Abstract:
An imaging apparatus achieves a high-speed automatic focus operation and a reduction in power consumption and prevents a miss focus due to an erroneous determination. The imaging apparatus has an automatic focus function, which detects a current position of a focal lens. A limited range near the detected current position of the focal lens is designated as a focus range. A movable range of the focal lens is changed in accordance with the designated focus range.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to an imaging technique and, more particularly, to an automatic focus system of an imaging apparatus such as a digital camera.  
         [0003]     2. Description of the Related Art  
         [0004]     Generally, an electronic imaging apparatus such as a digital still camera is provided with an automatic focus (AF) system, which automatically performs a focusing operation. As a method of automatic focusing, Japanese Patent Publication No. 39-5265 discloses a control method, which is referred to as a mountain-climbing AF control and used widely. In the mountain-climbing AF control, an integral value of high-frequency components or differences in intensity between adjacent pixels is acquired from an image signal obtained on an individual field or frame basis so as to set the thus-acquired integral value to an AF evaluation value which indicates a degree of focus. Since edge portions of a photographing object are sharp in a focused state, the AF evaluation value is large. On the other hand, the AF evaluation value is small in an unfocused state. When an AF control is being performed, a focal lens, which constitutes a part of an imaging optical system, is moved so as to sequentially acquire the AF evaluation value, and, then, the lens if stopped when the AF evaluation value is at a maximum where it is regarded as a focal point.  
         [0005]     Since an accurate focus is required generally for an imaging apparatus such as a digital still camera which takes a still picture as compared with an imaging apparatus such as a video camera which takes a motion picture, an AF operation is performed each time of taking a picture or a focusing operation is repeatedly performed always in a recording mode.  
         [0006]     However, if the mountain-climbing control AF is applied in each shoot, an amount of movement of the focus lens is large in a distance measurement range in a macro mode or a telephoto side of a zoom camera, which results in a problem in that a time lag, that is, a shutter release time lag, is generated from a time when the operator performs a shoot start request until the shoot is actually performed.  
         [0007]     Additionally, in a digital still camera equipped with a zoom lens of which zoom magnification can be changed by changing a focal distance, it is possible to select one of two kinds of picture range, one is a macro mode which uses, as a picture range, a distance range closer than a predetermined distance and the other is a normal picture mode which uses, as a picture range, a distance range farther than the above-mentioned predetermined distance. However, in the macro mode where a distance between a photographing object and the focal lens is extremely small and an amount of movement of the focal lens in a total distance measurement range is large, and, thus, the above-mentioned shutter release time lag is generated, which results in a problem that the shutter is not released at the right moment.  
         [0008]     Furthermore, since the movement of the focal lens by a motor is always performed as an AF scan operation on an entire object area each time of taking a picture, a large amount of electric power is consumed, which results in decrease in a service life of a battery cell, which is normally used as a power source. Additionally, if, for example, a picture of an object is taken with wire-netting in between, the AF evaluation value has a peak at the wire-netting when an entire object area is subjected to the AF scan operation, which results in an erroneous operation such that a photographing object is faded. As measures for solving such a problem, Japanese Laid-Open Patent Application No. 54-113334 suggests a technique to limit a range to perform a focal determination. Additionally, Japanese Laid-Open Patent Applications No. 2003-230039, No. 2003-262786 and No. 9-211298 disclose techniques to limit the AF scan range, that is, a range of movement of a focal lens by a motor, to a specific range.  
         [0009]     Specifically, Japanese Laid-Open Patent Application No. 54-113334 discloses a technique to perform a focal position detection by selecting a desired distance range from among all distance ranges, but only data in the desired range from among all scan area is used, which may be effective in preventing an erroneous determination but there is little contribution to a reduction in a time period spent on the AF operation and a reduction in an electric power consumption. Japanese Laid-Open Patent Application No. 2003-230039 discloses a technique to perform an AF scan while limiting a focus range when a release operation is performed while operating a specific button. The technique of this patent document is to limit the AF scan range to a predetermined range. Japanese Laid-Open Patent Application No. 2003-262786 discloses a technique to divide a focus range and to perform an AF scan on a designated one of the divided ranges. The technique of this patent document divides a focus range, and performs an AF scan on one of the divided focus ranges so as to perform an AF scan on the other divided focus range only when the focusing operation to said one of the divided focus range cannot be completed. Japanese Laid-Open Application No. 9-211298 disclose a technique to select and set whether to divide a range of movement of a focal lens into a motion permitted area and a motion prohibited area so as to prevent the focal lens to move into the motion prohibited area.  
       SUMMARY OF THE INVENTION  
       [0010]     It is a general object of the present invention to provide an improved and useful imaging apparatus in which the above-mentioned problems are eliminated.  
         [0011]     A more specific object of the present invention is to provide an imaging apparatus and a focusing method, which achieve a high-speed automatic focusing operation and a reduction in power consumption and prevent a miss focus due to an erroneous determination so as to improve convenience of use.  
         [0012]     Another object of the present invention is to provide an imaging apparatus and a focus control method, which achieve a high-speed automatic focusing operation and a reduction in power consumption and contribute to stabilization of a result of the automatic focus operation.  
         [0013]     In order to achieve the above-mentioned objects, there is provided according to the present invention an imaging apparatus having an automatic focus function, comprising: focal lens position detecting means for detecting a current position of a focal lens; focus range designating means for designating as a focus range a limited range near the current position of the focal lens detected by the focal lens position detecting means; and focal lens movable range changing means for changing a movable range of the focal lens in accordance with the focus range designated by the focus range designating means.  
         [0014]     According to the above-mentioned invention, a high-speed automatic focusing operation and a reduction in power consumption can be achieved, and a miss focus due to an erroneous determination can be prevented, which improves convenience of use. That is, a time required for AF can be reduced by designating the AF scan range, and power consumption can be reduced by narrowing the range for operating the focus motor. Further, focus can be made to a photographing object in a desired range even when the photographing object has a large depth that provides a plurality of peaks when a mountain climbing AF is performed.  
         [0015]     The imaging apparatus according to the present invention may further comprise: previous focus result storing means for storing a result of a previous focusing operation; and alarm displaying means for displaying an alarm when no focus is acquired according to the result of a previous focusing operation that has been stored in the previous focus result storing means when designation of the focal range is made by the focus range designation means. Accordingly, photographing can be effectively prevented from being performed under an erroneously focused state. That is, by announcing a user that an area is limited under an unfocused state due to a previous AF result, the user recognizes the state, which can prevent repetition of photographing under the unfocused state.  
         [0016]     The imaging apparatus according to the present invention may further comprise focus range displaying means for displaying the focus range at present. Accordingly, a failure in photographing can be effectively prevented. That is, since the focus range is displayed, a failure in photographing can be suppressed. Additionally, when a range is not limited, it is indicated that the entire range is the focus area, and, thus, it can be recognized beforehand that a time lag in the focusing operation will be increased.  
         [0017]     The imaging apparatus according to the present invention may further comprise focus range position changing means for changing a position of the limited focus range designated by the focus range designating means. According to the above-mentioned structure, a high-speed automatic focusing operation and a reduction in power consumption can be achieved, and a miss focus due to an erroneous determination can be prevented, which improves convenience of use. Additionally, it can easily respond also to position change of a photographing object or a change in photographing conditions.  
         [0018]     The imaging apparatus according to the present invention may further comprise focus range designation canceling means for canceling the setting of the limited focus range designated by the focus range designating means. According to the above-mentioned structure, a high-speed automatic focusing operation and a reduction in power consumption can be achieved further effectively, and a miss focus due to an erroneous determination can be prevented, which improves convenience of use. Additionally, it can be respond to a case where AF is desired to be performed again for all areas.  
         [0019]     The imaging apparatus according to the present invention may further comprise macro mode switching means for switching between a macro mode and a normal mode, the macro mode for taking a picture of a photographing object at an extremely short distance, the normal mode for taking a picture of a photographing object at a normal distance larger than the extremely short distance, wherein the focus range designating means is permitted to designate the focus range only when the macro mode is set. Accordingly, a high-speed AF operation and a reduction in power consumption can be achieved effectively by matching the photographing mode between the normal mode and the macro mode for photographing a picturing object at an extremely short distance, and a false focus due to an erroneous determination can be prevented, which improves usability. That is, in the normal photographing mode, it can easily respond to a change in a picturing object by eliminating designation of a focus range, and a possibility of failure in photographing can be reduced.  
         [0020]     The imaging apparatus according to the present invention may further comprise: macro mode switching means for switching between a macro mode and a normal mode, the macro mode for taking a picture of a photographing object at an extremely short distance, the normal mode for taking a picture of a photographing object at a normal distance larger than the extremely short distance; and means for automatically canceling, when the focus range is designated by the focus range designating means, the designation of the focus range. Accordingly, a high-speed AF operation and a reduction in power consumption can be achieved effectively by matching the photographing mode between the normal mode and the macro mode for photographing a picturing object at an extremely short distance, and a false focus due to an erroneous determination can be prevented, which improves usability. That is, in the ON/OFF switching of the macro mode, there is high possibility of not only the photographing range having been changed but also a picturing object having been changed, and, thus, if is eliminated to forget a cancellation of designation of the range by automatically canceling the limitation of the range. Thereby, a possibility of failure in photographing can be effectively reduced.  
         [0021]     The imaging apparatus according to the present invention may further comprise: zooming means for changing a focal distance of an imaging lens; zoom magnification changing means for changing a zoom magnification achieved by the zooming means; and means for automatically canceling, when the focus range is designated by the focus range designating means, the designation of the focus range. Accordingly, a high-speed AF operation and a reduction in power consumption can be achieved effectively by corresponding to a change in a zoom position of the zoom lens, and a false focus due to an erroneous determination can be prevented, which improves usability. That is, in such a case there is a high possibility that a photographing condition of a picturing object has changed, and, thus, it is possible that the photographing is failed when a picture is taken without change. Thus, the possibility of failure in photographing can be suppressed by canceling the limitation of the range.  
         [0022]     The imaging apparatus according to the present invention may further comprise: zooming means for changing a focal distance of an imaging lens; and zoom magnification changing means for changing a zoom magnification achieved by the zooming means; and means for changing the focus range in accordance with the zoom magnification set by the zoom magnification changing means.  
