Abstract:
An image pickup apparatus capable of performing favorable focusing operation when image signals are read out from an image pickup device at a high speed while flickers are occurring. Flickers are detected from image signals outputted from the image pickup device, while the image signals outputted from the image pickup device is stored. The image signals outputted from the image pickup device and the image signals stored in a first memory are synthesized depending on the flicker detection result. A position of a focus lens is adjusted by selectively using any one of the image signals outputted from the image pickup device and the synthesized image signal. When the image pickup device performs reading images in a cycle shorter than a flicker cycle, image synthesis is performed depending on the brightness of the image signals outputted from the image pickup device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image pickup apparatus and control method therefor, which take a measure for flickers in an arbitrary cycle in an image pickup device and accomplish autofocus for favorable focusing operation. 
     2. Description of the Related Art 
     Conventionally, an image pickup apparatus such as a digital video camera or a digital still camera employs a contrast detection method as a method for performing automatic focusing operation using an image signal obtained from an image pickup device such as a CCD. 
     In the contrast detection method, images are picked up sequentially by moving the position of a focus lens within a predetermined range, a focal point evaluation signal is computed from each image signal, the focusing position is obtained from a focal point evaluation value corresponding to the focal point evaluation signal. A focal point evaluation value is computed so as to be larger as a focal point gets closer to the focusing focal point such as by using a BPF (band-pass filter). Then, the position of the focus lens is controlled toward the position where the focal point evaluation value becomes the maximum so that the focusing state is achieved. 
     In the contrast detection method, a direction to move a focus lens is obtained from sequential focal point evaluation values. As such, the method has a problem in that favorable focusing operation is not possible if S/N ratios of image signals being the bases of the focal point evaluation values for comparison are largely different from one another. Particularly, it is greatly affected by flickers of fluorescent light. 
       FIG. 9  is a diagram illustrating an effect of fluorescent flickers when a cycle of readout from an image pickup device is 1/60 sec. and a flicker cycle due to a power supply frequency is 1/50 sec. 
     In  FIG. 9 , reference numeral  901  denotes a cycle of readout from an image pickup device, reference numeral  902  denotes accumulation time for the image pickup device, reference numeral  903  denotes brightness changes due to flickers of fluorescent light, reference numeral  904  denotes a cycle of sampling focal point evaluation values, and reference numeral  905  denotes image signals outputted from the image pickup device. 
     In  FIG. 9 , the brightness changes due to the flickers of fluorescent light during the accumulation time for the image pickup device differ from one another for respective read out image signals. Accordingly, sequential image signals have different S/N ratios, so that a focal point evaluation value varies even if an object does not change. As a result, favorable focusing operation cannot be performed. 
     There is a further problem in that while an S/N ratio is steadily low such as at a low-light intensity, a change in an image signal is minor due to a noise effect so that the favorable focusing operation cannot be performed similarly to the brightness changes due to flickers. 
     To solve the above problems, the following techniques have been proposed. 
     It is determined whether or not the light intensity is low by comparing a focal point evaluation value gained from an image signal and a comparison value. If the light intensity is sufficient, accumulation time for an image pickup device is modified; and if the light intensity is low, a cycle of sampling focal point evaluation values is modified. This eliminates a flicker effect (for example, see Japanese Patent Laid-Open Patent Publication (Kokai) No. 8-79595). 
     There has been also proposed a technique capable of eliminating a flicker effect by adjusting a gain of an image signal subjected to wave detection and computation for cancelling a flicker effect by using the computation result (for example, see Japanese Patent Laid-Open Patent Publication (Kokai) No. 2001-186407). 
