with no distinction of the regions of interest, it is referred to as “the reference value Z.” 
     More specifically, as shown in  FIG. 2 , the estimation value comparison unit  7  sequentially reads the previous reference values Z1 to Zn of the frame Z of the corresponding regions of interest stored in the estimation value storage unit  6  at timings at which the estimation value calculation unit  5  outputs each of the estimation values A1 to An of the frame A. The estimation value comparison unit  7  sequentially compares magnitudes of the estimation value A and the reference value Z at each of the regions of interest. The estimation value comparison unit  7  sequentially outputs the comparison result representing whether or not the estimation value A is larger than the reference value Z to the diaphragm control unit  8 . 
     Provisionally, the case in which the diaphragm unit  2   b  is driven by one step in the negative direction (the opening direction) from the previous frame Z to the current frame A will be assumed. The estimation value comparison unit  7  first outputs the comparison result obtained by comparing magnitude correlations of the reference value Z1 of an initial (first) region of interest and the estimation value A1 to the diaphragm control unit  8 .  FIG. 2  shows the case in which the magnitude comparison result of the reference value Z1 and the estimation value A1 is that “the reference value Z1&gt;the estimation value A1.” Here, the estimation value comparison unit  7  outputs, for example, the signal representing that the magnitude comparison result (hereinafter referred to as “a comparison result signal”)=“0” to the diaphragm control unit  8  as the comparison result in the first region of interest. 
     Next, the estimation value comparison unit  7  compares the magnitude correlations of the reference value Z2 of the second region of interest and the estimation value A2. The estimation value comparison unit  7  outputs the comparison result in the second region of interest to the diaphragm control unit  8 .  FIG. 2  shows the case in which the magnitude comparison result of the reference value Z2 and the estimation value A2 is that “the reference value Z2&lt;the estimation value A2.” Here, the estimation value comparison unit  7  outputs, for example, the comparison result signal=“1” to the diaphragm control unit  8  as the comparison result in the second region of interest. 
     Hereinafter, similarly, the estimation value comparison unit  7  repeats comparisons of the magnitude correlations of the reference values Z and the estimation values A of the regions of interest. The estimation value comparison unit  7  sequentially outputs the comparison result signal=“1” when “the estimation value A&gt;the reference value Z” is true and the comparison result signal=“0” when false to the diaphragm control unit  8  as the comparison results in the regions of interest. In this way, the estimation value comparison unit  7  sequentially outputs the comparison results obtained by comparing the magnitude correlations of the reference values Z and the estimation values A with respect to all of n regions of interest to the diaphragm control unit  8 . 
     The diaphragm control unit  8  counts the number of regions of interest in which it is determined that the estimation value A of the frame A is larger than the reference value Z of the frame Z, i.e., “the estimation value A&gt;the reference value Z,” in the comparison results sequentially input from the estimation value comparison unit  7 . For example, in  FIG. 2 , the number of the regions of interest in which the comparison result signal=“1” is counted. The diaphragm control unit  8  determines a direction in which the diaphragm unit  2   b  is driven based on the counted result and the preset constant. 
     More specifically, if the counted result is equal to or larger than a preset constant M, the diaphragm control unit  8  determines that the current frame A has a larger number of focused regions of interest than the previous frame Z. Here, the diaphragm control unit  8  controls the diaphragm unit  2   b  to be driven by one step in the same direction in which it was previously driven. On the other hand, if the counted result is smaller than the preset constant M, the diaphragm control unit  8  determines that the current frame A has a smaller number of focused regions of interest than the previous frame Z. Here, the diaphragm control unit  8  controls the diaphragm unit  2   b  to be driven by one step in an opposite direction to that in which it was previously driven. In  FIG. 2 , as a result of determination in the current frame A, the case in which the diaphragm unit  2   b  is controlled to be driven by one step in the positive direction (the diaphragm direction) is shown. 
     Subsequently, the imaging unit  3  outputs the image signal according to the electric signal obtained by photoelectrically converting the optical image of the object of the next frame B captured by the optical system  2  to the estimation value calculation unit  5 . The estimation value calculation unit  5  calculates estimation values B1 to Bn corresponding to the regions of interest in the frame B. The estimation value calculation unit  5  sequentially outputs the estimation values B1 to Bn to the estimation value storage unit  6  and the estimation value comparison unit  7 . 
     The estimation value comparison unit  7  replaces the reference values Z1 to Zn of the above-mentioned frame Z with the estimation values A1 to An of the frame A stored in the estimation value storage unit  6  in the previous frame A, respectively. The estimation value comparison unit  7  compares the reference value A with the estimation value B of the frame B input from the estimation value calculation unit  5 . The estimation value comparison unit  7  outputs the comparison result signal corresponding to each of the regions of interest to the diaphragm control unit  8 . Accordingly, the diaphragm control unit  8  controls driving of the diaphragm unit  2   b  based on the determination result in the frame B.  FIG. 2  shows the case in which the diaphragm unit  2   b  is controlled to be further driven by one step in the positive direction (the diaphragm direction) based on the determination result in the frame B, and further, the diaphragm unit  2   b  is controlled to be driven by one step in the negative direction (the opening direction) based on the determination result in the next frame C. 
     In this way, in the endoscope system  100 , as the estimation values in the region of interest are calculated and the calculated estimation value and the estimation value (the reference value) of the previous frame are compared at each frame imaged by the solid state imaging device  3   a , the opening of the diaphragm unit  2   b  in the next frame is controlled. Accordingly, in the endoscope system  100 , the opening of the diaphragm unit  2   b , i.e., the range in which the object is focused on, follows a variation in the distance at which the object is distributed, and the focused image focused on the entire image can be photographed. 
     The entire control operation of the diaphragm unit  2   b  in the endoscope system  100  according to the first embodiment will be described.  FIG. 4  is a view schematically showing an example of the entire diaphragm control operation in the endoscope system  100  according to the first embodiment.  FIG. 4  schematically shows a relation between the diaphragm position and the focus range in the optical system  2  in which a focal position is set (fixed) to a central position of the distance to the object serving as the photographing target and the entire range of the object position is focused on when the diaphragm unit is maximally narrowed. In addition,  FIG. 4  schematically shows the relation between the object position and the focus range in the frames when the moving object is photographed using the optical system  2  as time elapses. 
     A variation in focus range by the control of the diaphragm unit  2   b  in the endoscope system  100  will be described using  FIG. 4 . The object position shown in  FIG. 4  is a position of the object in a depth direction. The focus range shown in  FIG. 4  is a range of focusing in the depth direction. 
     A relation between the diaphragm position and the focus range in the optical system  2  of the endoscope system  100  will be described. The optical system  2  can control eight diaphragm positions A to H shown in  FIG. 4 . Focus ranges at the diaphragm positions are ranges shown in  FIG. 4 . More specifically, the diaphragm position A, i.e., the focus range when the size of the opening of the diaphragm unit  2   b  is minimized, is a range of object positions 1 to 16, i.e., from a near point closest to the distance to the object to a far point farthest from the distance to the object. The focus range at the diaphragm position B is a range of the object positions 2 to 15. The focus range at the diaphragm position C is a range of the object positions 3 to 14. The focus range at the diaphragm position D is a range of the object positions 4 to 13. The focus range at the diaphragm position E is a range of the object positions 5 to 12. The focus range at the diaphragm position F is a range of the object positions 6 to 11. The focus range at the diaphragm position G is a range of the object positions 7 to 10. The diaphragm position H, i.e., the focus range when the size of the opening of the diaphragm unit  2   b  is maximized, is a range of the object positions 8 to 9. 
     As described above, the focal position of the optical system  2  is fixed to a position of a center of a distance to the object serving as the photographing target, i.e., a position of a black spot a of a center of the focus range in each of the diaphragm positions. 
     In the endoscope system  100 , the image of each frame is photographed using the optical system  2 . Here, in the endoscope system  100 , according to the above-mentioned diaphragm control operation, the driving control of the diaphragm unit  2   b  is performed at each frame, and the diaphragm position when the image of the next frame is photographed is varied. 
       FIG. 4  shows the case in which the object disposed at the range of the object position (the range of the position of the object in the depth direction) shown by a thick-bordered box in a frame F1 is photographed using the opening of the diaphragm unit  2   b  as the diaphragm position A, and based on the determination result of the diaphragm control in the frame F1, the opening of the diaphragm unit  2   b  upon photographing of a frame F2 is controlled to be driven to the diaphragm position B by one step in the negative direction (the opening direction). Next, the case in which the object disposed at the range of the object position shown by the thick-bordered box in the frame F2 is photographed at the diaphragm position B of the opening of the diaphragm unit  2   b , and based on the determination result of the diaphragm control in the frame F2, the opening of the diaphragm unit  2   b  upon photographing of a frame F3 is controlled to be driven to the diaphragm position A by one step in the positive direction (the diaphragm direction) is shown. Next, the case in which the object disposed at the range of the object position shown by the thick-bordered box in the frame F3 is photographed at the diaphragm position A of the opening of the diaphragm unit  2   b , and based on the determination result of the diaphragm control in the frame F3, the opening of the diaphragm unit  2   b  upon photographing of a frame F4 is at the diaphragm position A with no modification is shown. Next, the case in which the object disposed at the range of the object position shown by the thick-bordered box in the frame F4 is photographed at the diaphragm position A of the opening of the diaphragm unit  2   b , and based on the determination result of the diaphragm control in the frame F4, the opening of the diaphragm unit  2   b  upon photographing of a frame F5 is controlled to be driven to the diaphragm position B by one step in the negative direction (the opening direction) is shown. 
     Similarly, based on the determination result of the diaphragm control in each of the frames, the diaphragm unit  2   b  is controlled to be driven at the diaphragm position at which the next frame is photographed. Accordingly, as shown in  FIG. 4 , the diaphragm position can be controlled to follow the object position. 
     In the endoscope system  100 , as described above, the focal position of the optical system  2  is fixed to a position of the black spot a of the center of the focus range. For this reason, in the endoscope system  100 , the object position is controlled to be included in the focus range. That is, while the focused image cannot be photographed normally when the object is in the vicinity of the near point or the far point, a depth of field is increased by controlling the opening of the diaphragm unit  2   b  in the diaphragm direction according to the diaphragm control operation in the endoscope system  100 . Accordingly, in the endoscope system  100 , as shown in  FIG. 4 , even when the object position is deviated toward the near point or the far point, the focused image in which the object is included within the focus range can be photographed. 
     As described above, in the endoscope system  100  according to the first embodiment, the entire image of each of the frames is divided into the plurality of regions of interest. Based on the estimation values of the divided regions of interest, the opening of the diaphragm unit  2   b  when the image of the next frame is imaged is controlled to follow a variation in the distance at which the object is distributed. Accordingly, the high resolution image can be photographed at the appropriate depth of field according to the distance to the object without excessively increasing the depth of field and decreasing the resolution of the photographed image. 