         [0023]     Accordingly, a high-speed AF operation and a reduction in power consumption can be achieved effectively by corresponding to a change in a zoom position of the zoom lens, and a false focus due to an erroneous determination can be prevented, which improves usability. That is, although the shutter release time lag is prevented from being changed due to a change in the zoom magnification by causing an amount of operation of a focus motor to be substantially constant, which improves usability, there is a possibility of reducing the focus detection range and a there may be a problem associated with a focus rate. Thus, it is possible to prevent the focus rate from decreasing while preventing a large increase in the focusing time on the telephoto side by changing the focus detection in response to a zoom position in such a degree that the focusing time does not change greatly.  
         [0024]     The imaging apparatus according to the present invention may further comprise: expanded distance re-measurement selecting means for selecting whether to perform a distance measurement again by expanding a distance measurement range when a focusing operation is performed in a state where the limited range is designated by the focus range designating means; and means for performing the focusing operation again with a range larger than the focus range set by the focus range designating means when no focus is acquired with the limited range set by the focus range designating means and the expanded distance measurement with the expanded distance is permitted by the expanded distance re-measurement selecting means. Accordingly, it is possible to flexibly and effectively respond to a case in which a focus position cannot be detected. That is, by performing the AF operation immediately after a recordable state is set, an effect of reducing a shutter release time lag can be obtained.  
         [0025]     Additionally, there is provided according to another aspect of the present invention a focusing method of an imaging apparatus, comprising: a focal lens position detecting step of detecting a current position of a focal lens; a focus range designating step of designating as a focus range a limited range near the current position of the focal lens; and a focal point detecting step of detecting a focal point by moving the focal lens within a range corresponding to the focus range designated in the focus range designating step.  
         [0026]     According to the above-mentioned invention, a high-speed AF operation and a reduction in power consumption can be achieved effectively in an imaging apparatus, and a false focus due to an erroneous determination can be prevented, which improves usability.  
         [0027]     Additionally, there is provided according to another aspect of the present invention a computer readable recording medium storing a program for causing a computer to execute: a focal lens position detecting procedure for detecting a current position of a focal lens; a focus range designating procedure for designating as a focus range a limited range near the current position of the focal lens; and a focal point detecting procedure for detecting a focal point by moving the focal lens within a range corresponding to the focus range designated in the focus range designating step.  
         [0028]     According to the above-mentioned invention, a high-speed AF operation and a reduction in power consumption can be achieved effectively in an imaging apparatus, and a false focus due to an erroneous determination can be prevented, which improves usability.  
         [0029]     Additionally, there is provided according to another aspect of the present invention an imaging apparatus in which one of a macro mode and a normal mode is selectable, the macro mode for setting a photographing range to a distance range shorter than a predetermined range, the normal mode for setting the photographing range to a distance longer than the predetermined range, the imaging apparatus comprising: focus control means for sequentially detecting a focal point while moving a focal lens and stopping the focal lens at the focal point so as to perform an automatic focusing operation; and range control means for causing the focus control means to perform the automatic focusing operation by limiting a movable range of the focal lens to a range narrower than an entire focus range with an immediately preceding focal point when taking a picture in the macro mode and a predetermined condition is satisfied.  
         [0030]     According to the above-mentioned invention, a high-speed automatic focusing operation can be achieved when the macro mode is selected in the imaging apparatus which can be selectably switched between the macro mode for taking a picture of a picturing object at an extremely short distance and the normal photographing mode for taking a picture of a picturing object at a normal distance, and power consumption can be reduced, and, further, an automatic focusing on a picturing object in a desired distance range can be achieved.  
         [0031]     In the imaging apparatus according to the present invention, the predetermined condition may include that the automatic focus operation was performed at least one time after the macro mode was set, and the range control means includes means for causing the focusing operation to be performed on the limited range when the predetermined condition is satisfied. Accordingly, a failure in focusing in an automatic focusing operation immediately after setting the macro mode can be effectively prevented, and a reduction in a time spent on the automatic focusing operation and energy saving thereafter can be effectively achieved.  
         [0032]     In the imaging apparatus according to the above-mentioned invention, the range control means may includes: means for monitoring a focus state in the focus control means; and means for causing the focusing operation on the limited range when the predetermined condition is satisfied, the predetermined condition including that a focus state after an immediately preceding focusing operation is maintained base on the means for monitoring. Accordingly, especially when a focused state is maintained from the previous automatic focusing operation, a reduction of a time spent on the automatic focusing operation is effectively achieved, and a focus failure can be effectively prevented, which provides an easier operation and photographing with less failure.  
         [0033]     The imaging apparatus according to the above-mentioned invention may further comprise distance measurement means for measuring a distance to a photographing object, wherein the range control means comprises: means for monitoring a change in the distance measured by the distance measurement means; and means for causing the focusing operation on the limited range when the predetermined condition is satisfied, the predetermined condition including that the change in the distance after an immediately preceding automatic focusing operation is equal to or less than a predetermined value base on the monitoring by the means for monitoring. Accordingly, especially when a change in a distance to a picturing object from a previous focusing operation is small, a reduction of a time spent on the automatic focusing operation is effectively achieved, and a focus failure can be effectively prevented, which provides an easier operation and photographing with less failure.  
         [0034]     The imaging apparatus according to the above-mentioned invention may further comprise brightness distribution measuring means for measuring a brightness distribution in a picture screen, wherein the range control means may comprise: means for monitoring a change in the brightness distribution measured by the brightness distribution measuring means; and means for causing the focusing operation on the limited range when the predetermined condition is satisfied, the predetermined condition including that the change in the brightness distribution after an immediately preceding automatic focusing operation is equal to or less than a predetermined amount base on the monitoring by the means for monitoring. Accordingly, especially when a change in a brightness distribution from a previous automatic focusing operation is small, a reduction of a time spent on the automatic focusing operation is effectively achieved, and a focus failure can be effectively prevented, which provides an easier operation and photographing with less failure.  
         [0035]     The imaging apparatus according to the above-mentioned invention may further comprise an imaging optical system using a zoom optical system that optically changes a zoom magnification, wherein the range control means comprises: means for monitoring a change in the zoom magnification in the imaging optical system; and means for causing the focusing operation to be performed on an entire focus range when the zoom magnification has changed based on the monitoring of the means for monitoring. Accordingly, especially when a zoom magnification has changed, a reduction of a time spent on the automatic focusing operation and energy saving are effectively achieved, and a focus failure can be further effectively prevented, which provides an easier operation and photographing with less failure.  
         [0036]     Additionally, there is provided according to another aspect of the present invention an imaging apparatus in which one of a macro mode and a normal mode is selectable, the macro mode for setting a photographing range to a distance range shorter than a predetermined range, the normal mode for setting the photographing range to a distance longer than the predetermined range, the imaging apparatus comprising: focus control means for sequentially acquiring from an image signal an automatic focus evaluation value, which is acquired in correspondence with sharpness of an edge portion of a photographing object image, and stopping the focal lens at a focal point being set to a maximum point of the automatic focus evaluation value so as to perform an automatic focusing operation; and range control means for causing the focus control means to perform the automatic focusing operation by limiting a movable range of the focal lens to a range narrower than an entire focus range with an immediately preceding focal point when taking a picture in the macro mode and a predetermined condition is satisfied.  
         [0037]     According to the above-mentioned invention, a high-speed automatic focusing operation can be achieved when the macro mode is selected in the imaging apparatus which has a focusing function using an AF evaluation value and can be selectably switched between the macro mode for taking a picture of a picturing object at an extremely short distance and the normal photographing mode for taking a picture of a picturing object at a normal distance, and power consumption can be reduced, and, further, an automatic focusing on a picturing object in a desired distance range can be achieved.  
         [0038]     In the imaging apparatus according to the above-mentioned invention, the predetermined condition may include that the automatic focus operation was performed at least one time after the macro mode was set, and the range control means includes means for causing the focusing operation to be performed on the limited range when the predetermined condition is satisfied. Accordingly, a failure in focusing in an automatic focusing operation immediately after setting the macro mode can be effectively prevented, and a reduction in a time spent on the automatic focusing operation and energy saving thereafter can be effectively achieved.  
         [0039]     In the imaging apparatus according to the present invention, the range control means may include: means for monitoring a focus state in the focus control means; and means for causing the focusing operation on the limited range when the predetermined condition is satisfied, the predetermined condition including that a focus state after an immediately preceding focusing operation is maintained base on the means for monitoring. Accordingly, especially when a focus state is maintained from the previous automatic focusing operation, a reduction of a time spent on the automatic focusing operation is effectively achieved, and a focus failure can be effectively prevented, which provides an easier operation and photographing with less failure.  
         [0040]     The imaging apparatus according to the above-mentioned invention may further comprise distance measurement means for measuring a distance to a photographing object, wherein the range control means may comprise: means for monitoring a change in the distance measured by the distance measurement means; and means for causing the focusing operation on the limited range when the predetermined condition is satisfied, the predetermined condition including that the change in the distance after an immediately preceding automatic focusing operation is equal to or less than a predetermined value base on the monitoring by the means for monitoring. Accordingly, especially when a change in a distance to a picturing object from a previous focusing operation is small, a reduction of a time spent on the automatic focusing operation is effectively achieved, and a focus failure can be effectively prevented, which provides an easier operation and photographing with less failure.  
         [0041]     The imaging apparatus according to the above-mentioned invention may further comprise brightness distribution measuring means for measuring a brightness distribution in a picture screen, wherein the range control means comprises: means for monitoring a change in the brightness distribution measured by the brightness distribution measuring means; and means for causing the focusing operation on the limited range when the predetermined condition is satisfied, the predetermined condition including that the change in the brightness distribution after an immediately preceding automatic focusing operation is equal to or less than a predetermined amount base on the monitoring by the means for monitoring. Accordingly, especially when a change in a brightness distribution from a previous automatic focusing operation is small, a reduction of a time spent on the automatic focusing operation is effectively achieved, and a focus failure can be effectively prevented, which provides an easier operation and photographing with less failure.  
         [0042]     In the imaging apparatus according to the above-mentioned invention, the range control means may comprises: means for monitoring the automatic focus value in the focus control means; and means for causing the focusing operation on the limited range when the predetermined condition is satisfied, the predetermined condition including that the change in the automatic focus evaluation value after an immediately preceding automatic focusing operation is equal to or less than a predetermined value base on the monitoring by the means for monitoring. Accordingly, especially when a change in an automatic focus evaluation value from a previous automatic focusing operation is small, a reduction of a time spent on the automatic focusing operation is effectively achieved, and a focus failure can be effectively prevented, which provides an easier operation and photographing with less failure.  