     In recent yeas, apart from the above techniques, there has been proposed an image pickup apparatus which reads out image signals at a high speed and stores the signals.  FIG. 10  is a diagram illustrating an effect of fluorescent flickers when a cycle of readout from an image pickup device is 1/500 sec. and a flicker cycle due to a power supply frequency is 1/50 sec. during fast readout from the image pickup device. In  FIG. 10 , reference numeral  1001  denotes a cycle of readout from the image pickup device, reference numeral  1002  denotes accumulation time for the image pickup device, reference numeral  1003  denotes brightness changes due to fluorescent flickers, reference numeral  1004  denotes a cycle of sampling focal point evaluation values, and reference numeral  1005  denotes image signals outputted from the image pickup device. 
     Such fast readout shortens a cycle of sampling focal point evaluation values, thereby improving autofocus responsiveness. 
     However, the method of modifying accumulation time for an image pickup device disclosed in the above Japanese Patent Laid-Open Patent Publication (Kokai) No. 8-79595 cannot eliminate a flicker effect if a read-out cycle for the image pickup device exceeds two times of a power supply frequency. The method of modifying a cycle sampling focal point evaluation values also has a problem in that autofocus responsiveness largely decreases. 
     Additionally, the method of adjusting a gain disclosed in the above Japanese Patent Laid-Open Patent Publication (Kokai) No. 2001-186407 does not improve an S/N ratio. As such, the method has a problem in that an S/N ratio is very low in a flicker valleys indicated by a triangle “▴” in  FIG. 10  during fast readout so that favorable focusing operation is not possible. 
     SUMMARY OF THE INVENTION 
     The present invention provides an image pickup apparatus and control method therefor, which are capable of performing favorable focusing operation when image signals are read out from an image pickup device at a high speed while flickers are occurring. 
     In a first aspect of the present invention, there is provided an image pickup apparatus comprising a focus lens, an image pickup device, a flicker detection unit adapted to detect flickers from image signals outputted from the image pickup device, a storage unit adapted to store the image signals outputted from the image pickup device, an image synthesis unit adapted to synthesize the image signals outputted from the image pickup device and the image signals stored in the storage unit depending on a detection result by the flicker detection unit, and a focal point adjustment unit adapted to adjust a position of the focus lens by selectively using any one of the image signals outputted from the image pickup device and the image signal synthesized by the image synthesis unit, and when the image pickup device performs reading images in a cycle longer than a flicker cycle, the image synthesis unit performs image synthesis depending on the brightness of the image signals outputted from the image pickup device. 
     In a second aspect of the present invention, there is provided an image pickup apparatus comprising a detection unit adapted to detect a flicker from image signals outputted from an image pickup device, a storage unit adapted to store the image signals outputted from the image pickup device when the image pickup device performs reading images in a cycle longer than a flicker cycle, an image synthesis unit adapted to synthesize the image signals outputted from the image pickup device and the image signals stored in the storage unit depending on a detection result by the detection unit, and a focal point adjustment unit adapted to adjust a position of a focus lens by selectively using any one of the image signals outputted from the image pickup device and the image signal synthesized by the image synthesis unit. 
     In a third aspect of the present invention, there is provided an image pickup apparatus comprising a focus lens, an image pickup device, a flicker detection unit adapted to detect flickers from image signals outputted from the image pickup device, a correction unit adapted to correct the image signals outputted from the image pickup device dependent on a detection result by the flicker detection unit and a brightness level value of the image signal outputted from the image pickup device, and a focal point adjustment unit adapted to adjust a position of the focus lens based on the image signal corrected by the correction unit, and the image pickup device performs reading images in a cycle longer than a flicker cycle. 
     According to the present invention, if the image pickup device perform readout in a cycle shorter than a fluorescent flicker cycle, image synthesis is performed on image signals outputted from the image pickup device depending on the brightness of the image signals outputted from the image pickup device, and the position of a focus lens is adjusted by selectively using any one of the image signals outputted from the image pickup device and the synthesized image signals. As such, favorable focusing operation can be performed when image signals are read out from the image pickup device at a high speed while flickers are occurring. 
     Further features and advantages of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram schematically showing the configuration of an image pickup apparatus according to a first embodiment of the present invention. 