     In the endoscope system  100  according to the first embodiment, the case in which the diaphragm unit  2   b  is driven by one step at each of the frames has been described. However, the number of steps in which the diaphragm unit  2   b  is controlled to be driven is not limited to one step. For example, the driving control of the diaphragm unit  2   b  may be appropriately varied, such as controlling driving the diaphragm unit  2   b  in a plurality of steps, rather than one step, or not operating the diaphragm unit  2   b , according to the number of regions of interest in which it is determined that the estimation value of the current frame is larger than the reference value of the previous frame, i.e., “the estimation value&gt;the reference value.” For example, the driving control of the diaphragm unit  2   b  may be appropriately varied according to a difference between the reference value of the previous frame and the estimation value of the current frame when the estimation value comparison unit  7  compares the magnitude correlations. 
     In the endoscope system  100  according to the first embodiment, the case in which the estimation value comparison unit  7  compares the reference value of the previous frame and the estimation value of the current frame at each of the regions of interest has been described. However, a comparison method of the reference value and the estimation value in the estimation value comparison unit  7  is not limited to the above-mentioned comparison method. For example, the estimation value comparison unit  7  may incorporate the estimation values of all the regions of interest into one value by calculating a statistical value, and compare the incorporated reference value and the incorporated estimation values. 
     In the endoscope system  100  according to the first embodiment, the case in which the regions of interest set by the region setting unit  4  are regions having equal sizes with no gap as shown in  FIG. 3  has been described. However, the regions of interest set by the region setting unit  4  is not limited to the above-mentioned example. For example, various regions shown in  FIGS. 5A to 5D  may be set as the regions of interest.  FIG. 5A  shows an example of a region of interest divided into a plurality of discrete regions having gaps in an image of one frame.  FIG. 5B  shows an example of a region of interest divided into regions having irregular sizes that increase toward a center of the image when the region is divided into a plurality of discrete regions having gaps in an image of one frame.  FIG. 5C  shows an example of a region of interest divided into regions having irregular sizes that reduce toward a center of an image when the region is divided into a plurality of regions with no gap in the entire image of one frame.  FIG. 5D  shows an example of a region of interest as one region at a center of an image of one frame. In the endoscope system  100  according to the first embodiment, the estimation value calculation unit  5  calculates the estimation value at each of the regions of interest set by the region setting unit  4 . The estimation value comparison unit  7  compares the reference value of the previous frame and the estimation value of the current frame at each of the regions of interest. Accordingly, the region setting unit  4  sets the same region of interest with at least two frames without setting different regions of interest at each frame. 
     In the endoscope system  100  according to the first embodiment, the case in which the light source apparatus included in the illumination unit  1  is the xenon lamp  1   a  has been described. However, the light source apparatus is not limited to the xenon lamp  1   a . A halogen lamp, a Light Emitting Diode (LED), a laser, or the like, may be installed at the illumination unit  1  as the light source apparatus. In addition, a light adjustment unit may be installed in the illumination unit  1 . 
     Second Embodiment 
     Next, an endoscope system according to a second embodiment of the present invention will be described.  FIG. 6  is a block diagram showing an example of a schematic configuration of the endoscope system according to the second embodiment. An endoscope system  200  shown in  FIG. 6  includes an illumination unit  11 , an optical system  2 , an imaging unit  13 , a region setting unit  14 , an estimation value calculation unit  15 , an estimation value storage unit  16 , an estimation value comparison unit  17 , and a diaphragm control unit  18 . The endoscope system  200  may further include an image processing unit  9  and an image output unit  10 . In the endoscope system  200  according to the second embodiment, the optical system  2 , the image processing unit  9 , and the image output unit  10  are the same components as in the endoscope system  100  according to the first embodiment. The components of the endoscope system  200  according to the second embodiment different from the components of the endoscope system  100  according to the first embodiment may also include components having the same configuration as the endoscope system  100  according to the first embodiment. Accordingly, the same components and the same configurations as the endoscope system  100  according to the first embodiment will be designated by the same reference numerals, and a detailed description thereof will be omitted here. 
     The illumination unit  11  includes an LED  11   a  as a light source apparatus. The illumination unit  11  further includes a light adjustment unit  11   b  configured to adjust light emitted from the LED  11   a . The illumination unit  11  radiates the light from the LED  11   a  modulated by the light adjustment unit  11   b  to an object in a body photographed by the endoscope system  200 . 
     The imaging unit  13  has a configuration in which the gain adjustment unit  3   b  is removed from the imaging unit  3  installed in the endoscope system  100  according to the first embodiment. That is, the imaging unit  13  includes only the solid state imaging device  3   a  in the imaging unit  3  installed in the endoscope system  100  according to the first embodiment. The imaging unit  13  outputs an electric signal of each frame obtained by photoelectrically converting the optical image of the object captured by the solid state imaging device  3   a  through the optical system  2  to the estimation value calculation unit  15  and the image processing unit  9  as the image signal. 
     The region setting unit  14  sets the region of interest in which the image of one frame output from the imaging unit  13  is divided into, for example, a plurality of discrete regions disposed with gaps. Here, the region setting unit  14  sets the size of each of the regions of interest to an irregular size that increases toward a center of the image. The size of the region of interest set by the region setting unit  14  is determined, for example, to match a distortion aberration property of the optical lens  2   a  installed in the optical system  2 . 
     The estimation value calculation unit  15  detects an amount of a high frequency element, from which a noise element is removed, from the image signal input from the imaging unit  13  at each region of interest set by the region setting unit  14 . The estimation value calculation unit  15  calculates the estimation value at each of the regions of interest based on the detected amount of the high frequency element at each of the regions of interest. The estimation value calculation unit  15  outputs the one estimation value obtained by adding the estimation values to the estimation value storage unit  16  and the estimation value comparison unit  17 . 
     The estimation value storage unit  16  stores the one estimation value input from the estimation value calculation unit  15 . The estimation value storage unit  16  outputs the stored one estimation value to the estimation value comparison unit  17  as the reference value. The estimation value storage unit  16  can reduce a circuit scale because only the one estimation value is stored. 
     The estimation value comparison unit  17  reads the reference value serving as the one estimation value of the previous frame (the summed estimation value) from the estimation value storage unit  16  at the timing at which the estimation value calculation unit  15  outputs the estimation value of the current frame. The estimation value comparison unit  17  compares magnitudes of the read one reference value and the one estimation value input from the estimation value calculation unit  15 . The estimation value comparison unit  17  outputs one comparison result (comparison result signal) representing whether the one estimation value input from the estimation value calculation unit  15  is larger than the one reference value read from the estimation value storage unit  16  to the diaphragm control unit  18 . 
     The diaphragm control unit  18  determines whether the opening of the diaphragm unit  2   b  is controlled to be driven in the diaphragm direction or the opening direction based on the one comparison result input from the estimation value comparison unit  17 . More specifically, if the comparison result input from the estimation value comparison unit  17  is that “the estimation value&gt;the reference value,” the diaphragm control unit  18  determines that the diaphragm unit  2   b  is driven by one step in the same direction in which it was previously driven (any one of the diaphragm direction and the opening direction). Here, the diaphragm control unit  18  outputs a control signal for controlling driving of the diaphragm unit  2   b  in the determined direction. If the comparison result input from the estimation value comparison unit  17  is not that “the estimation value&gt;the reference value,” the diaphragm control unit  18  determines that the diaphragm unit  2   b  is driven by one step in an opposite direction to that in which it was previously driven. Here, the diaphragm control unit  18  outputs a control signal for controlling driving of the diaphragm unit  2   b  in the determined direction. 
     The image processing unit  9  outputs image-processed image data, in which the image signal of each of the frames input from the imaging unit  13  is converted into, for example, a format displayed on a monitor connected to the endoscope system  200 , to the image output unit  10 . The image output unit  10  outputs and displays the image data input from the image processing unit  9  to, for example, the monitor connected to the endoscope system  200  at each of the frames. 
     According to the above-mentioned configuration, in the endoscope system  200 , the image of the one frame imaged by the solid state imaging device  3   a  is divided into the plurality of regions of interest. Based on the one estimation value calculated from the image signals of the divided regions of interest, the opening of the diaphragm unit  2   b  when the image of the next frame is imaged is controlled. Accordingly, in the endoscope system  200 , like the endoscope system  100  according to the first embodiment, as the opening of the diaphragm unit  2   b  is controlled based on the distance at which the object is distributed, the focused image having high resolution photographed by increasing the opening of the diaphragm unit  2   b , or the same focused image as the related art photographed by reducing the opening of the diaphragm unit  2   b  can be photographed. 
     Next, an operation of the endoscope system  200  according to the second embodiment will be described.  FIG. 7  is a timing chart showing an example of a diaphragm control operation in the endoscope system  200  according to the second embodiment. As described above, in the endoscope system  200 , the estimation value calculation unit  15  calculates the estimation value of the region of interest at each of the frames imaged by the solid state imaging device  3   a . The endoscope system  200  outputs the one estimation value obtained by adding the estimation values. The endoscope system  200  controls driving of the diaphragm unit  2   b  based on the one estimation value. 
     In the following description, the case in which the region setting unit  14  sets the region of interest obtained by dividing the image of the one frame output from the imaging unit  13  into the plurality of discrete regions disposed with gaps and having irregular sizes that increase toward a center of the image will be described. In this case, the region of interest set by the region setting unit  14  is, for example, a region of interest disposed as shown in  FIG. 5B . As the region of interest is set as shown in  FIG. 5B , the number of regions of interest in the endoscope system  200  is reduced, and a circuit scale when the estimation value calculation unit  15  calculates the estimation value of each of the regions of interest can be reduced. 
     The imaging unit  13  outputs the image signal according to the electric signal obtained by photoelectrically converting the optical image of the object of the current frame A captured through the optical system  2  to the estimation value calculation unit  15 . In the endoscope system  200  according to the second embodiment, the light adjustment unit  11   b  configured to adjust light radiated by the LED  11   a  radiated to the object is installed in the illumination unit  11  as a means of correcting the brightness of the image, which is imaged by the solid state imaging device  3   a , varying depending on the opening and closing of the diaphragm unit  2   b  to an appropriate brightness. The imaging unit  13  controls the light adjustment unit  11   b  in the illumination unit  11  and increases the emission intensity of the LED  11   a  when the image of the frame A imaged by the solid state imaging device  3   a  is dark. Accordingly, the imaging unit  13  outputs the image signal adjusted to the constant brightness regardless of the opening and closing of the diaphragm unit  2   b  to the estimation value calculation unit  15  as the image signal of the frame A. 
     The estimation value calculation unit  15  calculates the estimation value in which the amount of the high frequency element, from which a noise element is removed, is detected from the image signal input from the imaging unit  13 , at each of the regions of interest set by the region setting unit  14  and obtained by dividing the image of the frame A into n regions discretely disposed with gaps having sizes that increase toward a center of the image. In “A” and “1 to n” of the estimation values A1 to An shown in  FIG. 7 , like the description of the operation of the endoscope system  100  according to the first embodiment, “A” represents the estimation value of the frame A and “1 to n” represent the corresponding regions of interest. The estimation value calculation unit  15  outputs the one estimation value Σ(A1:An) obtained by adding the calculated estimation values A1 to An corresponding to the regions of interest of the frame A to the estimation value storage unit  16  and the estimation value comparison unit  17 . 
     The estimation value storage unit  16  stores the one estimation value Σ(A1:An) of the frame A input from the estimation value calculation unit  15 . 