         [0043]     The imaging apparatus according to the above-mentioned invention may further comprise an imaging optical system using a zoom optical system that optically changes a zoom magnification, wherein the range control means may comprise: means for monitoring a change in the zoom magnification in the imaging optical system; and means for causing the focusing operation to be performed on an entire focus range when the zoom magnification has changed based on the monitoring of the means for monitoring. Accordingly, especially when a zoom magnification has changed, a reduction of a time spent on the automatic focusing operation and energy saving are effectively achieved, and a focus failure can be further effectively prevented, which provides an easier operation and photographing with less failure.  
         [0044]     Additionally, there is provided according to another aspect of the present invention a focus control method of an imaging apparatus in which one of a macro mode and a normal mode is selectable, the macro mode for setting a photographing range to a distance range shorter than a predetermined range, the normal mode for setting the photographing range to a distance longer than the predetermined range, the focus control method comprising: performing an automatic focusing operation by sequentially detecting a focal point while moving a focal lens, which constitute at least a part of an imaging optical system of the imaging apparatus, and stopping the focal lens at the focal point; and causing the automatic focusing operation to be performed by limiting a movable range of the focal lens to a range narrower than an entire focus range with an immediately preceding focal point set as a reference when the macro mode is set and a predetermined condition is satisfied.  
         [0045]     According to the above-mentioned invention, a high-speed automatic focusing operation can be achieved when the macro mode is selected in the imaging apparatus which can be selectably switched between the macro mode for taking a picture of a picturing object at an extremely short distance and the normal photographing mode for taking a picture of a picturing object at a normal distance, and power consumption can be reduced, and, further, an automatic focusing on a picturing object in a desired distance range can be achieved.  
         [0046]     Additionally, there is provided according to another aspect of the present invention a focus control method of an imaging apparatus in which one of a macro mode and a normal mode is selectable, the macro mode for setting a photographing range to a distance range shorter than a predetermined range, the normal mode for setting the photographing range to a distance longer than the predetermined range, the focus control method comprising: sequentially acquiring from an image signal an automatic focus evaluation value, which is acquired in correspondence to sharpness of an edge portion of a photographing object image while moving a focal lens, which constitute at least a part of an imaging optical system; controlling an automatic focusing operation by stopping the focal lens at a focal point being set as a maximum point of the automatic focus evaluation value; and causing the automatic focusing operation to be performed by limiting a movable range of the focal lens to a range narrower than an entire focus range with an immediately preceding focal point set as a reference when the macro mode is set and a predetermined condition is satisfied.  
         [0047]     According to the above-mentioned invention, a high-speed automatic focusing operation can be achieved when the macro mode is selected in the imaging apparatus which has a focusing function using an AF evaluation value and can be selectably switched between the macro mode for taking a picture of a picturing object at an extremely short distance and the normal photographing mode for taking a picture of a picturing object at a normal distance, and power consumption can be reduced, and, further, an automatic focusing on a picturing object in a desired distance range can be achieved.  
         [0048]     Additionally, there is provided according to another aspect of the present invention a computer readable recording medium storing a program for causing a computer to perform a focus control method of an imaging apparatus in which one of a macro mode and a normal mode is selectable, the macro mode for setting a photographing range to a distance range shorter than a predetermined range, the normal mode for setting the photographing range to a distance longer than the predetermined range, the focus control method comprising: performing an automatic focusing operation by sequentially detecting a focal point while moving a focal lens, which constitute at least a part of an imaging optical system of the imaging apparatus, and stopping the focal lens at the focal point; and causing the automatic focusing operation to be performed by limiting a movable range of the focal lens to a range narrower than an entire focus range with an immediately preceding focal point set as a reference when the macro mode is set and a predetermined condition is satisfied.  
         [0049]     Further, there is provided according to another aspect of the present invention a computer readable recording medium storing a program for causing a computer to perform a focus control method of an imaging apparatus in which one of a macro mode and a normal mode is selectable, the macro mode for setting a photographing range to a distance range shorter than a predetermined range, the normal mode for setting the photographing range to a distance longer than the predetermined range, the focus control method comprising: sequentially acquiring from an image signal an automatic focus evaluation value, which is acquired in correspondence to sharpness of an edge portion of a photographing object image while moving a focal lens, which constitute at least a part of an imaging optical system; controlling an automatic focusing operation by stopping the focal lens at a focal point being set as a maximum point of the automatic focus evaluation value; and causing the automatic focusing operation to be performed by limiting a movable range of the focal lens to a range narrower than an entire focus range with an immediately preceding focal point set as a reference when the macro mode is set and a predetermined condition is satisfied.  
         [0050]     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with t accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0051]      FIG. 1  is a block diagram of an outline of an entire system of the digital still camera, which is an imaging system.  
         [0052]      FIG. 2  is an illustrative plan view of the digital still camera;  
         [0053]      FIG. 3  is an illustrative front view of the digital still camera;  
         [0054]      FIG. 4  is an illustrative rear view of the digital still camera;  
         [0055]      FIG. 5  is an illustration of a display screen of an LCD monitor when a macro mode is set;  
         [0056]      FIG. 6  is an illustration of a display screen after at least one shoot was made or when a shutter release button is half-pressed;  
         [0057]      FIG. 7  is an illustration of a display screen after an OK switch is pressed;  
         [0058]      FIG. 8  is an illustration of a display screen showing a change result after changing a setting range;  
         [0059]      FIG. 9  is an illustration of a display screen giving an alarm in a macro mode;  
         [0060]      FIG. 10  is an illustration of a display screen when a finder mode is started after a power switch is turned on in a recording mode;  
         [0061]      FIG. 11  is an illustration of a display screen when a focus range is designated on a wide angle side in a normal mode;  
         [0062]      FIG. 12  is an illustration of a display screen when a focus range is designated on a telephoto side in the normal mode;  
         [0063]      FIG. 13  is a block diagram of an outline of an entire system of a digital still camera according to a second embodiment of the present invention;  
         [0064]      FIG. 14  is a graph showing a relationship between an inverse of a focal distance and a number of positions of a focal lens shown in Table 1;  
         [0065]      FIG. 15  is a display screen after at least one picture was taken or when a shutter release button is half-pressed;  
         [0066]      FIG. 16  is a display screen when the camera does not maintain a focused state;  
         [0067]      FIG. 17  is an illustration of a screen image immediately after performing an automatic focusing operation;  
         [0068]      FIG. 18  is an illustration of a screen image in a state where a distance to a photographing object changes after an automatic focusing operation was performed;  
         [0069]      FIG. 19  is an illustration of the blocks shown in  FIG. 17  by classifying them into three categories according to brightness data;  
         [0070]      FIG. 20  is an illustration of the blocks shown in  FIG. 18  by classifying them into three categories according a brightness data value;  
         [0071]      FIG. 21  is an illustration showing the blocks in which the brightness category was changed when a photographing object changes from that shown in  FIG. 17  and  FIG. 19  to that shown in  FIG. 18  and  FIG. 20 , respectively.  
         [0072]      FIG. 22  is a flowchart of a process to change an automatic focus scan range according to whether or not a focused state is maintained;  
         [0073]      FIG. 23  is a flowchart of a process of determining maintenance of focus in step S 20  of  FIG. 22  according to a change in a brightness distribution;  
         [0074]      FIG. 24  is an illustration of a screen that shows a range, within which a distance measurement sensor measures a distance;  
         [0075]      FIG. 25  is a flowchart of a process of step S 20  of  FIG. 22  to determine whether or not a focused state is maintained using a distance computed by the distance measurement sensor;  
         [0076]      FIG. 26  is an illustration of a screen image immediately after performing an automatic focusing operation;  
         [0077]      FIG. 27  is an illustration of a screen image in a state where a distance to a photographing object is changed from the state shown in  FIG. 26 ;  
         [0078]      FIG. 28  is an illustration showing a distribution of the blocks of which contrast is a value equal to or greater than a predetermined value in  FIG. 26 ;  
         [0079]      FIG. 29  is an illustration showing a distribution of the blocks of which contrast is a value equal to or greater than a predetermined value in  FIG. 27 ;  
         [0080]      FIG. 30  is an illustration showing blocks in black having a contrast value equal to or greater than 128 of which positions have changed when the state of the photographing object has changed from the state shown in  FIG. 26  to the state shown in  FIG. 27 ; and  
         [0081]      FIG. 31  is a flowchart of a process used for determining whether or not a focused state is maintained by detecting a number of blocks having a contrast value equal to or greater than 128 of which positions have changed. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0082]     A description will now be given, with reference to the drawings, of an imaging apparatus according to a first embodiment of the present invention. The imaging apparatus according to the first embodiment of the present invention is a digital still camera to which the present invention is applied.  FIG. 1  is a block diagram showing an outline of an entire system of the digital still camera, which is an imaging system.  FIG. 2  is an illustrative plan view of the digital still camera.  FIG. 3  is an illustrative front view of the digital still camera.  FIG. 4  is an illustrative rear view of the digital still camera.  
         [0083]     The digital still camera shown in  FIG. 1  comprises: an imaging lens system  1 ; a mechanical shutter  2 ; a solid imaging element (charge coupled device)  3 ; a correlation double sampling (CDS) circuit  4 ; an automatic gain control (AGC) circuit  5 ; an analog-to-digital (A/D) converter  6 ; a timing generator (TG)  7 ; a CCD interface (CCD-I/F)  8 ; a memory controller  9 ; a display-output control part  10 , a compression processing part  11 , a YUV conversion part  12 , a resizing processing part  13 ; a media interface (media I/F)  14 ; a central processing unit (CPU)  15 ; a read only memory (ROM)  16 ; a frame memory (SDRAM)  17 ; a liquid crystal display (LCD) display  18 , a motor driver  19 ; an operation part  20 ; an audio output unit  21 ; and a memory card  22 .  
         [0084]     The CDS circuit  4 , the AGC circuit  5 , the A/D converter  6  and the timing generator  7  together constitute a signal processing part  31  of a front end (F/E), and the CCD interface  8 , the memory controller  9 , the display output control part  10 , the compression processing part  11 , the YUV transducer  12 , the resizing processing part  13 , the media interface  14  and the CPU  15  are mounted in a digital-signal-processing IC (integrated circuit)  32 . The imaging lens system  1  is an optical system for forming an optical image of a photographing object on a light-receiving surface of the CCD solid imaging element  3 . The mechanical shutter  2  is interposed in an optical path between the imaging lens system  1  and the CCD solid imaging element so as to open and close the optical path to control exposure of the CCD solid imaging element  3 . The CCD solid imaging element  3  converts the optical image incident on the light-receiving surface in an exposed state into an electric signal, and temporarily retains the optical image and outputs and transfers the electric signal as image data. The CDS circuit  4 , the AGC circuit  5 , the A/D converter  6  and the timing generator  7  together constitute the signal processing part  31 , which processes the output signal of the CCD solid imaging element  3  in a front end. The CDS circuit  4  carries out a correlation double sampling of the output image signal of the CCD solid imaging element  3 . The AGC circuit  5  carries out an automatic gain control AGC on the correlation double sampling output of the CDS circuit  4 . The A/D converter  6  converts the analog output of the AGC circuit  5  into digital data.  