         FIG. 2  is a timing chart illustrating the operation of the image pickup apparatus in  FIG. 1 . 
         FIG. 3  is a flowchart showing a procedure of focus lens control (focal point adjustment) processing executed by the image pickup apparatus in  FIG. 1 . 
         FIG. 4  is a block diagram schematically showing the configuration of an image pickup apparatus according to a second embodiment of the present invention. 
         FIG. 5  is a timing chart illustrating the operation of the image pickup apparatus in  FIG. 4 . 
         FIG. 6  is a flowchart showing a procedure of focus lens control (focal point adjustment) processing executed by the image pickup apparatus in  FIG. 4 . 
         FIG. 7  is a block diagram schematically showing the configuration of an image pickup apparatus according to a third embodiment of the present invention. 
         FIG. 8  is a timing chart illustrating the operation of the image pickup apparatus in  FIG. 7 . 
         FIG. 9  is a diagram illustrating an effect of conventional fluorescent flickers when a cycle of readout from an image pickup device is 1/60 sec. and a flicker cycle due to a power supply frequency is 1/50 sec. 
         FIG. 10  is a diagram illustrating an effect of fluorescent flickers when a cycle of readout from an image pickup device is 1/500 sec. and a flicker cycle due to a power supply frequency is 1/50 sec. during fast readout from the image pickup device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following will describe embodiments of the present invention in detail with reference to the drawings. 
       FIG. 1  is a block diagram schematically showing the configuration of an image pickup apparatus according to a first embodiment of the present invention. 
     An object image enters an image pickup device  104  via a fixed lens  101 , an aperture  102  and a focus lens  103  for performing focal point adjustment, and is photoelectrically converted to be outputted as image signals by the image pickup device  104 . 
     The image signals outputted from the image pickup device  104  are subjected to co-related double sampling and A/D conversion and converted to digital signals in an analog front-end (AFE)  105 . The image signals converted to the digital signals by the analog front-end  105  are outputted to a first memory  106 , an exposure evaluation value calculation circuit  107 , and an image synthesis circuit  108 . 
     The first memory  106  stores the inputted image signals. The exposure evaluation value calculation circuit  107  integrates the brightness values of the image signals outputted from the analog front-end  105 , calculates the exposure evaluation values in frames, and outputs the values to a flicker detection circuit  109  and an exposure evaluation value determination circuit  110 . 
     The flicker detection circuit  109  detects whether or not the image signals are affected by a flicker component of fluorescent light by comparing the average value of the brightness values in the respective frames, and outputs the detection result to a first gain control circuit  111 , a second gain control circuit  112 , and a microcomputer  113 . 
     The first gain control circuit  111  generates a first flicker correction gain to correct flickers depending on the flicker component detection result and outputs the gain to a first flicker correction circuit  114 . The first flicker correction circuit  114  corrects the flicker component in the output of the first memory  106  with the first flicker correction gain and outputs the result to a camera signal processing circuit  115 . 
     The camera signal processing circuit  115  performs signal processing for imaging such as aperture correction, gamma correction or white balance on the image signal subjected to flicker correction by the first flicker correction circuit  114 . The output of the camera signal processing circuit  115  is stored as an output signal of a camera in a storage medium (not shown). 
     The exposure evaluation value determination circuit  110  compares the exposure evaluation value outputted (calculated) by the exposure evaluation value calculation circuit  107  with a predetermined value, and outputs the determination (comparison) result to the image synthesis circuit  108 , the second gain control circuit  112 , and the microcomputer  113 . 
     The image synthesis circuit  108  performs additive synthesis on the image signals outputted from the analog front-end  105  and the image signals stored in the second memory  116  described hereinafter depending on the determination result by the exposure evaluation value determination circuit  110 , and outputs those to the second memory  116  and a second flicker correction circuit  117  described hereinafter. 