     The estimation value comparison unit  17  compares the one estimation value Σ(A1:An) of the frame A input from the estimation value calculation unit  15  and the one reference value Σ(Z1:Zn), which is the summed one estimation value of the previous frame Z stored in the estimation value storage unit  16 . In “Z” and “1 to n” in the reference values Z1 to Zn shown in  FIG. 7 , like the description of the operation of the endoscope system  100  according to the first embodiment, “Z” represents the reference value of the frame Z, and “1 to n” represent the corresponding regions of interest. 
     More specifically, as shown in  FIG. 7 , the estimation value comparison unit  17  reads the one reference value Σ(Z1:Zn) of the frame Z stored in the estimation value storage unit  16  at the timing at which the estimation value calculation unit  15  outputs the one estimation value Σ(A1:An) of the frame A. The estimation value comparison unit  17  compares magnitudes of the estimation value Σ(A1:An) and the reference value Σ(Z1:Zn). The estimation value comparison unit  17  outputs the comparison result representing whether the estimation value Σ(A1:An) is larger than the reference value Σ(Z1:Zn) to the diaphragm control unit  18 . 
     Provisionally, the case in which the diaphragm unit  2   b  is driven by one step in the negative direction (the opening direction) from the previous frame Z to the current frame A is supposed. The estimation value comparison unit  17  outputs the comparison result obtained by comparing the magnitude correlations of the reference value Σ(Z1:Zn) and the estimation value Σ(A1:An) to the diaphragm control unit  18 .  FIG. 7  shows the case in which the magnitude comparison result of the reference value Σ(Z1:Zn) and the estimation value Σ(A1:An) is that “the reference value Σ(Z1:Zn)&lt;the estimation value Σ(A1:An).” Here, the estimation value comparison unit  17  outputs, for example, the comparison result signal=“1” representing the magnitude comparison result to the diaphragm control unit  18  as the comparison result in the frame A. 
     The diaphragm control unit  18  determines a direction in which the diaphragm unit  2   b  is driven based on the comparison result input from the estimation value comparison unit  17 . The diaphragm control unit  18  controls driving of the diaphragm unit  2   b  in the determined direction. More specifically, if the comparison result input from the estimation value comparison unit  17  is that “the reference value Σ(Z1:Zn)&lt;the estimation value Σ(A1:An),” i.e., the comparison result signal is “1,” the diaphragm control unit  18  determines that the current frame A has a larger number of focused regions of interest than the previous frame Z. Here, the diaphragm control unit  18  controls the diaphragm unit  2   b  to be driven by one step in the same direction as the direction in which the diaphragm unit  2   b  was previously driven.  FIG. 7  shows the case in which the diaphragm unit  2   b  is controlled to be driven by one step in the negative direction (the opening direction), as a result of determination in the current frame A. 
     The imaging unit  13  outputs the image signal according to the electric signal obtained by photoelectrically converting the optical image of the object of the next frame B captured through the optical system  2  to the estimation value calculation unit  15 . The estimation value calculation unit  15  calculates the estimation values B1 to Bn corresponding to the regions of interest in the frame B. The estimation value calculation unit  15  outputs the one estimation value Σ(B1:Bn) obtained by adding the estimation values B1 to Bn to the estimation value storage unit  16  and the estimation value comparison unit  17 . 
     The estimation value comparison unit  17  replaces the above-mentioned one reference value Σ(Z1:Zn) with the one estimation value Σ(A1:An) of the frame A stored in the estimation value storage unit  16  in the previous frame A. The estimation value comparison unit  17  performs magnitude comparison of the reference value Σ(A1:An) and the one estimation value Σ(B1:Bn) of the frame B input from the estimation value calculation unit  15 . The estimation value comparison unit  17  outputs the comparison result to the diaphragm control unit  18 .  FIG. 7  shows the case in which the magnitude comparison result of the reference value Σ(A1:An) and the estimation value Σ(B1:Bn) is that “the reference value Σ(A1:An)≧the estimation value Σ(B1:Bn).” Here, the estimation value comparison unit  17  outputs, for example, the comparison result signal=“0” representing the magnitude comparison result to the diaphragm control unit  18  as the comparison result in the frame B. 
     The diaphragm control unit  18  determines that the current frame B has a smaller number of focused regions of interest than the previous frame A if the comparison result input from the estimation value comparison unit  17  is that “the reference value Σ(A1:An)≧the estimation value Σ(B1:Bn),” i.e., the comparison result signal is “0.” Here, the diaphragm control unit  18  controls the diaphragm unit  2   b  to be driven by one step in an opposite direction to the direction in which the diaphragm unit  2   b  was previously driven.  FIG. 7  shows the case in which the diaphragm unit  2   b  is controlled to be driven by one step in the positive direction (the diaphragm direction), as a result of determination in the current frame B. 
     Thereafter, similarly, the diaphragm unit  2   b  is controlled to be driven based on the determination result in the next frame.  FIG. 7  shows the case in which the diaphragm unit  2   b  is controlled to be driven by one step in the negative direction (the opening direction) based on the determination result in the frame C. 
     In this way, in the endoscope system  200 , the estimation value of each of the regions of interest is calculated at each of the frames imaged by the solid state imaging device  3   a . As the one estimation value obtained by adding the calculated estimation values and the one estimation value (reference value) of the previous frame are compared, the opening of the diaphragm unit  2   b  in the next frame is controlled. Accordingly, in the endoscope system  200 , like the endoscope system  100  according to the first embodiment, the focused image in which the opening of the diaphragm unit  2   b , i.e., the range in which the object is focused on, follows a variation in the distance at which the object is distributed can be photographed. 
     The entire control operation of the diaphragm unit  2   b  in the endoscope system  200  according to the second embodiment is the same as that of the diaphragm unit  2   b  in the endoscope system  100  according to the first embodiment shown in  FIG. 4 , and thus a detailed description thereof will be omitted here. 
     As described above, in the endoscope system  200  according to the second embodiment, the entire image of each of the frames is divided into a plurality of regions of interest. Based on the one estimation value obtained by adding the estimation values of the divided regions of interest, the opening of the diaphragm unit  2   b  when the image of the next frame is imaged is controlled to follow a variation in the distance at which the object is distributed. Accordingly, like the endoscope system  100  according to the first embodiment, a high resolution image can be photographed at an appropriate depth of field according to the distance to the object without excessively increasing the depth of field and decreasing the resolution of the photographed image. 
     In the endoscope system  200  according to the second embodiment, the case in which the diaphragm unit  2   b  is driven by one step at each of the frames has been described. However, the number of steps in which the diaphragm unit  2   b  is controlled to be driven is not limited to the above-mentioned example. For example, in consideration of the magnitudes of the reference value Σ(Z1:Zn) and the estimation value Σ(A1:An), the driving control of the diaphragm unit  2   b  may be appropriately varied, such as control of non-operation of the diaphragm unit  2   b  or control of driving of the diaphragm unit  2   b  in a plurality of steps. 
     In the endoscope system  200  according to the second embodiment, the case in which the estimation value calculation unit  15  calculates the one estimation value obtained by adding the estimation values of the regions of interest has been described. However, a method of calculating the one estimation value is not limited to the above-mentioned example. The estimation value calculation unit  15  may calculate the one estimation value through a method other than summation of the estimation values of the regions of interest. For example, like the endoscope system  100  according to the first embodiment, the estimation value calculation unit  15  may output the calculated estimation values of the regions of interest. The estimation value comparison unit  17  may compare the reference value of the previous frame and the estimation value of the current frame at each of the regions of interest. 
     In the endoscope system  200  according to the second embodiment, the case in which the regions of interest set by the region setting unit  14  are a plurality of regions discretely disposed with gaps and having sizes that increase toward the center of the image has been described. However, the regions of interest set by the region setting unit  14  are not limited to the above-mentioned example. For example, like the endoscope system  100  according to the first embodiment, various regions such as disposition as shown in  FIG. 3  or  FIGS. 5A to 5D  may be set as the region of interest. 
     In the endoscope system  200  according to the second embodiment, the case in which the light source apparatus installed in the illumination unit  11  is the LED  11   a  and the light adjustment unit  11   b  configured to adjust the light emitted from the LED  11   a  is included has been described. However, the light source apparatus may be a halogen lamp, a xenon lamp, a laser, or the like. The light adjustment unit  11   b  is not installed in the illumination unit  11 , and alternatively, a gain adjustment unit configured to correct the brightness of the image, which is imaged by the solid state imaging device  3   a , varying depending on the opening and closing of the diaphragm unit  2   b  to an appropriate brightness may be installed in the imaging unit  13 . 
     Third Embodiment 
     Next, an endoscope system according to a third embodiment of the present invention will be described.  FIG. 8  is a block diagram showing an example of a schematic configuration of the endoscope system according to the third embodiment. An endoscope system  300  shown in  FIG. 8  includes an illumination unit  21 , an optical system  2 , an imaging unit  3 , a region setting unit  24 , an estimation value calculation unit  25 , an estimation value storage unit  26 , an estimation value comparison unit  27 , and a diaphragm control unit  28 . The endoscope system  300  may further include an image processing unit  9  and an image output unit  10 . In the endoscope system  300  according to the third embodiment, the optical system  2 , the imaging unit  3 , the image processing unit  9 , and the image output unit  10  are the same components as in the endoscope system  100  according to the first embodiment. The components of the endoscope system  300  according to the third embodiment different from the components of the endoscope system  100  according to the first embodiment may also include components having the same configuration as the endoscope system  100  according to the first embodiment. Accordingly, the same components and the same configurations as the endoscope system  100  according to the first embodiment will be designated by the same reference numerals, and a detailed description thereof will be omitted here. 
     The illumination unit  21  includes a xenon lamp  1   a  serving as a light source apparatus. The illumination unit  21  further includes a light adjustment unit  21   b  configured to adjust a quantity of the light emitted from the xenon lamp  1   a . The illumination unit  21  radiates the light from the xenon lamp  1   a , a quantity of which is adjusted by the light adjustment unit  21   b , to the object in a body photographed by the endoscope system  300 . 
     The imaging unit  3  is the same as that of the endoscope system  100  according to the first embodiment. The imaging unit  3  adjusts a level of the electric signal of each of the frames imaged by the solid state imaging device  3   a  by the gain adjustment unit  3   b . The imaging unit  3  outputs the adjusted electric signal to the estimation value calculation unit  25  and the image processing unit  9  as the image signal. 
     The region setting unit  24  sets, for example, region of interests divided into a plurality of discrete regions disposed with gaps in the image of the one frame output from the imaging unit  3 . Here, the region setting unit  24  sets the regions of interest to have equal sizes. 
     The estimation value calculation unit  25  detects an amount of the high frequency element, from which a noise element is removed, from the image signal after level adjustment input from the imaging unit  3  at each of the regions of interest set by the region setting unit  24 . The estimation value calculation unit  25  calculates the estimation value corresponding to the detected amount of the high frequency element of each of the regions of interest. The estimation value calculation unit  25  outputs the one estimation value obtained by performing a weighted average of the estimation values to the estimation value storage unit  26 , the estimation value comparison unit  27 , and the diaphragm control unit  28 . 