         [0085]     The timing generator  7  is responsive to a VD (vertical synchronization drive signal) and an HD signal (horizontal synchronization drive signal), which are synchronization drive signals given by the CCD interface  8  of the digital signal processing IC (hereinafter, referred to as signal processing IC)  32  and cooperates with the CPU  15  so as to provide a timing signal to the CCD solid imaging element  3 , the CDS circuit  4 , the AGC circuit  5  and the A/D converter  6  to cause those parts to appropriately synchronize with each other. The signal processing IC  32  stores in the frame memory  17  digital image data given through the A/D converter  6  of the signal processing part  31  in accordance with a control of the CPU  15 . The signal processing IC  32  applies necessary signal processing such as compression and YUV conversion to the digital image data and also stores the processed data in the frame memory  17 . The signal processing IC  32  displays on the LCD displays  18  the image data given from the A/D converter  6  or retrieved from the frame memory  17 . Additionally, the signal processing IC  32  performs a compression process, a YUV conversion process and a resizing process of the image data given from the A/D converter  6  or retrieved from the frame memory  17 . Further, the signal processing IC  32  stores the digital image data, retrieved from the frame memory  17 , in the memory card  22  through the media interface  14 .  
         [0086]     The CCD interface  8  receives the digital image data given from the A/D converter  6  of the signal processing part  31 , and stores the received data in the frame memory  17  through the memory controller  9 . The memory controller  9  controls various kinds of data to store in the frame memory  17  or read out from the frame memory  17  in accordance with a control of the CPU  15 , the various kinds of data including original RGB (RAW-RGB) data given through the CCD interface  8 , YUV data converted by the YUV conversion part  12 , and JPEG data and OSD image data that have been compressed by the compression processing part  11  according to the JPEG (Joint Photographic Experts Group) method. The display output control part  10  displays image data read out from the frame memory  17 , and outputs a TV output for causing an external TV (television), etc., to display the image data. The compression processing part  11  compresses the image data given from the A/D converter  6  or taken out from the frame memory  17  in accordance with a predetermined compression method such as, for example, the JPEG method. The YUV conversion part  12  performs a YUV conversion on the image data, which was given from the A/D converter  6  or was taken out from the frame memory  17 , in accordance with an automatic white balance (AWB) control value given from the CPU  15 . The resizing processing part  13  resizes the image data, which was given from the A/D converter  6  or was taken out from the frame memory  17 . The media interface  14  writes the image data, which was given from the A/D converter  6  or was taken out from the frame memory  17 , in the memory card  22  according to a control of the memory controller  9  and the CPU  15 .  
         [0087]     Namely, the memory controller  9  stores in the frame memory  17  the image data given from the A/D converter  6 , and retrieves the image data from the frame memory  17  and provides the image data to be displayed on the LCD display  18  through the display output control part  10 . The memory controller also retrieves the image data from the frame memory  17  and applies the compression process by the compressing processing part  11 , the YUV conversion process by the YUV conversion part  12  and the resizing process by the resize processing part  13 , and writes the data after the processing in the frame memory  17 , and further retrieves the data from the frame memory  17  and write the data in the memory card  22 . The ROM  16  stores the operating program of the CPU  15 , data, etc., and the CPU  15  performs various kinds of processing associated with a photography operation according to the program and data read from the ROM  16 . The frame memory  17  is a semiconductor memory such as a synchronous dynamic random access memory (SDRAM), and stores original RGB data, the YUV-converted YUV data and the JPEG-compressed JPEG data and the OSD image data. The LCD display  18  is a display device such as a liquid crystal display device or the like, and displays the image data, which is supplied from the A/D converter  6  or retrieved from the frame memory  17  and supplied through the display output control part  10 , and further displays necessary information thereon.  
         [0088]     Based on a control of the CPU  15 , the motor driver  19  drives a lens drive motor (not shown) of the imaging lens system  1  for focusing, zooming, etc., and drives a shutter drive motor (not shown) of the mechanical shutter  2  in association with the timing generator  7  for shutter opening/closing operation. The operation unit  20  includes a release switch for instructing a shoot, a mode switch for switching modes, and at least a part of other switches, keys, levers, dials and the like, and is operated to supply to the CPU  15  an operation instruction, a setting instruction, a selection instruction to the digital still camera. The audio output unit  21  generates a voice and sound such as an alarming sound, a voice announcement or the like. The memory card  22  is a small IC memory type recording medium equipped with a semiconductor non-volatile memory such as a flash memory, and is used as a removable external recording medium to the digital still camera. The memory card  22  is used, for example, by being removably attached to a slot provided in the digital still camera. The memory card  22  retrieves the image data, which was compressed according to the JPEG method in the frame memory  17  through the memory controller  9  in accordance with a control of the CPU  15 , and saves the image data as a result of photographing.  
         [0089]     Additionally, in  FIGS. 2 through 4 , the digital still camera is provided with an LCD monitor  18 A on a rear surface of a body thereof and a sub LCD  18 B is arranged on a top surface of the body. The LCD monitor  18 A and the sub LCD  18 B constitute the LCD display  18  shown in  FIG. 1 . The LCD monitor  18 A mainly displays an image, and sub LCD  18 B mainly displays various symbols indicating a film counter, a date/time and an operating state. Moreover, a shutter release  201  and a mode dial  202  are arranged on the top face of the body. Arranged on the back surface of the body are a wide side (WIDE) zoom switch  203 , a telephoto side (TELE) zoom switch  204 , a self-timer/cancel switch  205 , a menu switch  206 , a top/stroboscope switch  207 , a right switch  208 , a display switch  209 , a bottom/macro a switch  210 , a left/image check switch  211  and an OK switch  212 , which together constitute the operation part shown in  FIG. 1 . A power supply switch  101  is arranged on a lower part of the back surface of the body. A right side surface of the body when viewing a photographing object is provided with a memory card/battery lid  102 , which covers an accommodating part of the memory card  22  such as an SD card and a battery cell as a power source. Arranged on a front surface of the body are a stroboscope light-emitting part  103 , an objective surface of an optical finder  104 , a distance measurement unit  105 , a remote control light-receiving part  106  and a mirror body unit  107  of a photo lens. Also arranged on the back surface of the body are an ocular part of the optical finder  104 , an AF indicating light-emitting diode (LED)  108  and a stroboscope indicating LED  109 .  
         [0090]     It should be noted that the above-mentioned parts, mainly circuit parts, realize the following means under controls of the CPU  15 : focal lens position detecting means for detecting a current position of a focal lens; focus range designating means for designating as a focus range a limited range near the current position of the focal lens detected by the focal lens position detecting means; focal lens movable range changing means for changing a movable range of the focal lens in accordance with the focus range designated by the focus range designating means; previous focus result storing means for storing a result of a previous focusing operation; and alarm displaying means for displaying an alarm when no focus is acquired according to the result of a previous focusing operation that has been stored in the previous focus result storing means when designation of the focal range is made by the focus range designation means; focus range displaying means for displaying the focus range at present; focus range position changing means for changing a position of the limited focus range designated by the focus range designating means; focus range designation canceling means for canceling the setting of the limited focus range designated by the focus range designating means; macro mode switching means for switching between a macro mode and a normal mode, the macro mode for taking a picture of a photographing object at an extremely short distance, the normal mode for taking a picture of a photographing object at a normal distance larger than the extremely short distance; macro mode switching means for switching between a macro mode and a normal mode, the macro mode for taking a picture of a photographing object at an extremely short distance, the normal mode for taking a picture of a photographing object at a normal distance larger than the extremely short distance; means for automatically canceling, when the focus range is designated by the focus range designating means, the designation of the focus range; zooming means for changing a focal distance of an imaging lens; zoom magnification changing means for changing a zoom magnification achieved by the zooming means; means for automatically canceling, when the focus range is designated by the focus range designating means, the designation of the focus range; zooming means for changing a focal distance of an imaging lens; zoom magnification changing means for changing a zoom magnification achieved by the zooming means; means for changing the focus range in accordance with the zoom magnification set by the zoom magnification changing means; expanded distance re-measurement selecting means for selecting whether to perform a distance measurement again by expanding a distance measurement range when a focusing operation is performed in a state where the limited range is designated by the focus range designating means; and means for performing the focusing operation again with a range larger than the focus range set by the focus range designating means when no focus is acquired with the limited range set by the focus range designating means and the expanded distance measurement with the expanded distance is permitted by the expanded distance re-measurement selecting means.  
         [0091]     A description will now be given of an operation of the above-mentioned digital still camera. The digital still camera is started in a recording mode by operating the mode dial  202  shown in  FIG. 2  provided in the operation part  20  shown in  FIG. 1  to set the operation mode to the recording mode. When the mode dial  202  is set, the CPU  15  detects that the state of the mode switch contained in the operation part  20  of  FIG. 1  is turned on to the recording mode and the CPU  15  controls the driver  19  to move the imaging lens system  1  of the mirror body unit  107  to a position at which photographing can be carried out. Additionally, the CPU  15  causes a power to be supplied to each of the CCD solid imaging element  3 , the signal processing part  31 , the LCD display  18 , etc., so as to start operations of these parts. When those parts are turned on, an operation of a finder mode is started. In the finder mode, a light incident on the CCD solid imaging element  3  through the imaging lens system  1  is converted into an electric signal, and, in this case, the electric signal is sent to the A/D converter  6  through the CDS circuit  4  and the AGC circuit  5  sequentially as an analog RGB signal, which contains analog R, G and B signals. Each of the signals converted into the digital RGB signals by the A/D converter  6  is converted into YUV signals by the YUV conversion part  12  in the digital signal processing IC  32 , and is written in the frame memory  17  by the memory controller  9 . The YUV signal is read by the memory controller  9 , and it is output as a TV output through the display output control part  10 , or sent to the LCD display  18  so as to perform a display of the LCD monitor  18 A. Normally, the above-mentioned processing is performed at an interval of 1/30 second, and a display of the finder mode as a so-called electric finder is performed, which is updated every 1/30 second.  