     A second memory  116  stores the image signals outputted from the image synthesis circuit  108 . The second gain control circuit  112  synthesizes the result of detecting flickers in a plurality of frames depending on the determination result by the exposure evaluation value determination circuit  110  to generate a second flicker correction gain to correct the flickers, and outputs the gain to the second flicker correction circuit  117 . 
     The second flicker correction circuit  117  corrects the flickers of the output from the second memory  116  with the second flicker correction gain and outputs the result to a focal point evaluation value calculation circuit  118 . The focal point evaluation value calculation circuit  118  extracts a high-frequency component in the output of the second flicker correction circuit  117  such that a signal becomes larger as a focal point gets closer to the focusing focal point, and outputs the extracted high-frequency component as a focal point evaluation value to the microcomputer  113 . 
     The microcomputer  113  generates a control signal of the focus lens  103  from the determination result by the exposure evaluation value determination circuit  110 , the detection result by the flicker detection circuit  109 , and the focal point evaluation value by the focal point evaluation value calculation circuit  118 , and outputs the signal to a focal point adjustment part  119 . The focal point adjustment part  119  drives a driving apparatus (not shown) for driving the focus lens  103  depending on the control signal of the microcomputer  113  to adjust the position of the focus lens  103 . 
       FIG. 2  is a timing chart illustrating the operation of the image pickup apparatus in  FIG. 1 . 
     In  FIG. 2 , reference numeral  201  denotes a cycle of readout from the image pickup device  104 , reference numeral  202  denotes accumulation time for the image pickup device  104 , reference numeral  203  denotes brightness changes due to flickers of fluorescent light, reference numeral  204  denotes a cycle of sampling focal point evaluation values, reference numeral  205  denotes image signals outputted from the image pickup device  104 , and reference numeral  206  denotes values of a first flicker correction gain to eliminate an effect of the flickers of fluorescent light, which represents a correction gain to eliminate the effect of fluorescent flickers included in image signals outputted from the image pickup device  104 . 
     In a part corresponding to flicker valley labeled with a triangle “▴” in  FIG. 2 , the level for the output signal  205  of the image pickup device  104  is low so that the value of the first flicker correction gain is large. Reference numeral  207  denotes determination results by the exposure evaluation value determination circuit  110 , in which “o” indicates that an exposure evaluation value by the exposure evaluation value calculation circuit  107  is larger than a predetermined value, while “x” indicates that the exposure evaluation value by the exposure evaluation value calculation circuit  107  is equal to or smaller than the predetermined value or less. 
     Reference numeral  208  denotes image signals outputted from the image synthesis circuit  108 . A part represented with hatching represents an image signal stored in the second memory  116 . If the exposure evaluation value determination result  207  is “x”, the image synthesis circuit  108  performs additive synthesis on the image signals  205  outputted from the image pickup device  104  and image signals stored in the second memory  116 , and stores the result in the second memory  116 . If the determination result is “o”, the circuit  108  stores only the image signals outputted from the image pickup device  104  in the second memory  116 . 
     Reference numeral  209  denotes values of a second flicker correction gain to eliminate an effect of flickers of fluorescent light, which represents a correction gain to eliminate the effect of the flickers of fluorescent light included in image signals outputted from the image synthesis circuit  108 . 
     If the exposure evaluation value determination result  207  is “x”, the image synthesis circuit  108  performs additive synthesis processing on image signals in a superimposed manner. Accordingly, the second gain control circuit  112  synthesizes first flicker correction gains for the signals of the plurality of images subjected to the addition processing to generate second flicker correction gains for the image signals subjected to the additive synthesis. 
     The microcomputer  113  stores a plurality of the exposure evaluation value determination results  207  in an internal memory (not shown) for a predetermined number of times. Then, the microcomputer  113  generates a control signal of the focus lens  103  as in a flowchart in  FIG. 3 , described hereinafter, by using a detection result by the flicker detection circuit  109 , a focal point evaluation value by the focal point evaluation value calculation circuit  118 , and the determination results for a predetermined number of times stored in the internal memory. 