     The estimation value storage unit  26  stores the one estimation value input from the estimation value calculation unit  25 . The estimation value storage unit  26  outputs the stored one estimation value to the estimation value comparison unit  27  as a reference value. The estimation value storage unit  26  stores the estimation value corresponding to the plurality of frames required for detecting a position of the diaphragm unit  2   b  at which the estimation value becomes a maximum value (hereinafter referred to as “a maximum estimation value”) by the diaphragm control unit  28 . The estimation value storage unit  26  outputs the stored estimation value corresponding to the plurality of frames to the diaphragm control unit  28 . 
     The estimation value comparison unit  27  reads the reference value, which is the one estimation value of the previous one frame (the weight-averaged estimation value), from the estimation value storage unit  26  at the timing at which the estimation value calculation unit  25  outputs the estimation value of the current frame. The estimation value comparison unit  27  compares magnitudes of the read one reference value and the one estimation value input from the estimation value calculation unit  25 . The estimation value comparison unit  27  outputs the one comparison result (comparison result signal) representing whether the one estimation value input from the estimation value calculation unit  25  is larger than the one reference value read from the estimation value storage unit  26  to the diaphragm control unit  28 . 
     The diaphragm control unit  28  determines whether the opening of the diaphragm unit  2   b  is controlled to be driven in the diaphragm direction or the opening direction based on the one comparison result input from the estimation value comparison unit  27 . More specifically, the diaphragm control unit  28  determines that the diaphragm unit  2   b  is driven by one step in the same direction in which it was previously driven (any one of the diaphragm direction and the opening direction) if the comparison result input from the estimation value comparison unit  27  is that “the estimation value&gt;the reference value.” Here, the diaphragm control unit  28  outputs a control signal for controlling driving of the diaphragm unit  2   b  in the determined direction. The diaphragm control unit  28  determines that the diaphragm unit  2   b  is driven by one step in an opposite direction to that in which it was previously driven if the comparison result input from the estimation value comparison unit  27  is not “the estimation value&gt;the reference value.” Here, the diaphragm control unit  28  outputs a control signal for controlling driving of the diaphragm unit  2   b  in the determined direction. 
     The diaphragm control unit  28  detects a maximum estimation value based on the estimation value corresponding to the plurality of frames input from the estimation value storage unit  26  and the estimation value of the current frame input from the estimation value calculation unit  25 . A detection method of the maximum estimation value by the diaphragm control unit  28  is the same method as so-called climbing control widely used as a contrast type auto-focus operation in, for example, a digital camera or the like. For this reason, a detailed description of the detection method of the maximum estimation value by the diaphragm control unit  28  will be omitted here. 
     The diaphragm control unit  28  controls driving of the opening of the diaphragm unit  2   b  to reach the diaphragm position at which the estimation value is maximized when the maximum estimation value is detected. After that, the diaphragm control unit  28  pauses the driving control of the diaphragm unit  2   b  for a preset certain time T. Accordingly, the opening of the diaphragm unit  2   b  is held at the same diaphragm position for the certain time T. For this reason, a so-called wobbling operation performed after focusing at the diaphragm position at which the estimation value is maximized and moved (driven) forward and rearward in an optical axis direction of the optical lens  2   a  can be suppressed. In addition, the image quality of moving images by the endoscope system  300  can be improved. 
     In the endoscope system  300 , even in a period of the certain time T while the diaphragm position is held, the estimation value calculation unit  25  calculates the estimation value, and continues to update the estimation value stored in the estimation value storage unit  26 . The diaphragm control unit  28  drives the diaphragm unit  2   b  by one step in the diaphragm direction after the certain time T elapses. The diaphragm control unit  28  restarts the driving control of the diaphragm unit  2   b  based on the one comparison result input from the estimation value comparison unit  27 . 
     The driving control of the diaphragm unit  2   b  by the diaphragm control unit  28  after the certain time T elapses is not limited to the one step in the above-mentioned diaphragm direction. The driving control of the diaphragm unit  2   b  may be in a plurality of steps or in the opening direction. Calculation of the estimation value by the estimation value calculation unit  25  and update of the estimation value in the estimation value storage unit  26  may be paused for the certain time T in which the diaphragm position is held. 
     The image processing unit  9  outputs the image data obtained through the image processing of converting the image signal of each of the frames input from the imaging unit  3  into, for example, a format displayed on a monitor connected to the endoscope system  300 , to the image output unit  10 . The image output unit  10  outputs and displays the image data input from the image processing unit  9  to, for example, the monitor connected to the endoscope system  300  at each of the frames. 
     According to the above-mentioned configuration, in the endoscope system  300 , the image of the one frame imaged by the solid state imaging device  3   a  is divided into a plurality of regions of interest. Based on the one estimation value calculated from the image signal of the divided regions of interest, the opening of the diaphragm unit  2   b  when the image of the next frame is imaged is controlled. Accordingly, in the endoscope system  300 , like the endoscope system  100  according to the first embodiment or the endoscope system  200  according to the second embodiment, as the opening of the diaphragm unit  2   b  is controlled based on the distance at which the object is distributed, the focused image having high resolution photographed by increasing the opening of the diaphragm unit  2   b , or the same focused image as the related art photographed by reducing the opening of the diaphragm unit  2   b  can be photographed. 
     Next, an operation of the endoscope system  300  according to the third embodiment will be described.  FIG. 9  is a timing chart showing an example of a diaphragm control operation in the endoscope system  300  according to the third embodiment. As described above, in the endoscope system  300 , the estimation value calculation unit  25  calculates the estimation value of the region of interest at each of the frames imaged by the solid state imaging device  3   a . The endoscope system  300  outputs the one estimation value obtained through the weighted average of the estimation values. The endoscope system  300  controls driving of the diaphragm unit  2   b  based on the one estimation value. 
     In the following description, the case in which the region setting unit  24  sets the regions of interest obtained by dividing the image of the one frame output from the imaging unit  3  into the plurality of regions discretely disposed with gaps and having equal sizes will be described. In this case, the regions of interest set by the region setting unit  24  are, for example, regions of interest disposed as shown in  FIG. 5A . As the regions of interest are set as shown in  FIG. 5A , the number of regions of interest in the endoscope system  300  is reduced, and a circuit scale when the estimation value calculation unit  25  calculates the estimation value of each of the regions of interest can be reduced. In addition, correction of the estimation value due to a difference in size of the regions of interest is unnecessary, and the circuit scale can be further reduced. 
     The imaging unit  3  outputs the image signal according to the electric signal obtained by photoelectrically converting the optical image of the object of the current frame A captured through the optical system  2  to the estimation value calculation unit  25 . In the endoscope system  300  according to the third embodiment, as a means for correcting the brightness of the image, which is imaged by the solid state imaging device  3   a , varying depending on the opening and closing of the diaphragm unit  2   b  to an appropriate brightness, the light adjustment unit  21   b  is installed in the illumination unit  21  and the gain adjustment unit  3   b  is installed in the imaging unit  3 . When the image of the frame A imaged by the solid state imaging device  3   a  is dark, first, the imaging unit  3  primarily controls the light adjustment unit  21   b  in the illumination unit  21  and corrects the brightness such that the image of the frame A is brightened. After the light radiated by the illumination unit  21  is maximally brightened, the imaging unit  3  controls amplification of the electric signal output from the solid state imaging device  3   a  using the gain adjustment unit  3   b , and thus the noise element of the image of the frame A is reduced. Accordingly, the imaging unit  3  outputs the image signal controlled to the certain brightness regardless of the opening and closing of the diaphragm unit  2   b  to the estimation value calculation unit  25  as the image signal of the frame A. 
     Here, the estimation value calculation unit  25  calculates the estimation value obtained by detecting the amount of the high frequency element, from which the noise element is removed, from the image signal input from the imaging unit  3  at each of the regions of interest set by the region setting unit  24  and obtained by dividing the image of the frame A into n regions discretely disposed with gaps and having equal sizes. In “A” and “1 to n” in the estimation values A1 to An shown in  FIG. 9 , like the description of the operation of the endoscope system  200  according to the second embodiment, “A” represents the estimation value of the frame A and “1 to n” represent the corresponding regions of interest. K1 to Kn shown in  FIG. 9  are weighting coefficients applied to the estimation values A1 to An according to positions of the region of interest. The estimation value calculation unit  25  calculates the weighted average by multiplying the estimation values A1 to An corresponding to the regions of interest of the calculated frame A by the weighting coefficients K1 to Kn according to the positions of the regions of interest. The estimation value calculation unit  25  outputs the weighted average value to the estimation value storage unit  26 , the estimation value comparison unit  27 , and the diaphragm control unit  28  as one estimation value Σ(K1×A1:Kn×An). 
     In consideration of a property of symmetry in the image of the frame output from the imaging unit  3 , calculation of the weighted average can be simplified by setting the weighting coefficients K1 to Kn to be vertically or horizontally symmetric. 
     The estimation value storage unit  26  stores the one estimation value Σ(K1×A1:Kn×An) of the frame A input from the estimation value calculation unit  25 . 
     The estimation value comparison unit  27  compares the one estimation value Σ(K1×A1:Kn×An) of the frame A input from the estimation value calculation unit  25  and a reference value Σ(K1×Z1:Kn×Zn), which is one weighted average estimation value of the previous frame Z stored in the estimation value storage unit  26 . In “Z” and “1 to n” in the reference values Z1 to Zn shown in  FIG. 9 , like the description of the operation of the endoscope system  200  according to the second embodiment, “Z” represents the reference value of the frame Z and “1 to n” represent the corresponding regions of interest. 
     More specifically, as shown in  FIG. 9 , the estimation value comparison unit  27  reads the one reference value Σ(K1×Z1:Kn×Zn) of the frame Z stored in the estimation value storage unit  26  at the timing at which the estimation value calculation unit  25  outputs the one estimation value Σ(K1×A1:Kn×An) of the frame A. The estimation value comparison unit  27  compares magnitudes of the estimation value Σ(K1×A1:Kn×An) and the reference value Σ(K1×Z1:Kn×Zn). The estimation value comparison unit  27  outputs the comparison result representing whether the estimation value Σ(K1×A1:Kn×An) is larger than the reference value Σ(K1×Z1:Kn×Zn) to the diaphragm control unit  28 . 
     Provisionally, the case in which the diaphragm unit  2   b  is driven by one step in the negative direction (the opening direction) from the previous frame Z to the current frame A is supposed. The estimation value comparison unit  27  outputs the comparison result obtained by comparing the magnitude correlations of the reference value Σ(K1×Z1:Kn×Zn) and the estimation value Σ(K1×A1:Kn×An) to the diaphragm control unit  28 .  FIG. 9  shows the case in which the magnitude comparison result of the reference value Σ(K1×Z1:Kn×Zn) and the estimation value Σ(K1×A1:Kn×An) is that “the reference value Σ(K1×Z1:Kn×Zn)&lt;the estimation value Σ(K1×A1:Kn×An).” Here, the estimation value comparison unit  27  outputs, for example, the comparison result signal=“1” representing the magnitude comparison result to the diaphragm control unit  28  as the comparison result in the frame A. 