         [0092]     Additionally, an AF evaluation value which shows a degree of focus of a screen, an AE (automatic exposure) evaluation value which is a result of detection of brightness of the photographing object, and an AWB evaluation value which is a result of detection of a color of the photographing object are calculated from the digital RGB signals retrieved in the CCD interface  8  of the digital signal processing IC  32 . These values are read by the CPU  15  as feature data, and are used for each processing of AE, AF and AWB. The AF evaluation value is produced, for example, according to an output integral value of a high-frequency component extraction filter and an integral value of a difference in brightness between adjacent pixels. Since edge portions of the photographing object are sharp when it is in a focused state, a high-frequency component is highest. Using this, at the time of focus detection operation according to AF, the AF evaluation values at various focal lens positions are acquired so as to perform the AF control by determining the maximum position as a focal position. The AE evaluation value and the AWB evaluation value are produced from integral values of R, G, and B signals. For example, the screen is divided into 256 areas so as to calculate each of the RGB integral values. The CPU  15  reads the RGB integral values, and, in AE, calculates brightness of each area so as to determine an AE control value from a brightness distribution. In AWB, an AWB control value matching a color of the light source is determined based on the RGB distribution. The processes of AE and AWB are continuously performed during the finder mode.  
         [0093]     When the shutter release (button)  201  is operated, an AF operation which is a detection of a focal position and a still picture recording process are performed. When the shutter release is pressed, a still picture start signal is retrieved from the operation part  20  into the CPU  15 , and the CPU  15  performs the mountain-climbing AF by driving the imaging lens system  1  through the motor driver  19  in synchronization with a frame rate. If the focus range is all the areas from infinity to extremely close point, the focal lens moves to a focal position from the extreme close position to infinity or from infinity to extremely close position, and the CPU  15  reads the AF evaluation values of frames produced by the digital signal processing IC  32 . The focal lens is moved to a focal position by determining a position at which the AF evaluation value is maximum is the focal position. The analog RGB signals retrieved from the CCD solid imaging element  3  after completion of AF is converted into digital RGB signals, and are stored in the frame memory  17  through the digital signal processing IC  32 . The digital RGB signals are again read by the digital signal processing IC  32 , and are converted into YUV data, and is returned to the frame memory  17 .  
         [0094]     At the time of a still picture imaging, the YUV converted image data is sent to a compression processing part  11  comprising an image compression-decompression circuit in the digital signal processing IC  32 . The YUV data sent to the compression processing part  11  is compressed, and is returned to the frame memory  17 . The compressed data in the frame memory  17  is read through the digital signal processing IC  32 , and is stored in the data storage memory of the memory card  22 .  
         [0095]     A description will now be given of an operation of the digital still camera according to the present embodiment.  
                                                           TABLE 1                                       Focus Range                    Normal   Macro           (∞-30 cm)   (30-1 cm)                            Wide angle end   10 positions   120 positions           Number of focal           Positions           Telephoto end   70 positions   250 positions           Number of focal           Positions                      
 
         [0096]     Table 1 shows an example of a number of detection positions of the focal position with respect to a zoom position when an extremely close range of 1 cm-30 cm is set to a macro mode and a range distant from the extremely close range is set to a normal mode. In the macro range, if an entire range is scanned, a number of points is extremely large. For example, if there are 120 points, and if a screen updating interval of the finder mode is shifted by 1 point for each frame in 1/30 second, the AF scan takes 4 seconds. Therefore, a condition of the photographing object may change during the AF scan. As measures for solving the above-mentioned problem, a focal time is reduced by limiting the focus range (scanning area). When the down/macro switch  210  shown in  FIG. 4  is pressed in the finder mode state, the macro mode is set.  FIG. 5  shows an example of a display screen of the LCD monitor  18 A when the macro mode is set. A number of record pixels is set to, for example, 2048×1536. A horizontal bar B 1  located in a lower portion of the screen indicates a current focal position. Immediately after the macro mode is set, the focal distance is set to 30 cm.  
         [0097]     The shutter release  201  shown in  FIG. 2  is a two-stage switch so that a focusing operation is performed at a half-pressed position and a recording operation (photographing) is performed when the switch is further pressed.  FIG. 6  is the display screen after at least one picture was taken or when the shutter release  201  is half-pressed. In the case shown in  FIG. 6 , a focus is made about 10 cm, and thus, a mark “◯” indicating the focal position J 1  of the focal lens is moved to a position corresponding to about 10 cm.  
         [0098]     There are some methods which realize the detection of a current focal lens position. For example, in a case of driving a focal lens by a stepping motor, a detection of a reference point is performed as a resetting operation at a start time of a camera. A current position can be detected based on a number of pulses supplied to move the stepping motor after the resetting operation. Additionally, a resistance plate may be used to perform a position detection by an output of the resistance plate, which indicates a different resistance according to a position of the focal lens. When the OK switch  212  is pressed in a state shown in  FIG. 6 , a range near the current focal position is set to an AF scan range for a next time.  FIG. 7  shows a display screen after the OK switch  212  is pressed. A range indicated by a leftward and rightward arrow R 1  is set to the AF scan range for a next time.  
         [0099]      FIG. 8  is a screen display showing a change result after setting range is changed. Therefore, the change of the setting range is made by the left-and-right button  211  shown in  FIG. 4 , i.e., the left/image check switch  211  and the right switch  208 . In a state where an AF scan range is set as shown in  FIG. 7  or  FIG. 8 , the arrow R 1  indicating the AF scan range on the screen is moved by pressing the left-and-right button  211 . The range indicated by the arrow R 1  is set to the scan range at a next AF. In this state if the shutter release  201  is fully pressed to perform a photographing operation, an AF operation is performed prior to recording. If the scan range is limited to, for example, 15 steps, the time required for AF can be set to 0.5 second since the screen update is every 1/30 second, which greatly reduces a shutter time lag. Additionally, a period to operate the focus motor and a distance (an amount of rotation) can be reduced, which reduces power consumption.  
         [0100]     If the OK switch  212  is pressed in the state shown in  FIG. 7  or  FIG. 8 , the AF scan range is cancelled, and the display returns to the screen shown in  FIG. 6 . In the macro mode state, if the down/macro switch  210  is pressed again, it returns to the normal photographing mode such as shown in  FIG. 10 . Even if the scan range has been set as shown in  FIG. 7  or  FIG. 8 , the range setting is cancelled together with the macro mode being turned off. Additionally, if the OK switch  212  is pressed in the state shown in  FIG. 5 , which is an initial state after being set to the macro mode, an alarming screen is displayed such as shown in  FIG. 9 . This is because a focusing operation has not been performed after the macro mode was set and to prevent the scan range from being limited in a state where a distance is not known.  
         [0101]     A description will now be given of a case where the AF scan range is limited in the normal photograph mode. If the AF scan is performed on the entire range, a time period required for the focusing varies depending on a zoom position since a number of steps is greatly different between the wide angle end (WIDE end) and the telephoto end (TELE end) of the zoom. Therefore, when taking a picture of a moving object, a consideration must be given that a shutter release time lag varies according to a zoom position. If the distance is known approximately, the AF time can be reduced by limiting the scan range to a certain range. For example, if an amount of movement of the focal lens is maintained constant, the shutter release time lag can be set constant irrespective of the zoom position.  
         [0102]      FIG. 10  shows a display when the finder mode is started after the power switch  101  is turned on in the recording mode. The zoom position at the time of start is set to the wide angle end. The display of a bar B- 2  on the left side is a bar indication which indicates the zoom position, and it can be appreciated that it is operated at the wide angle end. If the OK switch  212  is pressed in the state of the display shown in  FIG. 10 , the screen is changed to a display shown in  FIG. 11 . The horizontal bar B 1  in a lower part of the screen indicates information regarding focus. A mark “◯” indicates a current position J 1  of the focal lens, and an arrow R 1  indicate a scan range at a next AF. At the wide angle end as shown in  FIG. 11 , since focus detection positions are only 10 points, 7 points are set to be the detection range. Since high-pass filter outputs of 7 frames are acquired with the detection range of 7 points, a time period required for the AF scan is about 0.25 seconds. Additionally,  FIG. 12  shows a case where the zoom is changed to the telephoto end. When a zoom magnification change is set by the zoom switches  203  and  204 , the CPU  15  drives the motor driver  19  to move the zoom lens. The zoom lens moves in a requested direction while the zoom switch  203  or  204  is pressed, and a mark “●” indicating a zoom position J 2  along a zoom bar B 2  moves in response to the moved position of the zoom lens as shown in  FIG. 12 .  
         [0103]     Although AF scan points are needed to be 7 points so as to prevent the AF scan time from being changed largely depending on a zoom position, 10 points are used in the present embodiment since the number of points is too small, if only 7 points are used, as compared to 70 points of the entire range of the telephoto end. When the entire area is canned, the scan time is 70× 1/30=2.3 seconds. On the other hand, if they are limited to 10 points, scan time can be reduced to 0.33 seconds. By doing such, it becomes possible to reduce the AF focus detection operation time, and the system having less difference in a time lag due to differences in zoom position can be achieved. Moreover, since an amount of operation of mechanical parts can be reduced, it can be achieved to reduce power consumption. Furthermore, since the focus range is clearly displayed on the screen, a photographer can check whether an intended range is set to the focus range. Since it can be checked beforehand to focus within the displayed range, if a picture is taken across a wire netting, it is free from care that a focus is made to the wire netting in front, which realizes the AF having little failure. Additionally, if the OK switch  212  is pressed in the state shown in  FIG. 11  or  FIG. 12 , the setting of the focus range is cancelled, and it returns to the state as shown in  FIG. 10 .  
         [0104]     In each example mentioned above, the number of AF focal points is determined so that the scan time periods are almost equal. However, there is a determining method in which a focal distance is within a range of a uniform ratio. For example, if a range is specified to cover 50% of the entire area, 5 points are set to the focal points on the zoom wide angle end, and 35 points are set on the zoom telephoto end. In the above-mentioned embodiment, the method of changing a distance range to the photographing object in order to equalize the focus time by equalizing the ranges of movement of the focal lens, and the method of changing the movable range of the focal lens by maintaining a distance range to a photographing object. However, depending on the zoom magnification, the difference in the number of points between the wide angle end and the telephoto end is larger. Therefore, if the zoom magnification is changed in the state where the focus range is designated, as shown in  FIG. 11  or  FIG. 12 , the designation of the focus range may be automatically cancelled.  