     As the above predetermined number of times, a period having a low S/N ratio value due to the flicker effect is calculated and set based on a flicker detection result and an exposure evaluation value determination result. 
       FIG. 3  is a flowchart showing a procedure of focus lens control (focal point adjustment) processing executed by the image pickup apparatus in  FIG. 1 . 
     The focus lens control processing is executed through the operation of the focal point adjustment part  119  by the microcomputer  113  in  FIG. 1 . 
     In  FIG. 3 , first, a result of detecting a flicker is determined (step S 301 ). If a flicker is not detected, a control signal of the focus lens  103  is generated using a focal point evaluation value (step S 302 ), followed by terminating the process. 
     If a flicker is detected, it is determined whether or not the previous determination result by the exposure evaluation value determination circuit  110  is “o”, i.e., the previous exposure evaluation value is larger than a predetermined value (step S 303 ). 
     If the previous exposure evaluation value is larger than the predetermined value, it is decided that the current focal point evaluation value is generated from an image signal with a sufficient S/N ratio, and the control signal of the focus lens  103  is generated using the previous focal point evaluation value (step S 302 ), followed by terminating the process. 
     If the previous exposure evaluation value is equal to or smaller than the predetermined value (NO to the step S 303 ), it is determined whether or not the current determination result of the exposure evaluation value determination circuit  110  is “o”, i.e., the current exposure evaluation value is larger than a predetermined value (step S 304 ). If the current exposure evaluation value is equal to or smaller than the predetermined value, it is decided that the focal point evaluation value is generated from a low S/N ratio image signal. Then, the generation of the focus lens control signal from the current focal point evaluation value is stopped and the previous focus lens control is kept (step S 305 ), followed by terminating the process. Namely, the microcomputer  113  temporarily prevents the focal point adjustment part  119  from adjusting the position of the focus lens  103  based on the low S/N ratio image signal synthesized by the image synthesis circuit  108 , when it is determined by the exposure evaluation value determination circuit  110  that the brightness level value is equal to or smaller than a predetermined value. 
     If the current exposure evaluation value is larger than the predetermined value, a predetermined times of past determination results stored in the internal memory are checked and it is determined whether or not a time period in which the S/N ratio is low (hereinafter referred to as “low S/N ratio period”) ends (step S 306 ). In the case where it is determined that low S/N ratio period ends, if all exposure evaluation values except for the current determination result are equal to or smaller than the predetermined value, it is determined that all the image signals during the low S/N ratio period are subjected to additive synthesis. Then, the control signal of the focus lens  103  is generated using the focal point evaluation value (step S 302 ), followed by terminating the process. 
     If the low S/N ratio period does not end (NO to the step S 306 ), then it is determined that the low S/N ratio period is lasting, the generation of the focus lens control signal from the focal point evaluation value is stopped and the immediately preceding focus lens control is kept (step S 305 ), followed by terminating the process. 
     During the sampling cycle  204  for a focal point evaluation value in  FIG. 2 , an up-arrow indicates the case where the microcomputer  113  generates the control signal of the focus lens  103  using a focal point evaluation value calculated by the focal point evaluation value calculation circuit  118 , while a right arrow indicates the case where there is no use of the focal point evaluation value. 
     In the present embodiment, the image signals are subjected to the additive synthesis during the low S/N ratio period due to the effect of flickers of fluorescent light, and then sampling of the focal point evaluation value is performed at a time. This prevents erroneous operations caused by using the focal point evaluation value calculated from the low S/N ratio image signal. 
     As described above, according to the present embodiment, the cycle of sampling a focal point evaluation value is maintained at a high speed during a favorable S/N ratio period and the sampling cycle is set to a low speed only during a low S/N ratio period. This enables favorable focusing operation during fast readout under fluorescent light without largely decreasing autofocus responsiveness by fast readout. 
     Next, a second embodiment of the present invention will be described. In this embodiment, the exposure evaluation value of the image signal synthesized by the image synthesis circuit  108  is calculated and a focal point evaluation value decided to have a favorable S/N ratio is used to control the focus lens  103  so that the number of sampling times of focal point evaluation values is increased. 
       FIG. 4  is a block diagram schematically showing the configuration of an image pickup apparatus according to the second embodiment of the present invention.  FIG. 5  is a timing chart illustrating the operation of the image pickup apparatus in  FIG. 4 . 
     An image pickup apparatus according to the second embodiment has fundamentally the same construction as the first embodiment described above, and therefore corresponding elements are designated by identical reference numerals and description thereof is omitted. Only different points will be described below. 
     In  FIG. 4 , a second exposure evaluation value calculation circuit  120  integrates the brightness values of image signals outputted from the second memory  116 , calculates an exposure evaluation value in a frame, and outputs the evaluation value to a second exposure evaluation value determination circuit  121 . 
     The second exposure evaluation value determination circuit  121  compares the exposure evaluation value outputted from the second exposure evaluation value calculation circuit  120  with a predetermined value and outputs the determination (comparison) result to the microcomputer  113 . 
     In  FIG. 5 , reference numeral  501  denotes a cycle of readout from the image pickup device  104 , reference numeral  502  denotes accumulation time for the image pickup device  104 , reference numeral  503  denotes brightness changes due to flickers of fluorescent light, reference numeral  504  denotes a cycle of sampling focal point evaluation values, reference numeral  505  denotes image signals outputted from the image pickup device  104 , reference numeral  506  denotes values of a first flicker correction gain to eliminate an effect of the flickers of fluorescent light, reference numeral  507  denotes determination results by the exposure evaluation value determination circuit  110 , reference numeral  508  denotes image signals outputted from the image synthesis circuit  108 , reference numeral  509  denotes values of a second flicker correction gain to eliminate the effect of the flickers of fluorescent light, and reference numeral  510  denotes determination results by the second exposure evaluation value determination circuit  121 , in which “o” indicates that an exposure evaluation value of the second exposure evaluation value calculation circuit  120  is larger than a second predetermined value, while “x” indicates that the exposure evaluation value of the second exposure evaluation value calculation circuit  120  is equal to or smaller than the second predetermined value. 
     The microcomputer  113  generates a control signal of the focus lens from a determination result by the exposure evaluation value determination circuit  110 , a detection result by the flicker detection circuit  109 , a focal point evaluation value by the focal point evaluation value calculation circuit  118 , and a determination result by the second exposure evaluation value determination circuit  121 . Then, the microcomputer  113  outputs the generated control signal to the focal point adjustment part  119 . The focal point adjustment part  119  generates the control signal of the focus lens  103  as in a flowchart in  FIG. 6 , described hereinafter, depending on a control signal of the microcomputer  113 . 
       FIG. 6  is a flowchart showing a procedure of focus lens control (focal point adjustment) processing executed by the image pickup apparatus in  FIG. 4 . 
     The focus lens control processing is executed through the operation of the focal point adjustment part  119  by the microcomputer  113  in  FIG. 4 . 
     In  FIG. 6 , first, a result of detecting a flicker is determined (step S 601 ). If a flicker is not detected, a control (adjustment) signal of the focus lens  103  is generated using a focal point evaluation value (step S 602 ), followed by terminating the process. 
     If a flicker is detected (YES to the step S 601 ), it is determined whether or not the determination result by the second exposure evaluation value determination circuit  121  is “o”, i.e., a second exposure evaluation value is larger than a second predetermined value (step S 603 ). 
     If the second exposure evaluation value is larger than the second predetermined value, it is determined that the current focal point evaluation value is generated from an image signal with a sufficient S/N ratio, and the control signal of the focus lens  103  is generated using the second focal point evaluation value (step S 602 ), followed by terminating the process. 
     If the second exposure evaluation value is equal to or smaller than the second predetermined value (NO to the step S 603 ), then it is determined that the focal point evaluation value is generated from a low S/N image signal, the generation of the focus lens control signal from the focal point evaluation value is stopped and the previous focus lens control is kept (step S 604 ), followed by terminating the process. 
     During the sampling cycle  504  for a focal point evaluation value in  FIG. 5 , an up-arrow indicates the case where the microcomputer  113  generates the control signal of the focus lens using a focal point evaluation value calculated by the focal point evaluation value calculation circuit  118 , while a right arrow indicates the case where there is no use of the focal point evaluation value. 
     According to the present embodiment, the exposure evaluation value (second exposure evaluation value) of the image signal synthesized by the image synthesis circuit  108  is calculated and a focal point evaluation value decided to have a favorable S/N ratio is used to control the focus lens  103 . This can increase the number of times of sampling the focal point evaluation values, thereby restraining a decrease in autofocus responsiveness by fast readout. 
     Next, a third embodiment of the present invention will be described. In this embodiment, the exposure evaluation value of an image signal synthesized by the image synthesis circuit  108  are calculated and a focal point evaluation value decided to have a favorable S/N ratio is used to control the focus lens  103 . Further, image signals that have been subjected to additive synthesis by the image synthesis circuit  108  are once reset so that erroneous determination is reduced when a shake occurs due to the additive synthesis. 
       FIG. 7  is a block diagram schematically showing the configuration of an image pickup apparatus according to the third embodiment of the present invention.  FIG. 8  is a timing chart illustrating the operation of the image pickup apparatus in  FIG. 7 . 
     An image pickup apparatus according to the second embodiment has fundamentally the same construction as the first embodiment described above, and therefore corresponding elements are designated by identical reference numerals and description thereof is omitted. Only different points will be described below. 
     In  FIG. 7 , the second exposure evaluation value calculation circuit  120  integrates the brightness values of image signals outputted from the second memory  116 , calculates an exposure evaluation value in a frame, and outputs the evaluation value to the second exposure evaluation value determination circuit  121 . 
     The second exposure evaluation value determination circuit  121  compares the exposure evaluation value outputted from the second exposure evaluation value calculation circuit  120  with a third predetermined value, and outputs the determination (comparison) result to the image synthesis circuit  108  and the microcomputer  113 . 
     In  FIG. 8 , reference numeral  801  denotes a cycle of readout from the image pickup device  104 , reference numeral  802  denotes accumulation time for the image pickup device  104 , reference numeral  803  denotes brightness changes due to flickers of fluorescent light, reference numeral  804  denotes a cycle of sampling focal point evaluation values, reference numeral  805  denotes image signals outputted from the image pickup device  104 , reference numeral  806  denotes values of a first flicker correction gain to eliminate an effect of the flickers of fluorescent light, reference numeral  807  denotes determination results by the exposure evaluation value determination circuit  110 , reference numeral  808  denotes image signals outputted from the image synthesis circuit  108 , reference numeral  809  denotes values of a second flicker correction gain to eliminate the effect of the flickers of fluorescent light, and reference numeral  810  denotes determination results by the second exposure evaluation value determination circuit  121 . If the second exposure evaluation value determination result  810  is “o”, i.e., the second exposure evaluation value is larger than the third predetermined value, additive synthesis of the image signals is once reset so that sampling of focal point evaluation values is performed. 
     According to the present embodiment, the exposure evaluation value of the image signal synthesized by the image synthesis circuit  108  is calculated and the focal point evaluation value decided to have a favorable S/N ratio is used to control the focus lens. Further, the image signals that have been subjected to additive synthesis by the image synthesis circuit  108  are once reset so that erroneous determination can be reduced when a shake occurs due to the additive synthesis. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions. 
     This application claims the benefit of Japanese Application No. 2007-129405, filed May 15, 2007, which is hereby incorporated by reference herein in its entirety.