     The diaphragm control unit  28  determines a direction in which the diaphragm unit  2   b  is driven based on the comparison result input from the estimation value comparison unit  27 . The diaphragm control unit  28  controls driving of the diaphragm unit  2   b  in the determined direction. More specifically, if the comparison result input from the estimation value comparison unit  27  is that “the reference value Σ(K1×Z1:Kn×Zn)&lt;the estimation value Σ(K1×A1:Kn×An),” i.e., the comparison result signal is “1,” the diaphragm control unit  28  determines that the current frame A has a larger number of focused regions of interest than the previous frame Z. Here, the diaphragm control unit  28  controls the diaphragm unit  2   b  to be driven by one step in the same direction in which it was previously driven. In  FIG. 9 , as a result of determination in the current frame A, the case in which the diaphragm unit  2   b  is controlled to be driven by one step in the negative direction (the opening direction) is shown. 
     The imaging unit  3  outputs the image signal according to the electric signal obtained by photoelectrically converting the optical image of the object of the next frame B captured through the optical system  2  to the estimation value calculation unit  25 . The estimation value calculation unit  25  calculates the estimation values B1 to Bn corresponding to the regions of interest in the frame B. The estimation value calculation unit  25  outputs the one estimation value Σ(K1×B1:Kn×Bn) obtained through weighted average by multiplying the calculated estimation values B1 to Bn by the weighting coefficients K1 to Kn to the estimation value storage unit  26 , the estimation value comparison unit  27 , and the diaphragm control unit  28 . 
     The estimation value comparison unit  27  replaces the above-mentioned one reference value Σ(K1×Z1:Kn×Zn) with the one estimation value Σ(K1×A1:Kn×An) of the frame A stored in the estimation value storage unit  26  in the previous frame A. The estimation value comparison unit  27  compares magnitudes of the reference value Σ(K1×A1:Kn×An) and the one estimation value Σ(K1×B1:Kn×Bn) of the frame B input from the estimation value calculation unit  25 . The estimation value comparison unit  27  outputs the comparison result to the diaphragm control unit  28 .  FIG. 9  shows the case in which the magnitude comparison result of the reference value Σ(K1×A1:Kn×An) and the estimation value Σ(K1×B1:Kn×Bn) is that “the reference value Σ(K1×A1:Kn×An)≧the estimation value Σ(K1×B1:Kn×Bn).” Here, the estimation value comparison unit  27  outputs, for example, the comparison result signal=“0” representing the magnitude comparison result to the diaphragm control unit  28  as the comparison result in the frame B. 
     The diaphragm control unit  28  determines that the current frame B has a smaller number of focused regions of interest than the previous frame A if the comparison result input from the estimation value comparison unit  27  is that “the reference value Σ(K1×A1:Kn×An)≧the estimation value Σ(K1×B1:Kn×Bn),” i.e., the comparison result signal is “0.” The diaphragm control unit  28  controls the diaphragm unit  2   b  to be driven by one step in an opposite direction to that in which it was previously driven. In  FIG. 9 , as a result of determination in the current frame B, the case in which the diaphragm unit  2   b  is driven by one step in the positive direction (the diaphragm direction) is shown. 
     Thereafter, similarly, based on the determination result in the next frame, the diaphragm unit  2   b  is controlled to be driven. Here, the diaphragm control unit  28  controls the opening of the diaphragm unit  2   b  at a diaphragm position of a maximum estimation value when the maximum estimation value is detected. The diaphragm control unit  28  pauses the driving control of the diaphragm unit  2   b  for a preset certain time T only and holds the opening of the diaphragm unit  2   b . The diaphragm control unit  28  restarts the driving control of the diaphragm unit  2   b  after the certain time T elapses.  FIG. 9  shows the case in which the estimation value Σ(K1×C 1 :Kn×Cn) in the frame C is the maximum estimation value and the driving control of the diaphragm unit  2   b  is paused for the certain time T only. 
     In this way, in the endoscope system  300 , the estimation value of each of the regions of interest is calculated at each of the frames imaged by the solid state imaging device  3   a . As the one estimation value obtained through the weighted average of the calculated estimation values and the one estimation value (reference value) of the previous frame are compared, the opening of the diaphragm unit  2   b  of the next frame is controlled. Accordingly, in the endoscope system  300 , like the endoscope system  100  according to the first embodiment and the endoscope system  200  according to the second embodiment, the focused image in which the opening of the diaphragm unit  2   b , i.e., the range in which the object is focused on, follows a variation in the distance at which the object is distributed can be photographed. 
     In the endoscope system  300 , when the diaphragm control unit  28  detects the maximum estimation value, the opening of the diaphragm unit  2   b  is controlled to be driven and held at the diaphragm position of the maximum estimation value. Accordingly, in the endoscope system  300 , frequent variation of the opening of the diaphragm unit  2   b , i.e., the range in which the object is focused on, due to variations such as slight noise in the electric signal of each of the frames imaged by the solid state imaging device  3   a  can be avoided. 
     The entire control operation of the diaphragm unit  2   b  in the endoscope system  300  according to the third embodiment will be described.  FIG. 10  is a view schematically showing an example of the entire diaphragm control operation in the endoscope system  300  according to the third embodiment. Like the entire diaphragm control operation in the endoscope system  100  according to the first embodiment shown in  FIG. 4 ,  FIG. 10  schematically shows a relation between the diaphragm position and the focus range in the optical system  2  in which the focal position is set (fixed) to a position of a center of a distance to the object serving as the photographing target and all the regions of interest is focused on when the diaphragm unit is maximally narrowed.  FIG. 10  schematically shows a relation between the object position and the focus range in each of the frames when the moving object is photographed using the optical system  2  as time elapses. 
     A variation in the focus range by the control of the diaphragm unit  2   b  in the endoscope system  300  will be described using  FIG. 10 . Like the entire diaphragm control operation in the endoscope system  100  according to the first embodiment shown in  FIG. 4 , the object position and the focus range shown in  FIG. 10  is a position of the object in the depth direction and a range of focusing in the depth direction. 
     Since a relation between the diaphragm position and the focus range in the optical system  2  of the endoscope system  300  is similar to the relation between the diaphragm position and the focus range in the endoscope system  100  according to the first embodiment shown in  FIG. 4 , a detailed description thereof will be omitted here. 
     In the endoscope system  300 , the image of each of the frames is photographed using the optical system  2 . According to the above-mentioned diaphragm control operation, the driving control of the diaphragm unit  2   b  is performed at each of the frames, and the diaphragm position when the image of the next frame is photographed is varied. In the endoscope system  300 , the driving control of the diaphragm unit  2   b  in the opening direction and the diaphragm direction is performed twice successively, and thus the maximum estimation value can be easily detected. 
       FIG. 10  shows the case in which the object in the range of the object position shown by the thick-bordered box in the frame F1 (a range of a position of the object in the depth direction) is photographed while the opening of the diaphragm unit  2   b  is at the diaphragm position A, and based on the determination result of the diaphragm control in the frame F1, the opening of the diaphragm unit  2   b  upon photographing of the frame F2 is controlled to be driven by one step in the negative direction (the opening direction) at the diaphragm position B. Similarly, based on the determination result of the diaphragm control in each of the frames, the diaphragm unit  2   b  is controlled to be driven to the diaphragm position at which the next frame is photographed. 
       FIG. 10  shows the case in which the frame F4 of the object in the range of the object position shown by the thick-bordered box in the frame F4 is photographed while the opening of the diaphragm unit  2   b  is at the diaphragm position B, and based on the determination result of the diaphragm control in the frame F4, the opening of the diaphragm unit  2   b  upon photographing of the frame F5 is controlled to be driven by one step in the positive direction (the diaphragm direction) to the diaphragm position A. Here,  FIG. 10  shows the case in which the estimation value of the frame F4 is detected as a maximum estimation value, and after the driving control to the diaphragm position A is performed, the driving control of the diaphragm unit  2   b  is paused for a certain time T only, i.e., the opening of the diaphragm unit  2   b  is held at the diaphragm position A of the maximum estimation value. 
       FIG. 10  shows the case in which, after the certain time T elapses, the object in the range of the object position shown by a thick-bordered box in a frame Fs is photographed while the opening of the diaphragm unit  2   b  is at the diaphragm position A, and based on the determination result of the diaphragm control in the frame Fs, the opening of the diaphragm unit  2   b  upon photographing of a frame Fs+1 is controlled to be driven by one step in the negative direction (the opening direction) to the diaphragm position B. 
     Similarly, based on the determination result and the estimation value of the diaphragm control in each of the frames, the pause or driving of the diaphragm unit  2   b  to the diaphragm position at which the next frame is photographed is controlled. Accordingly, as shown in  FIG. 10 , the diaphragm position can be controlled to follow the object position. Accordingly, in the endoscope system  300 , as shown in  FIG. 10 , even if the object position is deviated toward a near point or a far point, the focused image in which the object is received in the focus range can be photographed. 
     As described above, in the endoscope system  300  according to the third embodiment, the entire image of each of the frames is divided into the plurality of regions of interest. Based on the one estimation value obtained by adding the estimation values of the divided regions of interest, the opening of the diaphragm unit  2   b  when the image of the next frame is imaged is controlled to follow a variation in the distance at which the object is distributed. Accordingly, like the endoscope system  100  according to the first embodiment and the endoscope system  200  according to the second embodiment, the high resolution image can be photographed at an appropriate depth of field according to the distance to the object without excessively increasing the depth of field and decreasing the resolution of the photographed image. 
     In the endoscope system  300  according to the third embodiment, when the maximum estimation value is detected, the diaphragm position is held. Accordingly, frequent variations of the diaphragm position, i.e., the focus range in which the object is focused on, due to variations such as slight noise in the electric signal of each of the frames can be avoided, and the image of the stable depth of field can be photographed. 
     In the endoscope system  300  according to the third embodiment, the case in which the diaphragm unit  2   b  is driven by one step at each of the frames has been described. However, the number of steps in which the diaphragm unit  2   b  is controlled to be driven is not limited to the above-mentioned example. For example, in consideration of the magnitudes of the reference value Σ(K1×Z1:Kn×Zn) and the estimation value Σ(K1×A1:Kn×An), the driving control of the diaphragm unit  2   b  may be appropriately varied, such as control of non-operation of the diaphragm unit  2   b , control of driving the diaphragm unit  2   b  in a plurality of steps, or the like. 
     In the endoscope system  300  according to the third embodiment, the case in which the estimation value calculation unit  25  calculates the one estimation value obtained through weighted average of the estimation values of the regions of interest has been described. However, the method of calculating the one estimation value is not limited to the above-mentioned example. The one estimation value may be calculated through a method other than the weighted average of the estimation values of the regions of interest by the estimation value calculation unit  25 . For example, like the endoscope system  100  according to the first embodiment, the estimation value calculation unit  25  may output the calculated estimation values of the regions of interest. The estimation value comparison unit  27  may compare the reference value of the previous frame and the estimation value of the current frame at each of the regions of interest. 
     In the endoscope system  300  according to the third embodiment, the case in which the regions of interest set by the region setting unit  24  are the plurality of regions discretely disposed with gaps and having equal sizes has been described. However, the regions of interest set by the region setting unit  24  are not limited to the above-mentioned example. For example, like the endoscope system  100  according to the first embodiment or the endoscope system  200  according to the second embodiment, various regions may be set as the regions of interest, such as in the dispositions shown in  FIG. 3  or  FIGS. 5A to 5D . 