         [0105]     Additionally, it may be so constructed to set whether to detect a focal position by expanding the range even if it takes a long time so as to scan, for example, an entire range or limit the range to a set range, when focus cannot be acquired as a result of AF performed on a limited narrow scan range. If the range is set to be expandable, it can be automatically responded to a change in the photographing object even though the focusing time is increased, which reduces possibility of failure in taking a picture. However, when it is known beforehand that the focal point exists within a set range, it is needless to scan by expanding the range and an unnecessary power may be consumed. Additionally, if a focus is acquired at a point outside the limited range by scanning an entire range, the intended picture cannot be taken. Therefore, it is preferable that a selection can be made whether or not to expand the range when there is no focal point found in the range.  
         [0106]     Namely, providing means for selecting whether or not to perform a distance measurement again by expanding the distance measurement range when a focal point cannot be found as a result of focusing operation in a state where a narrow range is set, so as to perform a focusing operation on a range larger than the set range if a focal point cannot be acquired in the set narrow range and if an expanded distance re-measurement is permitted. It should be noted that the imaging apparatus according to the present invention can be achieved as a general purpose apparatus using a computer system, instead of constituting as an exclusive digital still camera. For example, an apparatus performing the above-mentioned process may be constructed by installing a program for causing a computer to perform the above-mentioned operation by reading the program from a recording medium such as a flexible disk, a CD-ROM, etc. According to the installation, the program is stored in a medium such as a hard disk or the like in the computer system, which constitutes the above-mentioned apparatus and used in practice. Moreover, the program concerned may be distributed to FTP (File Transfer Protocol) clients through a network by registering the program in an FTP server provided on the network such as the Internet. The program may be registered in an electronic bulletin board system (BBS) of a communication network so as to distribute the program through the network. Thus, the above-mentioned process can be achieved by executing the program under a control of an operating system (OS). Furthermore, above-mentioned process can be achieved by executing the program while the program is being transferred through a communication network.  
       Second Embodiment  
       [0107]     A description will now be given of a digital still camera as an imaging apparatus according to a second embodiment according to the present invention.  
         [0108]      FIG. 13  is a block diagram of an outline of an entire system of the digital still camera according to the second embodiment of the present invention. The digital still camera according to the second embodiment of the present invention has the same structure as the digital stile camera according to the first embodiment shown in  FIG. 1  except for a distance measurement sensor  23  being connected to the CPU  15 . In  FIG. 13 , parts that are the same as the parts shown in  FIG. 1  are given the same reference numerals, and descriptions there of will be omitted for the sake of simplification of the description.  
         [0109]     As shown in  FIG. 13 , the distance measurement sensor  23  is connected to the CPU  15 . The distance measurement sensor  23  together with the distance measurement unit  105  shown in  FIG. 3  constitute distance measurement means for periodically measuring a distance to a photographing object according to a so-called triangular surveying method. The CPU  15  monitors changes in a measured value of a distance to a photographing object by the distance measurement sensor  23  so as to determine whether or not a focused state is maintained.  
         [0110]     Parts provided on the digital still camera according to the present embodiment are the same as the parts shown in  FIGS. 2 through 4 , and, thus, the same reference numerals are given and the descriptions thereof will be omitted.  
         [0111]     It should be noted that the parts shown in  FIG. 13 , mainly circuit parts, realize the following means under a control of the CPU  15 : focus control means for sequentially detecting a focal point while moving a focal lens and stopping the focal lens at the focal point so as to perform an automatic focusing operation; range control means for causing the focus control means to perform the automatic focusing operation by limiting a movable range of the focal lens to a range narrower than an entire focus range with an immediately preceding focal point when taking a picture in the macro mode and a predetermined condition is satisfied; focus control means for sequentially acquiring from an image signal an automatic focus evaluation value, which is acquired in correspondence with sharpness of an edge portion of a photographing object image, and stopping the focal lens at a focal point being set to a maximum point of the automatic focus evaluation value so as to perform an automatic focusing operation; range control means for causing the focus control means to perform the automatic focusing operation by limiting a movable range of the focal lens to a range narrower than an entire focus range with an immediately preceding focal point when taking a picture in the macro mode and a predetermined condition is satisfied; means for causing the focusing operation to be performed on the limited range when the predetermined condition is satisfied; means for monitoring a focus state in the focus control means; means for causing the focusing operation on the limited range when the predetermined condition is satisfied, the predetermined condition including that a focus state after an immediately preceding focusing operation is maintained base on the means for monitoring; means for monitoring a change in the distance measured by the distance measurement means; means for causing the focusing operation on the limited range when the predetermined condition is satisfied, the predetermined condition including that the change in the distance after an immediately preceding automatic focusing operation is equal to or less than a predetermined value base on the monitoring by the means for monitoring; brightness distribution measuring means for measuring a brightness distribution in a picture screen; means for monitoring a change in the brightness distribution measured by the brightness distribution measuring means; means for causing the focusing operation on the limited range when the predetermined condition is satisfied, the predetermined condition including that the change in the brightness distribution after an immediately preceding automatic focusing operation is equal to or less than a predetermined amount base on the monitoring by the means for monitoring; means for monitoring a change in the zoom magnification in the imaging optical system; and means for causing the focusing operation to be performed on an entire focus range when the zoom magnification has changed based on the monitoring of the means for monitoring; means for monitoring the automatic focus value in the focus control means; and means for causing the focusing operation on the limited range when the predetermined condition is satisfied, the predetermined condition including that the change in the automatic focus evaluation value after an immediately preceding automatic focusing operation is equal to or less than a predetermined value base on the monitoring by the means for monitoring.  
         [0112]     A description will now be given of an operation of the above-mentioned digital still camera. The digital still camera is started in a recording mode by operating the mode dial  202  shown in  FIG. 2  provided in the operation part  20  shown in  FIG. 1  to set the operation mode to the recording mode. When the mode dial  202  is set, the CPU  15  detects that the state of the mode switch contained in the operation part  20  of  FIG. 1  is turned on to the recording mode and the CPU  15  controls the driver  19  to move the imaging lens system  1  of the mirror body unit  107  to a position at which photographing can be carried out. Additionally, the CPU  15  causes a power to be supplied to each of the CCD solid imaging element  3 , the signal processing part  31 , the LCD display  18 , etc., so as to start operations of these parts. When those parts are turned on, an operation of a finder mode is started. In the finder mode, a light incident on the CCD solid imaging element  3  through the imaging lens system  1  is converted into an electric signal, and, in this case, the electric signal is sent to the A/D converter  6  through the CDS circuit  4  and the AGC circuit  5  sequentially as an analog RGB signal, which contains analog R, G and B signals. Each of the signals converted into the digital RGB signals by the A/D converter  6  is converted into YUV signals by the YUV conversion part  12  in the digital signal processing IC  32 , and is written in the frame memory  17  by the memory controller  9 . The YUV signal is read by the memory controller  9 , and it is output as a TV output through the display output control part  10 , or sent to the LCD display  18  so as to perform a display of the LCD monitor  18 A. Normally, the above-mentioned processing is performed at an interval of 1/30 seconds, and a display of the finder mode as a so-called electric finder is performed, which is updated every 1/30 second.  
         [0113]     Additionally, an AF evaluation value which shows a degree of focus of a screen, an AE (automatic exposure) evaluation value which is a result of detection of brightness of the photographing object, and an AWB evaluation value which is a result of detection of a color of the photographing object are calculated from the digital RGB signals retrieved in the CCD interface  8  of the digital signal processing IC  32 . These values are read by the CPU  15  as feature data, and are used for each processing of AE, AF and AWB. The AF evaluation value is produced, for example, according to an output integral value of a high-frequency component extraction filter and an integral value of a difference in brightness between adjacent pixels. Since edge portions of the photographing object are sharp when it is in a focused state, a high-frequency component is highest. Using this, at the time of focus detection operation according to AF, the AF evaluation values at various focal lens positions are acquired so as to perform the AF control by determining the maximum position as a focal position. The AE evaluation value and the AWB evaluation value are produced from integral values of R, G, and B signals. For example, the screen is divided into 256 areas so as to calculate each of the RGB integral values. The CPU  15  reads the RGB integral values, and, in AE, calculates brightness of each area so as to determine an AE control value from a brightness distribution. In AWB, an AWB control value matching a color of the light source is determined based on the RGB distribution. The processes of AE and AWB are continuously performed during the finder mode.  
         [0114]     When the shutter release (button)  201  is operated, an AF operation which is a detection of a focal position and a still picture recording process are performed. When the shutter release is pressed, a still picture start signal is retrieved from the operation part  20  into the CPU  15 , and the CPU  15  performs the mountain-climbing AF by driving the imaging lens system  1  through the motor driver  19  in synchronization with a frame rate. If the focus range is all the areas from infinity to extremely close point, the focus lens moves to a focal position from the extreme close to infinity or from infinity to extremely close, and the CPU  15  reads the AF evaluation values of frames produced by the digital signal processing IC  32 . The focal lens is moved to a focal position by determining a position at which the AF evaluation value is maximum is the focal position. The analog RGB signals retrieved from the CCD solid imaging element  3  after completion of AF is converted into digital RGB signals, and are stored in the frame memory  17  through the digital signal processing IC  32 . The digital RGB signals are again read by the digital signal processing IC  32 , and are converted into YUV data, and is returned to the frame memory  17 .  
         [0115]     At the time of a still picture imaging, the YUV converted image data is sent to a compression processing part  11  comprising an image compression-decompression circuit in the digital signal processing IC  32 . The YUV data sent to the compression processing part  11  is compressed, and is returned to the frame memory  17 . The compressed data in the frame memory  17  is read through the digital signal processing IC  32 , and is stored in the data storage memory of the memory card  22 .  
         [0116]     A description will now be given of an operation of the digital still camera according to the present embodiment.  
         [0117]     The previously mentioned Table 1 shows an example of a number of detection positions of the focal position with respect to a zoom position when an extremely close range of 1 cm-30 cm is set to a macro mode and a range distant from the extremely close range is set to a normal mode.  
         [0118]     According to the Table 1, when comparing the numbers of positions of the focal lens at which the AF evaluation values are acquired between the normal mode and the macro mode, 10 positions are set for the normal mode while 120 positions are set for the macro mode at the wide angle end, and 70 positions are set for the normal mode while 250 positions are set for the macro mode. Thus, the number of positions of the focal lens at which the AF evaluation values are acquired when scanning an entire macro area is extremely larger than that of the normal area.  
         [0119]     The reason for the number of positions of the focal lens at which the AF evaluation values are acquired being larger than that of the normal are can be explained according to a Newton&#39;s formula (1) which represent a relationship between the principal point of the lens and image formation. 