     In the endoscope system  300  according to the third embodiment, the case in which the light source apparatus installed in the illumination unit  21  is the xenon lamp  1   a  and the light adjustment unit  21   b  configured to adjust and radiate a quantity of the light emitted from the xenon lamp  1   a  is included has been described. However, the light source apparatus may be a halogen lamp, an LED, a laser, or the like. The light adjustment unit  21   b  may not be installed in the illumination unit  21 . 
     Fourth Embodiment 
     Next, an endoscope system according to a fourth embodiment will be described.  FIG. 11  is a block diagram showing an example of a schematic configuration of the endoscope system according to the fourth embodiment. An endoscope system  400  shown in  FIG. 11  includes an illumination unit  11 , an optical system  22 , an imaging unit  3 , a region setting unit  34 , an estimation value calculation unit  35 , an estimation value storage unit  36 , an estimation value comparison unit  37 , and a diaphragm control unit  38 . The endoscope system  400  may further include an image processing unit  9  and an image output unit  10 . In the endoscope system  400  according to the fourth embodiment, the imaging unit  3 , the image processing unit  9 , and the image output unit  10  are the same components as those of the endoscope system  100  according to the first embodiment. The illumination unit  11  is the same component as that of the endoscope system  200  according to the second embodiment. The components of the endoscope system  400  according to the fourth embodiment different from the components of the endoscope system  100  according to the first embodiment may also include components including the same configuration as in the endoscope system  100  according to the first embodiment. Accordingly, the same components and the same configurations as the endoscope system  100  according to the first embodiment or the endoscope system  200  according to the second embodiment are designated by the same reference numerals, and a detailed description thereof will be omitted here. 
     The optical system  22  further includes a lens driving unit  2   c  in addition to the optical system  2  included in the endoscope system  100  according to the first embodiment. The lens driving unit  2   c  is interlocked with the opening of the diaphragm unit  2   b  to move the focal position of the optical lens  2   a . The optical system  22  delivers the object light having a focal position varied by the lens driving unit  2   c  to the imaging unit  3 . 
     The imaging unit  3  is similar to that in the endoscope system  100  according to the first embodiment. The imaging unit  3  adjusts the level of the electric signal of each of the frames imaged by the solid state imaging device  3   a  using the gain adjustment unit  3   b . The imaging unit  3  outputs the adjusted electric signal to the estimation value calculation unit  35  and the image processing unit  9  as the image signal. 
     The region setting unit  34  sets the regions of interest obtained by dividing the entire image of the one frame output from the imaging unit  3  into, for example, the plurality of regions with no gap. Here, the region setting unit  34  set the magnitudes of the regions of interest to irregular sizes that reduce toward a center of the image. 
     The estimation value calculation unit  35  detects an amount of a high frequency element, from which a noise element is removed, from the image signal after level adjustment input from the imaging unit  3  at each of the regions of interest set by the region setting unit  34 . The estimation value calculation unit  35  calculates the estimation value corresponding to the amount of the high frequency element of each of the detected regions of interest. The estimation value calculation unit  35  outputs the calculated estimation value to the estimation value storage unit  36 , the estimation value comparison unit  37 , and the diaphragm control unit  38 . 
     The estimation value storage unit  36  individually stores all of the estimation values of each of the regions of interest input from the estimation value calculation unit  35  for each frame. The estimation value storage unit  36  outputs each of the stored estimation values to the estimation value comparison unit  37  as the reference value. 
     The estimation value comparison unit  37  reads the reference value of the corresponding region of interest stored in the estimation value storage unit  36  at the timing at which the estimation value calculation unit  35  outputs the estimation value of the current frame. The estimation value comparison unit  37  compares the estimation value of each of the regions of interest input from the estimation value calculation unit  35  and the reference value of the corresponding region of interest read from the estimation value storage unit  36  at each of the regions of interest set by the region setting unit  34 . 
     In comparison of the estimation value and the reference value by the estimation value comparison unit  37 , comparison of magnitudes of the estimation value and the reference value and calculation of an absolute value of a difference between the estimation value and the reference value (|the estimation value−the reference value|) are performed. The estimation value comparison unit  37  outputs the comparison result (the comparison result signal) obtained by comparing the magnitudes of the estimation value and the reference value and the calculation result of |the estimation value−the reference value| to the diaphragm control unit  38 . 
     The diaphragm control unit  38  determines whether the opening of the diaphragm unit  2   b  installed in the optical system  2  is controlled to be driven in the diaphragm direction or the opening direction based on the calculation result and the comparison result of all the regions of interest input from the estimation value comparison unit  37 . More specifically, the diaphragm control unit  38  counts the number of regions of interest for which it is determined that “the estimation value&gt;the reference value” in the comparison result input from the estimation value comparison unit  37 . The diaphragm control unit  38  determines that the diaphragm unit  2   b  is driven by one step in the same direction in which it was previously driven (any one of the diaphragm direction and the opening direction) if the counted result is equal to or larger than the preset number. Here, the diaphragm control unit  38  outputs a control signal for controlling driving of the diaphragm unit  2   b  in the determined direction. The diaphragm control unit  38  determines that the diaphragm unit  2   b  is driven by one step in an opposite direction to that in which it was previously driven if the counted result is smaller than the preset number. Here, the diaphragm control unit  38  outputs a control signal for controlling driving of the diaphragm unit  2   b  in the determined direction. 
     The diaphragm control unit  38  determines whether the opening of the diaphragm unit  2   b  is held or not based on the estimation value of each of the regions of interest in the current frame input from the estimation value calculation unit  35 . Determining by the diaphragm control unit  38  whether the opening of the diaphragm unit  2   b  is held or not is performed based on the estimation value of all the regions of interest in the current frame input from the estimation value calculation unit  35  or the representative value obtained through weighted average of the estimation value of a predetermined number of the regions of interest. The diaphragm control unit  38  determines that the opening of the diaphragm unit  2   b  is held when a position of the diaphragm unit  2   b  at which the calculated representative value is maximized is detected. A detection method of the maximum representative value by the diaphragm control unit  38  is the same detection method of the maximum estimation value in the diaphragm control unit  28  installed in the endoscope system  300  according to the third embodiment. That is, in the diaphragm control unit  38 , instead of the estimation value used to detect the maximum estimation value by the diaphragm control unit  28  installed in the endoscope system  300  according to the third embodiment, the maximum representative value is detected using the representative value. Like the diaphragm control unit  28  installed in the endoscope system  300  according to the third embodiment, the diaphragm control unit  38  controls the opening of the diaphragm unit  2   b  to be at the diaphragm position at which the representative value is maximized when the maximum representative value is detected. After that, the diaphragm control unit  38  pauses the driving control of the diaphragm unit  2   b  and holds the diaphragm position for a preset certain time T. 
     The diaphragm control unit  38  may restart the driving control of the diaphragm unit  2   b  even while the driving control of the diaphragm unit  2   b  at the diaphragm position is paused, i.e., even in a state in which the certain time T does not elapse. Determining by the diaphragm control unit  38  whether the driving control of the diaphragm unit  2   b  is restarted or not is performed based on the calculation result of all the regions of interest input from the estimation value comparison unit  37 . More specifically, if the calculation result of all the regions of interest input from the estimation value comparison unit  37  does not exceed a set value P, which is preset, the diaphragm control unit  38  determines that the pause of the driving control of the diaphragm unit  2   b  is continued. If the calculation result exceeding the set value P, which is preset, in the calculation result of all the regions of interest input from the estimation value comparison unit  37  is at least one, the diaphragm control unit  38  determines that the driving control of the diaphragm unit  2   b  from the next frame is restarted, without waiting for the certain time T to elapse. The driving control of the diaphragm unit  2   b  after determining that the diaphragm control unit  38  restarts the driving control of the diaphragm unit  2   b  is the same driving control as the diaphragm unit  2   b  by the diaphragm control unit  28  installed in the endoscope system  300  according to the third embodiment. 
     The image processing unit  9  outputs image data obtained through image processing of converting the image signal of each of the frames input from the imaging unit  3  into, for example, a format displayed on a monitor connected to the endoscope system  400 , to the image output unit  10 . The image output unit  10  outputs and displays the image data input from the image processing unit  9  to, for example, the monitor connected to the endoscope system  400  at each of the frames. 
     According to the above-mentioned configuration, in the endoscope system  400 , the image of the one frame imaged by the solid state imaging device  3   a  is divided into the plurality of regions of interest. Based on the estimation value of the divided regions of interest, the opening of the diaphragm unit  2   b  when the image of the next frame is imaged is controlled. Accordingly, in the endoscope system  400 , like the endoscope systems according to the first to third embodiments, as the opening of the diaphragm unit  2   b  is controlled according to the distance at which the object is distributed, the high resolution focused image photographed by increasing the opening of the diaphragm unit  2   b  or the same focused image as the related art photographed by reducing the opening of the diaphragm unit  2   b  can be photographed. 
     Next, an operation of the endoscope system  400  according to the fourth embodiment will be described.  FIG. 12  is a timing chart showing an example of a diaphragm control operation in the endoscope system  400  according to the fourth embodiment. As described above, in the endoscope system  400 , the estimation value calculation unit  35  calculates the estimation value of the region of interest at each of the frames imaged by the solid state imaging device  3   a . The endoscope system  400  controls driving of the diaphragm unit  2   b  based on the estimation value of each of the regions of interest. 
     In the following description, the case in which the region setting unit  34  sets the regions of interest obtained by dividing the entire image of the one frame output from the imaging unit  3  into the plurality of regions disposed with no gap and having irregular sizes that reduce toward a center of the image will be described. In this case, the regions of interest set by the region setting unit  34  are, for example, the regions of interest shown in  FIG. 5C . As the regions of interest are set as shown in  FIG. 5C , in a state in which the region of the observation target in the endoscope system  400  is set as the entire image, the driving control of the diaphragm unit  2   b  that gives priority to the position of the center at which the object of interest is likely to be photographed can be performed. 
     The endoscope system  400  includes the lens driving unit  2   c  installed in the optical system  22 . The lens driving unit  2   c  is interlocked with the opening of the diaphragm unit  2   b  to move the focal position of the optical lens  2   a . The lens driving unit  2   c  is set to give priority to the resolution upon proximity observation in the endoscope system  400 . The lens driving unit  2   c  moves the focal position of the optical lens  2   a  such that, regardless of the diaphragm position at which the opening of the diaphragm unit  2   b  is disposed, the object disposed at the near point is included in the focus range and the depth of field which is increased by reducing the opening of the diaphragm unit  2   b  can be effectively used. 
     In the optical system installed in the endoscope system of the related art, as the opening of the diaphragm unit is increased, the focal position of the optical lens moves toward the far point. However, in the optical system  22  of the endoscope system  400 , conversely, as the opening of the diaphragm unit  2   b  is reduced, the focal position of the optical lens  2   a  moves toward the far point. That is, when the opening of the diaphragm unit  2   b  is reduced (driven in the diaphragm direction), the lens driving unit  2   c  is interlocked with movement (opening) of the diaphragm unit  2   b  and gradually moves the focal position of the optical lens  2   a  in the direction of the far point. The lens driving unit  2   c  becomes a pan-focus state in which the entire observation range of the endoscope system  400  is focused on when the magnitude of the opening of the diaphragm unit  2   b  is minimized. 
     The imaging unit  3  outputs the image signal according to the electric signal obtained by photoelectrically converting the optical image of the object of the current frame A captured through the optical system  22  to the estimation value calculation unit  35 . In the endoscope system  400  according to the fourth embodiment, as a means for correcting the brightness of the image, which is imaged by the solid state imaging device  3   a , varying depending on the opening and closing of the diaphragm unit  2   b  to an appropriate brightness, the light adjustment unit  11   b  is installed in the illumination unit  11  and the gain adjustment unit  3   b  is installed in the imaging unit  3 . When the image of the frame A imaged by the solid state imaging device  3   a  is dark, first, the imaging unit  3  primarily controls the light adjustment unit  11   b  in the illumination unit  11  and corrects the image of the frame A to be brightened. After the light radiated from the illumination unit  11  is brightened to a maximum value, the imaging unit  3  controls amplification of the electric signal output from the solid state imaging device  3   a  using the gain adjustment unit  3   b , and thus reduces the noise element of the image of the frame A. Accordingly, the imaging unit  3  outputs the image signal controlled to the certain brightness regardless of the opening and closing of the diaphragm unit  2   b  to the estimation value calculation unit  35  as the image signal of the frame A. 
     Here, the estimation value calculation unit  35  calculates the estimation value obtained by detecting the amount of the high frequency element, from which the noise element is removed, from the image signal input from the imaging unit  3  at each of the regions of interest set by the region setting unit  34  and obtained by dividing the entire image of the frame A into n regions disposed with no gap and having irregular sizes that reduce toward a center of the image in which the object of interest is likely to be photographed. In “A” and “1 to n” in the estimation values A1 to An shown in  FIG. 12 , like the description of the operation of the endoscope system  100  according to the first embodiment, “A” represents the estimation value of the frame A and “1 to n” represent the corresponding regions of interest. Also, in the following description, when the estimation value of the frame A is represented with no discrimination of the region of interest, it is referred to as “the estimation value A.” The estimation value calculation unit  35  sequentially outputs the calculated estimation values A1 to An corresponding to the regions of interest of the frame A to the estimation value storage unit  36 , the estimation value comparison unit  37 , and the diaphragm control unit  38 . 
     The estimation value storage unit  36  sequentially stores the estimation values A1 to An of the frame A input from the estimation value calculation unit  35  into storage regions in the estimation value storage unit  36  corresponding to the regions of interest, respectively. 
     The estimation value comparison unit  37  compares the estimation value A of the frame A input from the estimation value calculation unit  35  and the reference value Z serving as the estimation value stored in the estimation value storage unit  36  and corresponding to the same region of interest as the previous frame Z. In “Z” and “1 to n” in the reference values Z1 to Zn shown in  FIG. 12 , like the description of the operation of the endoscope system  100  according to the first embodiment, “Z” represents the reference value of the frame Z and “1 to n” represent the corresponding regions of interest. Also, in the following description, when the reference value of the frame Z is represented with no discrimination of the region of interest, it is referred to as “the reference value Z.” 
     More specifically, as shown in  FIG. 12 , the estimation value comparison unit  37  sequentially reads the reference values Z1 to Zn of the previous frame Z of the corresponding regions of interest stored in the estimation value storage unit  36  at the timing at which the estimation value calculation unit  35  outputs the estimation values A1 to An of the frame A. The estimation value comparison unit  37  sequentially compares magnitudes of the estimation value A and the reference value Z at each of the regions of interest. The estimation value comparison unit  37  sequentially outputs the comparison result (the comparison result signal) representing whether the estimation value A is larger than the reference value Z or not to the diaphragm control unit  38 . The estimation value comparison unit  37  calculates an absolute value of a difference between the estimation value A and the reference value Z (|the estimation value A−the reference value Z|). The estimation value comparison unit  37  sequentially outputs the calculation result of |the estimation value A−the reference value Z| to the diaphragm control unit  38 . 
     Provisionally, the case in which the diaphragm unit  2   b  is driven by one step in the negative direction (the opening direction) from the previous frame Z to the current frame A is supposed. First, the estimation value comparison unit  37  outputs the comparison result (the comparison result signal) obtained by comparing magnitude correlations of the reference value Z1 and the estimation value A1 of the initial (first) region of interest, and the calculation result to the diaphragm control unit  38 .  FIG. 12  shows the case in which the magnitude comparison result of the reference value Z1 and the estimation value A1 is that “the reference value Z1&gt;the estimation value A1.” Here, the estimation value comparison unit  37  outputs, for example, the comparison result signal=“0” representing the magnitude comparison result to the diaphragm control unit  38  as the comparison result in the first region of interest. The estimation value comparison unit  37  outputs the calculation result (|the estimation value A1−the reference value Z1|) to the diaphragm control unit  38 . 
     Next, the estimation value comparison unit  37  outputs the comparison result (the comparison result signal) in the second region of interest obtained by comparing the magnitude correlations of the reference value Z2 and the estimation value A2 of the second region of interest, and the calculation result to the diaphragm control unit  8 .  FIG. 12  shows the case in which the magnitude comparison result of the reference value Z2 and the estimation value A2 is that “the reference value Z2&lt;the estimation value A2.” Here, the estimation value comparison unit  37  outputs, for example, the comparison result signal=“1” to the diaphragm control unit  38  as the comparison result in the second region of interest. The estimation value comparison unit  37  outputs the calculation result (|the estimation value A2−the reference value Z2|) to the diaphragm control unit  38 . 
     Subsequently, similarly, the estimation value comparison unit  37  repeats comparison of the magnitude correlations of the reference value Z and the estimation value A of each of the regions of interest. The estimation value comparison unit  37  sequentially outputs the comparison result signal=“1” if “the estimation value A&gt;the reference value Z” is true or the comparison result signal=“0” if false to the diaphragm control unit  38  as the comparison result in each of the regions of interest. The estimation value comparison unit  37  outputs the calculation result (|the estimation value A−the reference value Z|) to the diaphragm control unit  38 . In this way, the estimation value comparison unit  37  sequentially outputs the comparison result obtained by comparing the magnitude correlations of the reference value Z and the estimation value A and the calculation result obtained by calculating the absolute value of a difference between the estimation value A and the reference value Z (|the estimation value A−the reference value Z|) with respect to all the n regions of interest to the diaphragm control unit  38 . 
     The diaphragm control unit  38  counts the number of regions of interest for which it is determined that the estimation value A of the frame A is larger than the reference value Z of the frame Z, i.e., “the estimation value A&gt;the reference value Z,” in the comparison results sequentially input from the estimation value comparison unit  37 . For example, in  FIG. 12 , the diaphragm control unit  38  counts the number of regions of interest in which the comparison result signal is “1.” The diaphragm control unit  38  determines a direction in which the diaphragm unit  2   b  is driven based on the counted result and a preset constant. 
     More specifically, the diaphragm control unit  38  determines that the current frame A has a larger number of focused regions of interest than the previous frame Z if the counted result is equal to or larger than a preset constant M. Here, the diaphragm control unit  38  controls the diaphragm unit  2   b  to be driven by one step in the same direction as the direction in which the diaphragm unit  2   b  was previously driven. Conversely, if the counted result is smaller than the preset constant M, the diaphragm control unit  38  determines that the current frame A has a smaller number of focused regions of interest than the previous frame Z. Here, the diaphragm control unit  38  controls the diaphragm unit  2   b  to be driven by one step in an opposite direction of the direction in which the diaphragm unit  2   b  was previously driven. An operation of the diaphragm control unit  38  is similar to that of the endoscope system  100  according to the first embodiment. 
     Here, the diaphragm control unit  38  detects a maximum representative value based on the representative value obtained through weighted average of all or a predetermined number of the estimation values A1 to An of the current frame A input from the estimation value calculation unit  35 . The diaphragm control unit  38  controls the opening of the diaphragm unit  2   b  to arrive at the diaphragm position at which the representative value is maximized. After that, the diaphragm control unit  38  pauses the driving control of the diaphragm unit  2   b  for the preset certain time T and holds the diaphragm position.  FIG. 12  shows the case in which the maximum representative value in the current frame A is detected, the diaphragm unit  2   b  is controlled to be driven by one step in the negative direction (the opening direction), and the driving control of the diaphragm unit  2   b  is paused and the diaphragm position is held for the preset certain time T. 
     Subsequently, the imaging unit  3  outputs the image signal according to the electric signal obtained by photoelectrically converting the optical image of the object of the next frame B captured through the optical system  22  to the estimation value calculation unit  35 . The estimation value calculation unit  35  calculates the estimation values B1 to Bn corresponding to the regions of interest in the frame B. The estimation value calculation unit  35  sequentially outputs the estimation values B1 to Bn to the estimation value storage unit  36 , the estimation value comparison unit  37 , and the diaphragm control unit  38 . 
     The estimation value comparison unit  37  replaces the reference values Z of the above-mentioned frame Z with the estimation values A of the frame A stored in the estimation value storage unit  36  in the previous frame A, respectively. The estimation value comparison unit  37  compares the reference value A and the estimation value B of the frame B input from the estimation value calculation unit  35 . The estimation value comparison unit  37  outputs the comparison result signal and the calculation result corresponding to the regions of interest to the diaphragm control unit  38 . Accordingly, the diaphragm control unit  38  controls driving of the diaphragm unit  2   b  based on the determination result in the frame B. However, in  FIG. 12 , since the driving control of the diaphragm unit  2   b  is paused and the diaphragm position is held, the driving control of the diaphragm unit  2   b  is not performed. 
     The diaphragm control unit  38  determines whether the driving control of the diaphragm unit  2   b  is restarted in the next frame B input from the estimation value calculation unit  35 . As described above, the determination is performed based on the calculation result of all the regions of interest input from the estimation value comparison unit  37 .  FIG. 12  shows the case in which the calculation result exceeding a set value P, which is preset, in the calculation result (|the estimation value B−the reference value A|) of all the regions of interest in the next frame B is at least one, and the diaphragm control unit  38  determines that the driving control of the diaphragm unit  2   b  is restarted from the next frame C without waiting for the certain time T to elapse. 
     The estimation value comparison unit  37  uses each of the estimation values B of the frame B stored in the estimation value storage unit  36  in the previous frame B as a reference value B. The estimation value comparison unit  37  compares the reference value B and an estimation value C of the next frame C input from the estimation value calculation unit  35 . The estimation value comparison unit  37  outputs the comparison result signal and the calculation result corresponding to each of the regions of interest to the diaphragm control unit  38 . Accordingly, the diaphragm control unit  38  controls driving of the diaphragm unit  2   b  based on the determination result in the frame C.  FIG. 12  shows the case in which the diaphragm unit  2   b  is controlled to be driven by one step in the positive direction (the diaphragm direction) based on the determination result in the frame C. 