 
Newton&#39;s formula:  Z×Z′=−f×f   (1) 
        Z: a distance (photographing object distance) from a principal point to a photographing object surface     Z″: a distance (imaging surface distance) from a principal point to an imaging surface     f: focal distance        
 
         [0123]     In order to acquire a ratio of changes in the imaging surface distance and the photographing object distance when the distance between the principal point and the picturing distance is changed, the Newton&#39;s formula (1) is differentiated according to the distance Z from the principal point to the photographing object to acquire formula (2).  
         [0124]     Differentiate the Newton&#39;s formula according to the distance Z from the principal point to the photographing object: 
 
 dZ/dZ′=f×f/Z×Z   (2) 
 
         [0125]     In the macro area, Z is small since the distance from the principal point to the photographing object is small, and according to the formula (2) when a value of Z×Z, which is a denominator, becomes small, a change in the imaging surface distance becomes large relative to the photographing object distance. That is, since the imaging surface distance changes greatly even if the photographing object distance changes small, the focal lens must be moved large so as to make a focus. Moreover, if the focal distance of the lens changes even when the picturing distance is the same, according to the formula (2), a value of f×f, which is a numerator, becomes large, a change in the imaging surface distance becomes large relative to the photographing object distance. Therefore, since the imaging surface distance changes greatly as the focal distance increases even when the photographing object distance changes little, the focal lens must be moved greatly so as to make a focus.  
         [0126]      FIG. 14  is a graph showing a relationship between the inverse of focus distance and the number of positions of the focus lens shown in Table 1. The reason for using the inverse of focus distance is because an amount of running out of the focal lens is smallest when focused on infinite. It should be noted that, in  FIG. 14 , a pulse number of running out of the focal lens when focused on infinite is set to “0”. Here, the AF evaluation value is calculated based on the RGB signals as mentioned above, and since the RGB signals are acquired at an updating timing of the screen, if the RGB signals are acquired at a screen updating interval of 1/30 second, a time required for acquiring the AF evaluation values at 120 positions is as long as 120× 1/30=4 seconds. Therefore, there is a problem in that a photo opportunity, that is, so-called shutter releasing chance, is missed such that a state of the photographing object is changed. As a measure for solving the above-mentioned problem, the focus time is reduced by limiting a focus range (scan area) under a specific condition, which enables taking a picture without missing a photo opportunity. For example, if an automatic focus scan is performed by limiting the focus range so that the positions of the focal lens at which the AF evaluation values are acquired are 15 positions which are ⅛ of 120 positions, the automatic focus scan is completed in 0.5 seconds, which reduces a time lag. Moreover, by doing so, an operating time of the motor for driving the focal lens is reduced, thereby reducing power consumption.  
         [0127]     A description will now be given of an operation of the digital still camera according to the present embodiment.  
         [0128]     When the down/macro switch  210  shown in  FIG. 4  is pressed in the finder mode state, the macro mode is set.  FIG. 5  shows an example of a display screen of the LCD monitor  18 A when the macro mode is set. A number of record pixels is set to, for example, 2048×1536. A horizontal bar B 1  located in a lower portion of the screen indicates a current focal position. That is, a mark “◯” indicates a focus position J 1  corresponding to a current position of the focal lens. Immediately after the macro mode is set, the focal distance is set to 30 cm. (However, in this sate, the automatic focusing operation has not been performed yet, the position J 1  is dos not accurately match the focal point.)  
         [0129]     The shutter release  201  shown in  FIG. 2  is a two-stage switch so that a focusing operation is performed at a half-pressed position and a recording operation (photographing) is performed when the switch is further pressed.  FIG. 15  is the display screen after at least one shoot was made or when the shutter release  201  is half-pressed. In the case shown in  FIG. 15 , a focus is made about 10 cm, and thus, the mark “◯” indicating the focal position J 1  of the focal lens is moved to a position corresponding to about 10 cm. In this case, the leftward and rightward arrows R 1  displayed together with “◯” indicate that the focused state is maintained. While the focused state is maintained, the arrows R 1  are always displayed, and the range indicated by the leftward and rightward arrows R 1 , which is narrower than the entire focus range indicated by the bar B 1 , is used as an object of a focusing operation at a next time. That is, the leftward and rightward arrows R 1  indicate that the camera is in the focused state and, simultaneously, indicate a limited range of a focusing operation at a next time.  
         [0130]      FIG. 16  shows a display screen when the camera does not maintain a focused state. This is realized by continuously monitoring the output of the distance measurement sensor  23  shown in  FIG. 13  using the distance measurement unit  105  shown in  FIG. 3  after execution of the last automatic focusing. At the time of the last mountain-climbing AF execution, the CPU  15  retains the output of the distance measurement sensor  23  in the RAM provided in the CPU  15 . Thereafter, the photographing object distance is measured by the distance measurement sensor  23  periodically, for example, every 0.1 second, and the measured value is compared with photographing object information retained in the RAM each time. If the photographing object distance changed, for example, more than 5 cm, it is no longer regarded that a state of the previous picturing time is maintained, and, thus, it is determined that a focused state is not maintained. These determination reference depends on a zoom position and a position of the focal lens, and is previously stored in the ROM  16  as table information. As shown in this  FIG. 16 , when the leftward and rightward arrow R 1  is not displayed, an overall focus range scan is performed in a subsequent automatic focusing operation.  
         [0131]     As shown in  FIG. 15  and  FIG. 16 , in order to indicate the focus point by a distance in centimeters, the distance must be acquired by converting from the position of the focal lens. There are various methods of conversion such as to use an equation based on a relationship between the inverse of focal distance and the number of positions of the focal lens shown in  FIG. 14 , or to store a relationship between the inverse of focal distance and the number of positions of the focal lens in the ROM  16  as conversion table information. There are some methods which realize the detection of a current focal lens position. For example, in a case of driving a focal lens by a stepping motor, a detection of a reference point is performed as a resetting operation at a start time of a camera. A current position can be detected based on a number of pulses supplied to move the stepping motor after the resetting operation. Additionally, a resistance plate may be used to perform a position detection by an output of the resistance plate, which indicates a different resistance according to a position of the focal lens. As also shown in Table 1, in the case of a zoom lens, even if the same distance range is subjected to the automatic focus scan, a number of moving pulses greatly differ depending on the zoom position. Moreover, many lenses have lower lens resolution as it goes further to the telephoto side. Therefore, when a zoom position is changed, the display is changed to that shown in  FIG. 16 , and an automatic focus scan is performed over the entire range as regarded that a focused state is not maintained when a subsequent automatic focusing operation is performed. Thus, failure picturing can be eliminated according to the above-mentioned operation.  
         [0132]      FIG. 17  is an illustration of a screen image immediately after performing an automatic focus operation.  FIG. 18  is an illustration of a screen image in a state where the distance to a photographing object changes after an automatic focus operation was performed. In the present embodiment, for example, the image data is 8-bit data, which can represent values 0-255. The image data comprises three primary colors R, G and B, and brightness data is calculated by the following equation. 
 Brightness  Y= 0.3 R+ 0.6 G+ 0.1 B   (3)  
         [0133]     In this case, since the image data is 8-bit data, the brightness data is also 8-bit data. In the present embodiment, as shown in  FIG. 17  and  FIG. 18 , the screen is divided into blocks of vertical  12 ×horizontal  16  so as to acquire the brightness data for each divided block. Acquisition of the brightness data can be achieved by calculating average values of R, G, B for each block in the CCD interface  8  shown in  FIG. 13 , reading the calculated average valued by the CPU  15 , and converting the average values into the brightness data according to the equation (3).  
         [0134]      FIG. 19  is an illustration of the blocks shown in  FIG. 17  by classifying them into three categories according the brightness data. Similarly,  FIG. 20  is an illustration of the blocks shown in  FIG. 18  by classifying them into three categories according the brightness data value. The blocks classified into the category A, which are indicated as “A” have the average brightness value of equal to or greater than 184. Similarly, the blocks classified into the category B, which are indicated as “B” have the average brightness value of equal to or greater than 92 and smaller than 184. The blocks classified into the category C, which are indicated as “C” have the average brightness value smaller tan 92 (that is, equal to or smaller than 91). Such a brightness detection is performed, for example, at intervals of 200 msec in response to a timing of performing an automatic exposure (AE) control. The CPU  15  saves the data of the brightness category of each block in the RAM, and compares the data of the brightness category with the data of previous brightness category each time the brightness detection is performed.  FIG. 21  is an illustration showing the blocks in which the brightness category was changed when the photographing object changed from that shown in  FIG. 17  and  FIG. 19  to that shown in  FIG. 18  and  FIG. 20 .  FIG. 21  indicates that the brightness category of 28 blocks was changed. The CPU  15  determines that the picturing state was changed and the focused state is not maintained if the number of blocks of which brightness category was changed exceeds 10% of the total.  
         [0135]      FIG. 22  is a flowchart of a process to change an automatic focus scan range according to whether or not a focused state is maintained. First, when a monitoring operation is started (step S 11 ), it is determined whether or not a focusing operation has been performed (step S 12 ). The fact whether or not a focusing operation has been performed is stored in the RAM. If a focusing operation is performed, information regarding a flag indicating the execution of the focusing operation is stored, and, thereafter, the information is retained until it is determined that a picturing condition was changed. It should be noted that the information is deleted when the operation is changed into an operation mode other than photographing such as a time of turning on a power or a time of replay. If it is determined, in step S 12 , that a focusing operation is not performed, the monitoring screen is updated (step S 13 ), wait for turning on the shutter release  201  (step S 14 ). If the shutter release  201  is not turned on, the routine returns to step S 13 , and the monitoring screen is repeatedly updated until the shutter release  201  is turned on. If the shutter release  201  is turned on, it means a new focusing operation is required, and, thus, an automatic focus is performed by an automatic focus scan on an entire area (step S 15 ). Thereafter, the above-mentioned information regarding execution of the focusing operation is stored (step S 16 ), and, then, a process of taking a still picture is performed (step S 17 ). After taking a picture, the routine returns to step S 11  to continue the monitoring operation.  
         [0136]     On the other hand, if it is determined, in step S 12 , that a focusing operation has been performed, the monitoring screen is updated (step S 18 ), and periodically check whether or not the focused state is maintained by detecting a state of photographing. That is, after updating the monitoring screen, data for determining whether or not the focused state is maintained is produced (step S 19 ), and, then, it is determined whether or not the focused state is maintained (step S 20 ). The determination of whether or not the focused state is maintained according to a change in the photographing condition will be explained later with reference to  FIG. 23 . If it is determined, in step S 20 , that the focused state is maintained, the routine proceeds to step S 21  where it is determined whether or not the shutter release  201  is turned on. If it is determined that the shutter release  201  is turned on, an automatic focus scan is performed on a scan range narrower than a normal range (step S 22 ). If it is determined, in step S 21 , that the shutter release  201  is not turned on, the routine returns to step S 18  so as to repeat the process of updating the monitoring screen (step S 18 ), producing focused state maintenance determination data (step S 19 ) and determining the focused state maintenance (step S 20 ). If it is determined, in step S 20 , that the focused state is not maintained, the information indicating execution of the focusing operation is cleared (step S 23 ), and the routine proceeds to step S 13  where a photographing operation according to an automatic focus scan is performed with a normal entire area to be an object.  
         [0137]      FIG. 23  is a flowchart of a process of determining maintenance of focus in step S 20  of  FIG. 22  according to a change in a brightness distribution. When the monitoring is started, the screen is updated every 1/30 second, and an automatic exposure (AE) is performed at intervals of 200 msec. Since the RGB average values, which are result of averaging of R, G and B, are acquired in the automatic exposure control (step S 31 ), average brightness of each block is calculated by using the acquired RGB average values (step S 32 ). Then, a brightness category map indicating a brightness distribution of the blocks, each of which is produced by dividing the screen and is classified into one of three categories (step S 33 ). The brightness distribution of the blocks classified into three categories was explained with reference to  FIGS. 9-12 . Then, the current brightness distribution is compared with the last brightness distribution so as to extract blocks of which brightness distribution was changed (step S 34 ). Then, it is determined whether or not a number of blocks of which brightness category was changed exceeds 10% of the total (step S 35 ). If it exceeds 10%, it is determined that the photographing condition was changed (step S 36 ), and the information indicating the focused state is cleared (step S 37 ) and the process is ended. In a subsequent focusing operation, a focus position is acquired by scanning an entire automatic focus scan range.  
         [0138]     On the other hand, if it is determined, in step S 35 , that the number of blocks of which brightness category was changed is equal to or less than 10% of the total and the focused state is maintained (step S 38 ), information indicating the maintenance of the focused state is set (step S 39 ), and, then, the process is ended. In a subsequent automatic focusing operation, a focus position is acquired by scanning only a narrow range near the current focus position. The narrow range near the focus position is set to, for example, 20% of the entire automatic focus scan range. As shown in Table 1, in the case of the wide angle end, since 120 positions are to be subjected to the automatic focus scan, the narrow range near the focus position is 24 positions. Thus, the automatic focus scan is performed on ±12 positions with the current position of the focal lens to be a center. It should be noted that it is preferable to store the various parameters in a non-volatile memory, which is rewritable in a narrow range. The parameters include an interval of acquiring the RGB average values, a threshold value for categorizing the brightness distribution, a number of blocks of which brightness category distribution is changed or a threshold value for determining maintenance of focus of a ratio of the changed blocks, and a ratio of the narrow range near the current focus position.  
         [0139]     As shown in  FIG. 3 , which is the front view of the camera, the digital still camera according to the present embodiment is provided with the distance measurement unit  105 . The distance measurement unit  105  has two optical units arranged side-by-side so that a distance to a photographing object can be calculated based on outputs of the two optical units in accordance of the principal of the triangular surveying method. In the recording mode, a distance measurement operation is repeatedly performed by the distance sensor  23  using the distance measurement unit  105 .  FIG. 24  shows a range, within which the distance measurement sensor  23  using the distance measurement unit  105  measures a distance, in the screen. The distance computed by the distance measurement sensor  23  is converted into a focal lens position. The data regarding a correspondence relationship between the distance acquired by the distance measurement sensor  23  and the focal lens position is stored in a rewritable non-volatile memory, and the CPU  15  converts the distance computed by the distance measurement sensor  23  into the focal lens position based on the stored data.  FIG. 25  is a flowchart of a process of step S 20  of  FIG. 22  to determine whether or not a focused state is maintained using the distance computed by the distance measurement sensor  23 . The switching of the AF scan range after determination of maintenance of the focus state was described with the flowchart of  FIG. 22 .  
         [0140]     When the monitoring is started, a distance measurement operation is performed at intervals almost the same as the intervals for performing the automatic exposure (AE) control (step S 51 ). Although there is a small difference depending on brightness of a photographing object, the distance measurement operation can compute the distance to a photographing object in about 100 msec. The computed distance to the photographing object is read (step S 52 ), and a focal lens position at which the photographing object is focused is acquired by using the conversion table of the focal lens position stored in the rewritable non-volatile memory (step S 53 ). Then, the CPU  15  compares the current position of the focal lens with the focal lens position calculated from the distance to the photographing object by using the distance measurement sensor  23  (step S 54 ). In the present embodiment, the focal lens is controlled, for example, by a stepping motor, and a focal lens position can be known by counting a number of pulses corresponding to the travel of the focal lens from a reference position at a start time. Alternatively, without using a stepping motor, a position of the focal lens can be detected from a resistance value of a resistance plate, which outputs a resistance value corresponding to a focal lens position.  
         [0141]     If the difference between the current focal lens position and the focal lens position computed from the distance to the photographing object computed using the distance measurement sensor  23  is larger than the narrow range of the automatic focus scan range (step S 55 ), it is determined that the focus state is not maintained (step S 56 ), and, thus, the information indicating the maintenance of the focus state is cleared (step S 57 ) and the process is ended. In a subsequent focusing operation, a focus position is acquired by scanning the entire automatic focus scan range (step S 56 ). Here, the narrow rage of the automatic focus scan range is, as mentioned before, a scan range used when a focus is maintained and corresponds to a position range of 20% of the entire scan range. At the wide angle end, since the entire range includes 120 positions, 24 positions correspond to the narrow range of the automatic focus scan range. Thus, there is a shift larger than ±12 positions from the current position, it is determined that the focus is not maintained. On the other hand, if it is determined, in step S 55 , that the result acquired from the distance measurement sensor  23  is within the range of ±12 positions from the current position (step S 58 ), the information indicating the maintenance of the focus state is set (step S 59 ), and the process is ended. In the subsequent automatic focusing operation, a focus position is acquired by scanning only the narrow range near the current focus position.  
         [0142]      FIG. 26  is an illustration of a screen image immediately after performing the automatic focusing operation.  FIG. 27  is an illustration of a screen image in a state where a distance to a photographing object is changed from the state shown in  FIG. 26 . In the present embodiment, the screen is divided into blocks of vertical  12 ×horizontal  16 , and an average value of a difference between adjacent pixels is acquired with respect to G pixels in each block. For example, if 150 pixels are included in one block, 50 pixels are assigned to G pixel since an image is constituted by R, G and B pixels. With respect to the 50 pixels, differences are acquired between adjacent pixels, and sum the acquired differences are summed. The result of the summing is divided by 50, which is the number of pixels, so as to acquire an average of differences of G pixels for each block. In this case, if the G pixel is 8-bit data, the average value of the differences of G pixels is also 8-bit data and its maximum value is 255. The values of G pixels are acquired in the CCD interface  8  shown in the diagram of imaging system of  FIG. 13 . Here, the fact that the values of G pixels differ greatly between pixels means that the G component changes greatly between the pixels, in other words, there is a high contrast. Namely, acquiring an average of differences of G pixels of each block corresponds to acquiring a contrast for each block. Thus, in the following description, the average of differences of G pixels may be referred to as a contrast.  
         [0143]      FIG. 28  shows a distribution of the blocks of which contrast is a value equal to or greater than a predetermined value in  FIG. 26 .  FIG. 29  shows similarly a distribution of the blocks of which contrast is a value equal to or greater than a predetermined value in  FIG. 27 . In the present embodiment, a change in a state of a photographing object is detected according to whether or not a block having a contrast value of equal to or greater than 128 has moved. In  FIGS. 28 and 29 , portions to which “◯” are given are the blocks having a contrast value of equal to or greater than 128. Also in this case, detection of the contrast for each block is performed at intervals of 200 msec according to the interval for performing the automatic exposure (AE) control. The CPU  15  saves information, which indicates positions of the blocks having a contrast value equal to or greater than 128, in a RAM so as to compare with positions of blocks having a previous contrast value equal to or greater than 128 at the time of detecting a contrast.  FIG. 30  shows the blocks in black having a contrast value equal to or greater than 128 of which positions have changed when the state of the photographing object has changed from the state shown in  FIG. 26  to the state shown in  FIG. 27 . In the case of  FIG. 30 , 34 blocks have changed. When the number of blocks having a contrast value equal to or greater than 128 of which positions have changed exceeds, for example, 10% of the total, the CPU  15  determines that a focus state is not maintained.  
         [0144]      FIG. 31  is a flowchart of a process used for determining whether or not a focused state is maintained by detecting a number of blocks having a contrast value equal to or greater than 128 of which positions have changed. Switching of the AF scan range after the determination of maintenance of the focused state was explained with reference to the flowchart of  FIG. 22 . When the monitoring is started, an average of differences of each block, that is, a contrast of each block is detected (step S 71 ) at intervals substantially equal to the intervals 200 ms of the automatic exposure (AE) control, and blocks having a contrast equal to or greater than 128 are determined (step S 72 ). Based on the determination, a contrast block map, which shows a distribution of the blocks having a contrast equal to or greater than 128 is produced (step S 73 ). Then, a current contrast block map is compared with a previous contrast block map, and the number of blocks having a contrast equal to or greater than 128 of which positions have changed is counted (step S 74 ). Here, it is determined whether or not the number of blocks having a contrast equal to or greater than 128 exceeds 10% of the total (step S 75 ). If it is determined that the number of blocks having a contrast equal to or greater than 128 exceeds 10% of the total, it is determined that the focused state is not maintained and the photographing state has changed (step S 76 ). Thus, information indicating maintenance of the focused state is cleared (step S 77 ), and the process is ended. In a subsequent focusing operation, a focal position is acquired by scanning the entire automatic focus scan range. On the other hand, if it is determined that the number of blocks having a contrast equal to or greater than 128 is less than 10% of the total, it is determined that the focused state is maintained and the photographing state has changed (step S 78 ). Thus, information indicating maintenance of the focused state is set (step S 79 ), and the process is ended. In a subsequent focusing operation, a focal position is acquired by scanning only a narrow range near the current focal position.  
         [0145]     The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.  
         [0146]     The present application is based on Japanese priority applications No. 2004-007154 filed Jan. 14, 2004 and No. 2004-166442 filed Jun. 3, 2004, the entire contents of which are hereby incorporated herein by reference.