     In this way, in the endoscope system  400 , the estimation value of each of the regions of interest is calculated at each of the frames imaged by the solid state imaging device  3   a . As the calculated estimation value and the estimation value (the reference value) of the previous frame are compared, the opening of the diaphragm unit  2   b  in the next frame is controlled. Accordingly, in the endoscope system  400 , like the endoscope systems according to the first to third embodiments, the focused image in which the opening of the diaphragm unit  2   b , i.e., the range in which the object is focused on, follows a variation in the distance at which the object is distributed can be photographed. 
     In the endoscope system  400 , the diaphragm control unit  38  controls holding of the diaphragm position due to the pause of the driving control of the diaphragm unit  2   b  and restarting of the driving control of the diaphragm unit  2   b  based on the estimation value of all the regions of interest or the representative value obtained through weighted average of the estimation value of a predetermined number of the regions of interest. Accordingly, in the endoscope system  400 , like the endoscope system  300  according to the third embodiment, frequent variations of the opening of the diaphragm unit  2   b , i.e., the range in which the object is focused on, due to variations such as slight noise in the electric signal of each of the frames imaged by the solid state imaging device  3   a  can be avoided. In the endoscope system  400 , since the driving control of the diaphragm unit  2   b  can be restarted even before the certain time T for which the diaphragm position is held has elapsed, even if the position of the object imaged by the solid state imaging device  3   a  is largely varied, the diaphragm position following the object position can be controlled at an early stage. 
     The entire control operation of the diaphragm unit  2   b  in the endoscope system  400  according to the fourth embodiment will be described.  FIG. 13  is a view schematically showing an example of the entire diaphragm control operation in the endoscope system  400  according to the fourth embodiment.  FIG. 13  schematically shows a relation between the diaphragm position and the focus range in the optical system  22  in which the focal position is interlocked with the opening of the diaphragm unit  2   b  to gradually move in a direction of the far point, and the range of the entire object position is focused on when the diaphragm unit is maximally narrowed. Like the entire diaphragm control operation in the endoscope system  100  according to the first embodiment shown in  FIG. 4  and the entire diaphragm control operation in the endoscope system  300  according to the third embodiment shown in  FIG. 10 ,  FIG. 13  schematically shows the relation between the object position and the focus range in each of the frames when the moving object is photographed using the optical system  22  as time elapses. 
     A variation in the focus range by the control of the diaphragm unit  2   b  in the endoscope system  400  will be described using  FIG. 13 . Like the entire diaphragm control operation in the endoscope system  100  according to the first embodiment shown in  FIG. 4  and the entire diaphragm control operation in the endoscope system  300  according to the third embodiment shown in  FIG. 10 , the object position and the focus range shown in  FIG. 13  are also a position of the object in the depth direction and a range of focusing in the depth direction. 
     First, a relation between the diaphragm position and the focus range in the optical system  22  of the endoscope system  400  will be described. The optical system  22  can be controlled in eight diaphragm positions, i.e., the diaphragm positions A to H shown in  FIG. 13 . The focus range at each of the diaphragm positions is a range shown in  FIG. 13 . More specifically, the diaphragm position A, i.e., the focus range when the size of the opening of the diaphragm unit  2   b  is minimal, is a range of the object positions 1 to 16, i.e., a range from the near point at which the distance to the object is nearest to the far point at which the distance to the object is farthest. The focus range at the diaphragm position B is a range of the object positions 1 to 14. The focus range at the diaphragm position C is a range of the object positions 1 to 12. The focus range at the diaphragm position D is a range of the object positions 1 to 10. The focus range at the diaphragm position E is a range of the object positions 1 to 8. The focus range at the diaphragm position F is a range of the object positions 1 to 6. The focus range at the diaphragm position G is a range of the object positions 1 to 4. The diaphragm position H, i.e., the focus range when the size of the opening of the diaphragm unit  2   b  is maximal, is a range of the object positions 1 to 2. 
     As described above, the focal position of the optical system  22  is interlocked with the opening of the diaphragm unit  2   b  to gradually move to the position of the black spot a shown in  FIG. 13 . In the optical system  22 , when the opening of the diaphragm unit  2   b  is at any one of the diaphragm positions, the nearest point is always included in the focus range. 
     In the endoscope system  400 , the image of each of the frames is photographed using the above-mentioned optical system  22 . According to the above-mentioned diaphragm control operation, the driving control of the diaphragm unit  2   b  is performed at each of the frames, and the diaphragm position when the image of the next frame is photographed is varied. 
       FIG. 13  shows the case in which the object disposed at the range of the object position (the range of the position of the object in the depth direction) shown by the thick-bordered box in the frame F1 is photographed while the opening of the diaphragm unit  2   b  is at the diaphragm position A, and based on the determination result of the diaphragm control in the frame F1, the opening of the diaphragm unit  2   b  upon photographing of the frame F2 is controlled to be driven by one step in the negative direction (the opening direction) to the diaphragm position B. Similarly, based on the determination result of the diaphragm control in each of the frames, the diaphragm unit  2   b  is controlled to be driven to the diaphragm position at which the next frame is photographed. 
       FIG. 13  shows the case in which the frame F4 of the object disposed at the range of the object position shown by the thick-bordered box in the frame F4 is photographed while the opening of the diaphragm unit  2   b  is at the diaphragm position B, and based on the determination result of the diaphragm control in the frame F4, the opening of the diaphragm unit  2   b  upon photographing of the frame F5 is controlled to be driven by one step in the positive direction (the diaphragm direction) to the diaphragm position A. Here,  FIG. 13  shows the case in which the maximum representative value is detected based on the estimation value of the frame F4, and after controlling driving to the diaphragm position A, the driving control of the diaphragm unit  2   b  is paused for the certain time T only, i.e., the opening of the diaphragm unit  2   b  is held at the diaphragm position A of the maximum estimation value. 
       FIG. 13  shows the case in which it is determined that the driving control of the diaphragm unit  2   b  is restarted from the next frame Ft without waiting for the certain time T to elapse. The case in which the object disposed at the range of the object position shown by the thick-bordered box in the frame Ft is photographed while the opening of the diaphragm unit  2   b  is at the diaphragm position A, and based on the determination result of the diaphragm control in the frame Ft, the opening of the diaphragm unit  2   b  upon photographing of the frame Ft+1 is controlled to be driven by one step in the negative direction (the opening direction) at the diaphragm position B is shown. 
     Similarly, based on the determination result of the diaphragm control in each of the frames and the representative value obtained through weighted average of the estimation values, driving of the diaphragm unit  2   b  to the diaphragm position at which the next frame is photographed and pausing of the diaphragm unit  2   b , and restarting of the driving control of the diaphragm unit  2   b  are controlled. Accordingly, as shown in  FIG. 13 , the diaphragm position can be controlled to follow the object position. Accordingly, in the endoscope system  400 , as shown in  FIG. 13 , if the object position is deviated toward the near point, in a state in which the opening of the diaphragm unit  2   b  is increased, the focused image in which the object is received in the focus range can be photographed. 
     As described above, in the endoscope system  400  according to the fourth embodiment, the entire image of each of the frames is divided into the plurality of regions of interest. Based on the estimation value of the divided regions of interest, the opening of the diaphragm unit  2   b  when the image of the next frame is imaged is controlled to follow a variation in the distance at which the object is distributed. Accordingly, like the endoscope systems according to the first to third embodiments, a high resolution image can be photographed at an appropriate depth of field according to the distance to the object without excessively increasing the depth of field and decreasing the resolution of the photographed image. 
     In the endoscope system  400  according to the fourth embodiment, when the maximum representative value is detected, even before the diaphragm position has been held for the certain time T, the driving control of the diaphragm unit  2   b  is restarted. Accordingly, frequent variations in the diaphragm positions, i.e., the focus range in which the object is focused on, due to a variation in slight noise or the like in the electric signal of each of the frames can be avoided. In addition, if the position of the object is largely varied, the focused image that follows the object position can be photographed at an early stage. 
     In the endoscope system  400  according to the fourth embodiment, the case in which the lens driving unit  2   c  installed in the optical system  22  is interlocked with movement (opening) of the diaphragm unit  2   b  to gradually move the focal position of the optical lens  2   a  in a direction of the far point, i.e., sets a high value on the near point, has been described. However, the control method of the focal position by the lens driving unit  2   c  is not limited to the above-mentioned comparison method. A high value may be set for a middle near point or the far point in the control method of the focal position. 
     In the endoscope system  400  according to the fourth embodiment, the case in which the diaphragm unit  2   b  is driven by one step at each of the frames has been described. However, the number of steps in which driving of the diaphragm unit  2   b  is controlled is not limited to one step. For example, in consideration of the magnitudes of the reference value Z and the estimation value A, the driving control of the diaphragm unit  2   b  may be appropriately varied, such as control of non-operation of the diaphragm unit  2   b , control of a plurality of steps of driving the diaphragm unit  2   b , or the like. 
     In the endoscope system  400  according to the fourth embodiment, the case in which the estimation value comparison unit  37  compares the reference value of the previous frame and the estimation value of the current frame at each of the regions of interest has been described. However, the comparison method of the reference value and the estimation value in the estimation value comparison unit  37  is not limited to the above-mentioned comparison method. For example, like the endoscope system  200  according to the second embodiment or the endoscope system  300  according to the third embodiment, as the estimation value comparison unit  37  calculates statistical values, the estimation values of all the regions of interest are combined as one, and the combined one reference value and the estimation value may be compared. 
     In the endoscope system  400  according to the fourth embodiment, the case in which the regions of interest set by the region setting unit  34  are a plurality of regions disposed with no gap and having irregular sizes that reduce toward a center of the image has been described. However, the regions of interest set by the region setting unit  34  are not limited to the above-mentioned example. For example, like the endoscope systems according to the first to third embodiments, various regions may be set as the regions of interest, such as in the dispositions shown in  FIG. 3  or  FIGS. 5A to 5D . 
     In the endoscope system  400  according to the fourth embodiment, the case in which the light source apparatus installed in the illumination unit  11  is the LED  11   a  and the light adjustment unit  11   b  configured to adjust the light emitted from the LED  11   a  is included has been described. However, the light source apparatus may be a halogen lamp, a xenon lamp, a laser, or the like. The light adjustment unit  11   b  may not be installed in the illumination unit  11 . 
     As described above, according to the embodiments of the present invention, the image of each of the frames is divided into a plurality of regions of interest. Based on the estimation values of the divided regions of interest, the opening of the diaphragm unit when the image of the next frame is imaged is controlled to follow a variation in the distance at which the object is distributed. Accordingly, the size of the opening of the diaphragm unit can be increased as much as possible. In addition, the image of the appropriate depth of field that follows the distance to the object can be photographed without decreasing the resolution of the photographed image. 
     According to the embodiments of the present invention, based on the estimation values of the regions of interest, the holding of the diaphragm position and the restarting of the driving of the diaphragm unit are controlled. Accordingly, a variation in the focus range to the object due to the slight noise or the like in the electric signal of each of the frames can be reduced. In addition, the image that follows the position of the object can be photographed at the stable depth of field. 
     In this way, according to the embodiments of the present invention, as the diaphragm unit is controlled, the image in the pan-focus state in which the entire observation range observed in the endoscope apparatus is focused on can be photographed. 
     While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit and scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims.