Abstract:
A focus detecting device for determining the focus state of an imaging optical system, of a type for time-sequentially entering light beams, transmitted by different areas of an imaging optical system, into a sensor and detecting the phase difference between the image signal obtained from the sensor means corresponding to such time-sequentially entered light beams. 
     The device determines the positional difference in the vertical direction of the light beams (images) time-sequentially entering the sensor, and determines the storage positions of the outputs (image signals) from the sensor corresponding to the light beams (images) of the respective areas, thereby enabling exact focus detection even in case the images are received in vertically different positions on the sensor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improvement on a focus detecting device that is adapted to be equipped on a digital camera or the like and which time-sequentially divides the photographing light beam passing through an optical system into at least two different areas, converts optical images, time-sequentially focused on image sensor means through respective areas, into image signals and detects the phase difference of the image signals thereby calculating the focus state of the optical system. 
     2. Related Background Art 
     Among the automatic focusing devices employed in the conventional camera that uses silver halide-based film of the single lens reflex type, there are widely employed those base on the phase difference detecting system. 
     FIG. 48 is a view showing the arrangement of the optical system of a single lens reflex type, provided with a conventional focus detecting device of the phase difference detecting system, wherein a light beam  9   a  emerging from a photographing lens  1  is partly reflected by a main mirror  2 , consisting of a half mirror, as a light beam  9   b  toward a focusing screen  3  and focuses an image of the object on a matted face thereof. The photographer observes the object image on the focusing screen, through eyepiece lenses  5   a ,  5   b  and a pentagonal roof prism  4 . 
     On the other hand, among the light beam  9   a , a part  9   e  transmitted by the main mirror  2  is reflected by a sub mirror  6  and is guided as a light beam  9   f  to a focus detecting device  7 , which detects, by means of the light beam  9   f , the focus state (defocus amount) of the photographing lens  1  relative to a silver halide-based photographic film  8 . 
     The detected defocus amount is larger than a predetermined range of focus so that the photographing lens is judged as in an out-of-focus state, and an unrepresented control circuit so drives a focusing lens of the photographing lens  1  as to cancel the defocus amount, thereby performing the focusing operation. 
     In the following there will be explained, with reference to FIGS. 49A to  49 C, the focus detecting principle of the conventional focus detecting device. 
     FIG. 49A shows an in-focus state, wherein light beams  16   a ,  16   b  passing respectively through two different pupils of a photographing lens  10  are focused on a primary focal plane  14 , and images of the object on such primary focal plane are refocused by secondary imaging lenses  12   a ,  12   b  on a sensor plane  13  having two line sensors for each of the refocused images. A field lens  11  is provided in the vicinity of the primary image plane of the photographing lens  10 , thus efficiently guiding the light beam of a predetermined image height to the sensor plane  13  and preventing the loss in the amount of light resulting from the increase in the image height. The two light beams  16   a ,  16   b , passing through the different pupils of the photographing lens  10 , are in general limited by unrepresented diaphragms positioned immediately in front of or immediately behind the secondary imaging lenses  12   a ,  12   b , and the photographing lens  10  is not provided with a member for dividing the pupil. Assuming that δ 0  is the relative distance (phase difference) of the positions of the two images in the in-focus state, the amount of defocus in the current state and the direction thereof can be known from the difference between δ 0  and the actually given phase difference. 
     FIG. 49B shows a state in which the lens is focused in front by a defocus amount d 1 , wherein the phase difference δ 1  of the two images becomes smaller than δ 0 , and the difference (δ 0 −δ 1 ) increases with the increase of d 1 . 
     FIG. 49C shows a state in which the lens is focused in the back by a defocus amount d 2 , wherein the phase difference δ 2  of the two images becomes larger than δ 0 , and the difference (δ 2 −δ 0 ) increases with the increase of d 1 . In this manner the detection of the phase difference of the two images focused on the sensor plane  13  allows the camera to detect the focus state of the photographing lens, or the magnitude and the direction of the defocus amount. 
     However, in the prior art, the single lens reflex camera becomes inevitably bulky for securing the space for the focus detecting device and becomes expensive by the cost required therefor. Also, the accuracy of focusing is deteriorated as the relative positional relationship between the photographic film and the focus detecting device is varied by a change in the temperature or by a time-dependent variation in the quick return mirror. 
     The image taking apparatus, such as a digital camera, employ a solid-state image pickup device as the image sensor and can avoid the above-mentioned drawbacks by employing such solid-state image pickup device as the focus detecting sensor. A method for utilizing the solid-state image pickup device for the focus detecting sensor, for example employed in video cameras, consists of effecting the focusing operation, based on the detected contrast of the object image on the solid-state image pickup device, but such method is incapable of high-speed and highly precise focusing, as the exact defocus amount cannot be detected. 
     The highly precise high-speed focusing, without the above-mentioned drawbacks, can however, be achieved by providing the photographing lens with pupil dividing means for rendering transmissive either one of the two different pupil areas, and detecting the relative positional difference, or the phase difference, between the object image obtained by the light beam transmitted by such transmissive pupil area and an object image obtained by the light beam transmitted by the other pupil area, thereby detecting the defocus amount. 
     However, in such conventional configuration, the image signal of the object image formed by the light beam transmitted by a pupil area is stored at first and then that of the object image formed by the light beam transmitted by the other pupil area is stored later, so that the two image signal storage operations are mutually different in time. If the object image on the image pickup sensor moves in a period from the storage of the first image signal to that of the second image signal, there will result a detection error because of such image movement. 
     This drawback will be explained with reference to FIGS. 13A and 13B, which illustrate a case where the object image on the image pickup sensor moves in the vertical direction by a hand vibration or by the movement of the object itself. 
     If the camera is inclined downwards by a hand vibration after the storage of a first object image L shown in FIG. 13A, the object image R to be stored next moves upwards as shown in FIG.  13 B. In such case, the image signal Rc of an image pickup position the same as that for the image signal Lc, is in fact generated by looking at the object from a lower position, and therefore the camera assumes the image signals indicate a different shape. The correlation calculation utilizing the image signals of different forms leads to an erroneous phase difference. 
     In this manner the vertical movement of the object image on the image pickup element, caused by hand vibration or by movement of the object, will result in an error in the detected phase difference. 
     SUMMARY OF THE INVENTION 
     One aspect of the application is to provide an apparatus capable of determining the light receiving position of each image in the vertical direction of an image pickup element (sensor means) according to the vertical moving position of the image on the image pickup element, storing image signals in such determined positions, and detecting the phase difference between thus fetched image signals. 
     One aspect of the application is to provide a focus detecting device provided with image pickup means for taking an image; an image taking optical system; pupil shape changing means for time-sequentially dividing an image taking light beam, transmitted by the image taking optical system, into at least two different areas and focusing the divided light beams on the image pickup means; and calculation means for converting the optical images, time-sequentially formed on the image pickup means respectively through the different areas, into image signals and calculating the focus state of the image taking optical system by detecting the phase difference of the image signals, the device comprising movement amount detecting means for detecting the amount of movement of the optical image on the image pickup element, in a direction perpendicular to the direction of arrangement of the above-mentioned areas (for example in the vertical direction of a camera), and image position changing means for changing the position of the image signal to be employed in the calculation of the phase difference to be executed in the calculation means. 
     One aspect of the application is to provide an apparatus for calculating a correlation value indicating the level of coincidence of time-sequentially obtained two images by a correlation calculation, determining the positions of the image signals showing a high level of coincidence based on such correlation value and calculating the phase difference utilizing the image signals of such positions. 
    
    
     Still other objects of the present invention, and the features thereof, will become fully apparent from the following description which is to be taken in conjunction with the attached drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A,  1 B and  1 C are schematic views showing light beams passing through different pupil areas and focus states thereof on a CCD surface, in an in-focus state in cameras constituting embodiments of the present invention; 
     FIGS. 2A,  2 B and  2 C are schematic views showing light beams passing through different pupil areas and focus states thereof on a CCD surface, in a front-focused state in the cameras constituting the embodiments of the present invention; 
     FIGS. 3A,  3 B and  3 C are schematic views showing light beams passing through different pupil areas and focus states thereof on a CCD surface, in a rear-focused state in the cameras constituting the embodiments of the present invention; 
     FIG. 4 is a perspective view showing a portion for changing the pupil area, equipped in the cameras constituting the embodiments of the present invention; 
     FIG. 5 is a longitudinal cross-sectional view of the configuration shown in FIG. 4; 
     FIG. 6 is a block diagram showing the principal electrical configuration of the cameras constituting the embodiments of the present invention; 
     FIGS. 7A,  7 B and  7 C are views showing a pupil area varying operation in the cameras constituting the embodiments of the present invention; 
     FIGS. 8,  9  and  10  are views for explaining the driving method for the CCD shown in FIG. 6; 
     FIGS. 11 and 12 are charts for explaining a phase difference detecting operation by correlation calculation in the cameras of the embodiments of the present invention; 
     FIGS. 13A and 13B are views showing the situation in the case when the camera vibrates in the vertical direction at the photographing operation; 
     FIGS. 14 and 16 are charts showing the change in the correlation in the cameras of the embodiments of the present invention; 
     FIG. 15 is a chart showing the method of interpolation in the cameras of the embodiments of the present invention; 
     FIG. 17 is a flow chart showing the process for calculating the shift range in the cameras of the embodiments of the present invention; 
     FIGS. 18 and 19 are charts showing the method for detecting the hand vibration in the camera of a first embodiment of the present invention; 
     FIG. 20 is a flow chart showing the control sequence in the camera of the first embodiment of the present invention; 
     FIG. 21 is a flow chart showing an example of the focus detecting operation in the camera of the first embodiment of the present invention; 
     FIG. 22 is a flow chart showing an example of the defocus amount calculating operation in the camera of the first embodiment of the present invention; 
     FIG. 23 is a flow chart showing another example of the focus detecting operation in the camera of the first embodiment of the present invention; 
     FIG. 24 is a flow chart showing another example of the defocus amount calculating operation in the camera of the first embodiment of the present invention; 
     FIG. 25 is a chart showing the method for detecting a hand vibration in a camera of a second embodiment of the present invention; 
     FIG. 26 is a flow chart showing an example of the focus detecting operation in the camera of the second embodiment of the present invention; 
     FIG. 27 is a flow chart showing an example of the defocus amount calculating operation in the camera of the second embodiment of the present invention; 
     FIG. 28 is a flow chart showing another example of the focus detecting operation in the camera of the second embodiment of the present invention; 
     FIG. 29 is a flow chart showing another example of the defocus amount calculating operation in the camera of the second embodiment of the present invention; 
     FIG. 30 is a flow chart showing another example of the focus detecting operation in the camera of the second embodiment of the present invention; 
     FIG. 31 is a flow chart showing another example of the defocus amount calculating operation in the camera of the second embodiment of the present invention; 
     FIG. 32 is a chart showing the method for detecting a hand vibration in the camera of a third embodiment of the present invention; 
     FIG. 33 is a flow chart showing an example of the focus detecting operation in the camera of the third embodiment of the present invention; 
     FIG. 34 is a flow chart showing an example of the defocus amount calculating operation in the camera of the third embodiment of the present invention; 
     FIG. 35 is a flow chart showing another example of the focus detecting operation in the camera of the third embodiment of the present invention; 
     FIG. 36 is a flow chart showing another example of the defocus amount calculating operation in the camera of the third embodiment of the present invention; 
     FIG. 37 is a flow chart showing another example of the focus detecting operation in the camera of the third embodiment of the present invention; 
     FIG. 38 is a flow chart showing another example of the defocus amount calculating operation in the camera of the third embodiment of the present invention; 
     FIG. 39 is a chart showing the relationship between the time interval of the storage of the image signals and the calculation method in the camera of a fourth embodiment of the present invention; 
     FIG. 40 is a flow chart showing the focus detecting operation in the camera of the fourth embodiment of the present invention; 
     FIG. 41 is a flow chart showing a part of the control sequence for calculating the number of the image signals, in the camera of the fourth embodiment of the present invention; 
     FIG. 42 is a continuation of the flow chart shown in FIG. 41; 
     FIG. 43 is a flow chart showing the details of the operation “image signal storage  2 ” in FIG. 40; 
     FIG. 44 is a flow chart showing the details of the operation “image signal storage  3 ” in FIG. 40; 
     FIG. 45 is a flow chart showing the details of the operation “image signal storage  4 ” in FIG. 40; 
     FIG. 46 is a flow chart showing the details of the operation “image signal storage  5 ” in FIG. 40; 
     FIG. 47 is a flow chart showing the defocus amount calculating operation of the camera of the fourth embodiment of the present invention; 
     FIG. 48 is a view showing the arrangement of the optical system in an ordinary single lens reflex camera; and 
     FIGS. 49A,  49 B and  49 C are views showing the principle of focus detection in a single lens reflex camera of the configuration shown in FIG.  48 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     The present invention will be explained in detail, based on the embodiment as illustrated. 
     FIGS. 1A to  1 C,  2 A to  2 C and  3 A to  3 C are views showing the detecting principle of a focus detecting device constituting a second embodiment of the present invention. 
     FIGS. 1A to  1 C illustrate light beams in an in-focus state. In a state shown in FIG. 1A, the light beam  23   a  emerging from a photographing lens  20  is focused, on the optical axis  24  thereof, on the light receiving face of an image pickup device  22 , with a zero defocus amount. Also when a diaphragm  21   b  having an aperture (first pupil area) in the upper part of the photographing lens  20  is inserted as shown in FIG. 1B, the light beam  23   b  emerging therefrom still enters, on the light receiving face of the image pickup device  22 , a position on the optical axis  24  of the photographing lens. Also when a diaphragm  21   c  having an aperture (second pupil area) in the lower part of the photographing lens  20  is inserted as shown in FIG. 1C, the light beam  23   c  emerging therefrom still enters, on the light receiving face of the image pickup device  22 , a position on the optical axis  24  of the photographing lens. 
     Thus, in the in-focus state, the light beam transmitted by the first pupil area (hereinafter simply represented as “first pupil”) and that transmitted by the second pupil area (hereinafter simply represented as “second pupil”) enter the same position on the image pickup device  22 , so that the phase difference between the two images is zero. 
     FIGS. 2A to  2 C illustrate the light beams in a front-focused state. In a state shown in FIG. 2A, the light beam  25   a  emerging from the photographing lens  20  is focused at a position which is in front, by d a , of the light receiving face of the image pickup device  22 , so that the defocus amount is d a . FIG. 2B shows a state in which a diaphragm  21   b  having an aperture (first pupil) in the upper part of the photographing lens  20  is inserted. The light beam  25   b  emerging from the photographing lens  20  enters, on the light receiving face of the image pickup device  22 , a position that is below the optical axis  24  of the photographing lens by a distance δ a /2. Also in the case of a diaphragm  21   c  having an aperture (second pupil) in the lower part of the photographing lens  20  being inserted as shown in FIG. 2C, the light beam  25   c  emerging from the photographing lens  20  enters, on the light receiving face of the image pickup device  22 , a position that is above the optical axis  24  of the photographing lens by a distance δ a /2. 
     Thus, in such rear-focused state, the light beams respectively transmitted by the first and second pupils generate a phase difference δ a  on the image pickup device. 
     FIGS. 3A to  3 C illustrate the light beams in a rear-focused state. In a state shown in FIG. 3A, the light beam  26   a  emerging from the photographing lens  20  is focused at a position behind, by d b , the light receiving face of the image pickup device  22 , so that the defocus amount is d b . FIG. 3B shows a state in which a diaphragm  21   b  having an aperture (first pupil) in the upper part of the photographing lens  20  is inserted. The light beam  26   b  emerging from the photographing lens  20  enters, on the light receiving face of the image pickup device  22 , a position above the optical axis  24  of the photographing lens by a distance δ b /2. Also in the case of a diaphragm  21   c  having an aperture (second pupil) in the lower part of the photographing lens  20  being inserted as shown in FIG. 3C, the light beam  26   c  emerging from the photographing lens  20  enters, on the light receiving face of the image pickup device  22 , a position below the optical axis  24  of the photographing lens by a distance δ b /2. 
     Thus, in such a rear-focused state, the light beams respectively transmitted by the first and second pupils generate a phase difference δ b  on the image pickup device. 
     By taking the phase difference as positive in case the object image formed by the light beam transmitted by the second pupil is positioned above that formed by the light beam transmitted by the first pupil, the phase difference in the front-focused state shown in FIGS. 2A to  2 C is represented by “+δ a ” while that in the rear-focused state shown in FIGS. 3A to  3 C is represented by “−δ b ”. Thus the magnitude and the direction of the defocus amount can be known from the magnitude and the sign of the phase difference. 
     FIG. 4 is a perspective view showing a part of the optical system of a camera employing a focus detecting device constituting an embodiment of the present invention, and FIG. 5 is a longitudinal cross-sectional view of the optical system shown in FIG.  4 . 
     In these drawings, numerals  60 ,  61  indicate a photographing lens, in which a focusing lens  61  is driven by an unrepresented lens driving motor to effect focusing. There are also shown an optical low-pass filter  62 , an infrared cut-off filter  63 , a solid-state image pickup device  64 , composed of a CCD, for converting an optical image into an image signal, a focus detecting diaphragm  65 , which can be inserted into and retracted from the optical path of the photographing lens by a motor  66 , and a focus detecting light shading board  67 , which can be inserted into and retracted from the optical path of the photographing lens by a motor  68 . 
     FIG. 6 is a schematic block diagram showing the electrical configuration of the camera with the focus detecting device of the present embodiment. 
     Referring to FIG. 6, a solid-state image pickup device  70 , composed of a CCD (corresponding to the CCD  64  shown in FIG. 4) releases an analog image signal, which is converted by an A/D converter  71  into a digital signal and sent to a digital signal processing unit  72 . There are also provided a VRAM  73  constituting memory means for storing image data to be displayed on an electronic view finder (EVF)  76 ; a D/A converter  74  for converting the image data from the VRAM  73  into an analog signal; and an LCD driver  75  for controlling the electronic view finder (EVF)  76  for displaying therein the image released from the VRAM  73 . 
     There are also provided a buffer memory  77  composed, for example, of a DRAM for temporarily storing the digital image signal and various data; a memory  78  for storing the photographed image data; a system control unit  79  for controlling the entire camera; a display member  80  for displaying the setting of the camera and the control state thereof; an LCD driver  81  for driving the display member  80 ; a CCD driver  82  for driving the CCD  70 ; a lens control unit  83  for controlling the diaphragm and the light shading board for focus detection, the diaphragm for photographing and the focusing lens; and an operation switch  84  for setting the photographing mode and for detecting the shutter releasing operation. 
     The foregoing represents the principal configuration of the camera of the present embodiment. 
     FIGS. 7A to  7 C illustrate the function of the focus detecting diaphragm  65  and the focus detecting light shading board  67  shown in FIGS. 4 and 5. When the focus detecting operation is not conducted, the focus detecting diaphragm  65  and the light shading board  67  are retracted, as shown in FIG. 7A, from a pupil area  69  corresponding to the full-open photographing diaphragm (passing area of the effective light beam). When the focus detecting operation is conducted, the focus detecting diaphragm  65  is inserted, by the motor  66 , into the optical path of the photographing lens as shown in FIGS. 7B and 7C, while the focus detecting light shading board  67  is moved by the motor  68  to cover either an aperture  65   a  or  65   b  of the diaphragm  65 . 
     An image signal obtained by the light beam transmitted by the left pupil, in the state shown in FIG. 7B, is represented by L, while an image signal obtained by the light beam transmitted by the right pupil, in the state shown in FIG. 7C, is represented by R. 
     In the following there will be explained the driving method for the solid-state image pickup device or CCD, with reference to FIGS. 8,  9  and  10 . 
     In the focus detecting device of the present embodiment, utilizing the pupil division on a time-shared basis, the image signal L formed by the light beam transmitted by the left pupil is stored at first and then the image signal R formed by the light beam transmitted by the right pupil is stored, and the time interval of the storage of the image signals L and R is desirably as short as possible since a shorter interval reduces the error in the focus detection, resulting from the movement of the object image caused, for example, by hand vibration. 
     The read-out of the pixel data, if executed on the entire area of the CCD as in the photographing operation, will take a long time, thus prolonging the time interval between the storages of the image signals. For this reason, the storage of the image signal for focus detection is executed by the pixel read-out of a higher speed, as will be explained in the following. 
     FIG. 8 is a schematic view of an interline CCD, wherein shown are pixels  31 , vertical charge transfer elements  32 , a horizonal charge transfer element  33 , and an output unit  34 . 
     Signal charges, obtained by photoelectric conversion in the pixels, are transferred to the vertical charge transfer elements  32 , and are transferred in succession toward the horizontal charge transfer element  33  according to 4-phase driving pulses φV 1 , φV 2 , φV 3 , φV 4 . The horizontal charge transfer element  33  transfers the signal charges of a horizontal row, transferred from the vertical signal transfer elements  32 , in succession toward the output unit  34  according to 2-phase driving pulses φH 1 , φH 2 , and the signal charges are converted in the output unit  34  into voltages and outputted. 
     FIG. 9 is a schematic view of the image taking area of the CCD. In the present embodiment, for achieving a high-speed read-out operation, the signal charges are read at a normal speed only in an area  41  used for the focus detection, and are sweep transferred at a higher speed in other areas  42 ,  43 . 
     FIG. 10 is a timing chart of a vertical synchronization period, in case the vertical charge transfer elements  32  of the aforementioned CCD are 4-phase driven. 
     A vertical synchronization signal VD indicates the vertical blanking period by a low potential state, while a horizontal synchronization signal HD indicates the horizontal blanking period by a low potential state. There are also shown 4-phase drive pulses φV 1 , φV 2 , φV 3  and φV 4 , for the vertical charge transfer elements  32 , and read-out pulses  51 ,  52  for transferring the signal charges, obtained by the photoelectric conversion in the pixels  31 , to the vertical charge transfer elements  32 . Among the 4-phase drive pulses, those  53  and  54  are high-speed sweep drive pulses for high-speed transfer of the signal charges in the vertical charge transfer elements  32  in the areas  42 ,  43  in FIG. 9, and are generated at a higher rate than in the ordinary drive pulses. 
     By sweeping out the signal charges at a high speed in the areas not used for focus detection, it is rendered possible to reduce the time required for reading out the image signal, thereby preventing the deterioration of the precision of the focus detection and achieving the focus detecting operation at a higher speed. 
     In the following there will be explained a method for determining the phase difference by correlation calculation, with reference to FIGS. 11 and 12. 
     FIG. 11 shows the image signal L formed by the light beam transmitted by the left pupil and the image signal R formed by the light beam transmitted by the right pupil, and the two image signals have a phase difference δ. The image signal L is composed of values l 1 -l 23 , respectively corresponding to the signal charges of the pixels, while the image signal R is similarly composed of values r 1 -r 23 . At first a correlation C(τ) is calculated from the image signals L and R, according to the following formula:                C        (   τ   )       =       ∑     i   =   1     10          max        [       l        (     i   +   6     )       ,     r        (     i   +   τ   +   6     )         ]                 (   1   )                                
     wherein max[l(i+6), r(i+τ+6)] means to select the larger of l(i+6) and r(i+τ+6), and τ is varied from −6 to +6. 
     FIG. 12 shows the change of the correlation C(τ), which becomes smaller as the value τ approaches the phase difference δ. An interpolating calculation is made on the correlations C( 3 ), C( 4 ) and C( 5 ) to calculate C(δ) corresponding to the minimum correlation between C( 3 ) and C( 4 ), and a value τ corresponding to C(δ) with the minimum correlation indicates the phase difference δ. In this manner the phase difference of the image signals L and R can be calculated. The maximum and minimum values of τ and the number of the pixel data are not limited to those in the foregoing embodiment. 
     In the following there will be explained a method for correcting the phase difference, in case the object image moved in the vertical direction on the CCD, by a hand vibration. 
     FIGS. 13A and 13B show a case where the camera is inclined downwards by the hand vibration, in a period from the storage of the object image L formed by the light beam transmitted by the left pupil (FIG. 13A) to that of the object image R formed by the light beam transmitted by the right pupil (FIG.  13 B), wherein the object image moves upwards as illustrated. In such case, an image signal R c , at a position the same as that of the image signal L c , looks at a lower position of the object than in the image signal L c , thus assuming a different shape. The correlation calculation, based on the image signals of such different shapes, will lead to an erroneous phase difference. 
     Consequently correlation calculations are conducted between the image signal L c  and the image signals R a , R b , R c , R d  and R e  to obtain minimum correlation values C a (δ), C b (δ), C c (δ), C d (δ) and C e (δ) and phase differences δ a , δ b , δ c , δ d , and δ e . Among these minimum correlation values C a (δ)-C e (δ), the smallest one indicates the highest correspondence of the two images. Thus the image signals providing the smallest one in the minimum correlations are considered to be obtained from the same portion of the object, and the phase difference in such state can be used to minimize the error resulting from the hand vibration in the vertical direction. Consequently, the movement of the image signal to be employed in the correlation calculation, based on the above-explained analysis, constitutes the basic principle of the correcting method for reducing the error in the focus detection resulting from the hand vibration in the vertical direction. 
     FIG. 14 shows the plotting of the minimum values C a (δ), C b (δ), C c (δ), C d (δ) and C e (δ) of the correlations between the image signal L c  shown in FIG.  13 A and the image signals R a , R b , R c , R d  and R e  shown in FIG. 13B, wherein C b (δ) is smallest because the image signal R b  looks at a position the same as that of the image signal L c . Consequently, the error caused by the hand vibration in the vertical direction can be significantly reduced by employing the phase difference δ b  between the image signals L c  and R b  for calculating the defocus amount. 
     In the foregoing embodiment, the correction is made in the pitch of a pixel line of the CCD, but the actual image movement by the hand vibration does not occur in such pitch. Thus, for correction at a higher precision, image signals of a pitch, for example, of 0.5 lines may be generated by interpolation. 
     FIG. 15 shows a method of generating an image signal R ab  of a pitch of 0.5 lines from the image signals R a  and R b . 
     The pixel outputs R a1 -R an  of the image signal R a  and those R b1 -R bn  of the image signal R b  are respectively averaged as R ab1 -R abf  as represented by: 
     
       
           R   ab1 =( R   a1   +R   b1 )/2 
       
     
     and 
     
       
           R   abi =( R   ai   +R   bi )/2  i= 1 to n 
       
     
     For a finer interpolation with a pitch smaller than 0.5 lines, the above-mentioned averaging can be replaced by a weighted averaging. In this manner it is rendered possible to achieve finer correction in the vertical direction, thereby improving the accuracy of focus detection. 
     However, in an object showing small change in shape in the vertical direction (object with a low spatial frequency), the minimum value C(δ) of the correlation does not vary much by the position of the image signal. Also, the value of C(δ) varies to a certain extent by the noise component of the image signal. Consequently, in such case, the image signal at a position where the C(δ) becomes smallest may be of an erroneous position. 
     Such erroneous judgment may occur in case the difference between the correlation C c (δ) at a position corresponding to the image signal L c  and the smallest C b (δ) is smaller than a predetermined value. In such case, shown in FIG. 16, the image signal R c  is adopted, and such method cannot improve the precision but can prevent further deterioration of the precision by the adverse effect of the erroneous judgment. This method is particularly effective in case the object has a pattern f diagonal lines, because, in such case, the value C(δ) remains the same irrespective of the position, while the phase difference δ varies significantly depending on the position, whereby the possibility of erroneous judgment is rather high and such erroneous judgment leads to a significant deterioration in the precision of focus detection. Also, in such object, the error in the phase difference caused by the vertical movement behaves similarly as that caused by the horizontal movement, and can be corrected similarly by the method for reducing the error in the horizontal direction, to be explained later. 
     The vertical moving amount of the image signals L and R, caused by the hand vibration, becomes larger with the focal length of the photographing lens and with the interval of the storages of the image signals L and R. Based on these facts, the size of the calculating area, employed for the correction in the vertical direction, can be selected according to the focal length information of the photographing lens and the interval of storage of the image signals, determined from the image accumulating time and the driving time of the focus detecting light shading board, in order to dispense with the calculation in an unnecessarily wide area, thereby reducing the calculation time. Also, the amount of the image signal to be read can be reduced, thereby decreasing the required memory capacity and reducing the time required for signal read-out. 
     FIG. 17 is a flow chart of a “shift range calculation” subroutine for setting the calculation area, which is activated at a step  2402  through a step  2401 . 
     At first a step  2402  calculates the image accumulation time TC from the luminance information of the object, then a step  2403  calculates the driving time TD of the focus detecting light shading board  67 , and a next step  2404  calculates the interval TA of storage of the image signals as the sum of the above-mentioned accumulation time TC and the drive time TD. A step  2405  detects the focal length FA of the photographing zoom lens by reading a zoom encoder thereof. 
     A step  2406  discriminates whether the storage interval TA of the image signals is smaller than 10 msec, and, if smaller, the sequence proceeds to a step  2407 , but, if not, the sequence proceeds to a step  2412 . The step  2407  discriminates whether the focal length FA is smaller than 30 mm, and, if smaller, the sequence proceeds to a step  2410 , but, if not, the sequence proceeds to a step  2408 . The step  2410  enters 1 as LS in order to set the calculation area so as to include an upper line and a lower line. Then, a step  2808  discriminates whether FA is smaller than 60 mm, and, if smaller, the sequence proceeds to a step  2411 , but, if not the sequence proceeds to a step  2409 . The step  2411  enters 2 as LS in order to set the calculation area so as to include two upper lines and two lower lines, and the step  2409  enters 3 as LS. 
     Therefore, if TA is 10 msec and FA&lt;30 mm, there is selected LS=1. If 30 mm≦FA&lt;60 mm, there is selected LS=2, and, if FA≧60 mm, there is selected LS=3. Similarly, in steps  2412  to  2417 , if 10 msec≦TA≦20 msec and FA&lt;30 mm, there is selected LS=2. If 30 mm≦FA&lt;60 mm, there is selected LS=4, and, if FA≧60 mm, there is selected LS=5. Similarly, in steps  2418  to  2422 , if TA≧20 msec and FA&lt;30 mm, there is selected LS=3. If 30 mm≦FA&lt;60 mm, there is selected LS=5, and, if FA≧60 mm, there is selected LS=6. 
     The variation of the calculation area LS according to the focal length of the photographing lens and the storage interval of the image signals allows one to dispense with the unnecessary calculation and the unnecessary image signal read-out. The calculation area LS is naturally made smaller (with fewer number of lines) in case of a shorter focal length of the photographing lens or a shorter storage interval of the image signals, which reduces the influence of the hand vibration. 
     FIG. 18 is a chart showing the principle of eliminating the detection error, caused by the hand vibration, in an assumed case in which the object image moves from left to right at a constant speed because of the hand vibration. It is assumed that the image signals are stored at a constant interval. L 1 , R 2  and L 3  indicate the actually stored image signals, and R 1 , L 2  and R 3  indicate the other image signals if they are stored at the same timings. 
     As the object image moves by a distance δ m  in the period from the storage of the image signal L 1  to that of the image signal R 2 , the phase difference δ 12  between the image signals L 1  and R 2  becomes larger than the true phase difference δ 11  by δ m , so that: 
     
       
         δ 12 =δ 11 +δ m   (2) 
       
     
     Also the object image similarly moves by a distance δ m  in the period from the storage of the image signal R 2  to that of the image signal L 3 , so that the phase difference δ 23  between the image signals R 2  and L 3  becomes smaller than the true phase difference δ 11  by δ m , so that: 
     
       
         δ 23 =δ 11 −δ m   (3) 
       
     
     The true phase difference δ 11  can be determined from δ 12  and δ 23  according to the following equation:                    δ   =         (       δ   12     +     δ   23       )     /   2                =       (       δ   11     +     δ   m     +     δ   11     -     δ   m       )     /   2                                =     δ   11                     (   4   )                                
     The calculation of the phase difference δ in this manner allows the method to eliminate the error in focus detection, resulting from the object movement of a constant speed, caused by the hand vibration. Since the aforementioned condition stands in case the interval of image signal storages is sufficiently short, this method is very effective when the driving speed of the focus detecting light shading board  67  is fast and the image accumulation time is short. 
     In the foregoing description, it is assumed that the time required for driving the focus detecting light shading board  67  from the right pupil to the left pupil of the focus detecting diaphragm  65  is equal to that from the left pupil to the right pupil and that the image accumulation time of the CCD remains the same for all the three image signals, so that the storages of the image signals are realized with the same interval, but there may be a fluctuation in the driving time of the focus detecting light shading board  67  or in the image accumulation time. In the following there will be explained, therefore, a calculation method in case the interval of the storages of the image signals is varied. 
     FIG. 19 shows the relationship between the image signal storage time and the position of the object image, wherein T 12  is the interval of the storages of the image signals L 1  and R 2 , T 23  is the interval of the storages of the image signals R 2  and L 3 , and δ 12  and δ 23  are respectively the phase differences between L 1  and R 2  and between R 2  and L 3 . 
     The inclination of a solid line, representing the image signals L is indicated by: 
     
       
         (δ 23 −δ 12 )/( T   12   +T   23 ) 
       
     
     and the phase difference between L 1  and L 2  is represented by: 
       T   12 (δ 23 −δ 12 )/( T   12   +T   23 ) 
     Also the phase difference δ between L 2  and R 2  is represented by:              δ   =             T   12          (       δ   23     -     δ   12       )           T   12     +     T   23         +     δ   12       =           T   12     ·     δ   23       +       T   23     ·     δ   12             T   12     +     T   23                   (   5   )                                
     corresponding to a weighted average by the intervals T 12  and T 23 . 
     In the following there will be explained the function of the auto focusing device of the camera of the above-explained configuration, with reference to the appended flow charts. 
     FIG. 20 is a flow chart showing the main control sequence of the entire camera. When a power switch is turned on to activate the various circuits, the sequence proceeds from a step  001  to a step  002  to execute display on the view finder. More specifically, the image signal is stored by the CCD and displayed on the electronic view finder (EVF) after various signal processings. A next step  003  detects the state of a switch SW 1  which is to be turned on by the depression by a first stroke of a shutter release button, and, if it is off, the sequence proceeds to a step  004  to initialize a flag JF for detecting the state of the focusing. If the switch SW 1  is on, the sequence proceeds to a step  005 . 
     A step  005  calculates the luminance of the object, based on the output of the image signal stored by the CCD, the gain of the signal processing circuit, the image accumulation time of the CCD and the F-number of the photographing lens. A next step  006  calculates the appropriate shutter speed (exposure time) and aperture stop (F-value), based on the calculated object luminance, the photographing mode of the camera and the exposure correcting information, and stores thus calculated values in a predetermined memory area. At the shutter releasing operation to be explained later, the shutter and the diaphragm are controlled according to the data thus stored in the memory area. 
     A next step  007  discriminates the state of the flag JF for detecting the state of focusing. If JF=1, indicating that the focusing is already completed and the photographing lens is in the in-focus state, the sequence proceeds to a step  012 , but, if JF=0, indicating that the in-focus state is not yet reached, the sequence proceeds to a step  008  for effecting the focusing operation. 
     A step  008  is a “focus detection” subroutine for detecting the defocus amount of the photographing lens, which will be explained later in detail. A next step  009  compares the defocus amount detected in the step  008  with a permissible defocus amount determined from the diameter of a permissible fluctuation circle and the F-number of the photographing lens, and, if the former is smaller, indicating an in-focus state, the sequence proceeds to a step  010  to enter 1 in the flag JF thereby memorizing the in-focus state, and then the sequence proceeds to a step  012 . 
     On the other hand, if the step  009  identifies that the in-focus state has not been reached, a step  011  executes the focusing operation by driving the focusing lens group of the photographing lens so as to cancel the defocus amount detected in the foregoing step  008 , and then the sequence returns to the step  002 . 
     In this manner, the focusing operation is repeated until the in-focus state is reached or the switch SW 1  is turned off. 
     When the in-focus state is reached, a step  012  discriminates the state of a switch SW 2  which is to be turned on by the depression of the shutter release button by a second stroke, and, if it is off, the sequence returns to the step  002 , but, if it is on, the sequence proceeds to a step  013  for effecting the shutter releasing operation. 
     A step  013  controls the lens diaphragm to a diaphragm value calculated in the step  006 , and a next step  014  closes the shutter, resets the charges in the CCD and controls the shutter with the shutter time calculated in the step  006 , thereby executing the exposure operation of the CCD. A next step  015  drives the CCD to release the image signal and applies a predetermined signal processing thereon. A next step  016  executes a compression process, and the compressed image signal is stored in the memory medium in a step  017 . 
     A step  018  opens the shutter to restore the initial state, and then the sequence returns to the step  002 . 
     In the following there will be given an explanation of the “focus detection” subroutine, with reference to a flow chart shown in FIG.  29 . 
     When the focus detection subroutine is called in the step  008  in FIG. 20, the sequence proceeds, through a step  101 , to a step  102 . 
     A step  102  moves the focus detecting diaphragm  65 , which has been in a state shown in FIG. 7A, into the optical path of the photographing lens, and a next step  103  moves the focus detecting light shading board  67 , which has been in a state shown in FIG. 7A, into the optical path of the photographing lens. After the steps  102  and  103 , the focus detecting diaphragm  65  and the focus detecting light shading board  67  are shifted to a state shown in FIG. 7B, whereby the light beam transmitted by the left aperture  65   a  of the diaphragm  65  alone is focused on the CCD. 
     A step  104  executes the image accumulation in the CCD, and a next step  105  reads the image signal L 1  accumulated in the step  104  and stores it in a predetermined memory area. Different from the ordinary read-out operation, the read-out operation of the image signal L 1  is executed only in an area necessary for the focus detection and the image signal in the unnecessary areas is discarded at a high speed as explained in the foregoing, whereby the reduction in the image signal read-out time can be realized. 
     A next step  106  moves again the focus detecting light shading board  67  to a state shown in FIG. 7C, whereby the light beam transmitted by the right aperture  65   b  of the diaphragm  65  alone is focused on the CCD. A next step  107  executes the image accumulation as in the foregoing step  104 , and a step  108  reads and stores the image signal R 2  in a predetermined memory area, as in the foregoing step  105 . 
     A next step  109  moves again the focus detecting light shading board  67  to a state shown in FIG. 7B, whereby the light beam transmitted by the left aperture  65   a  of the diaphragm  65  alone is focused on the CCD. Then, a next step  110  executes the image accumulation, and a step  111  reads and stores the image signal L 3  in a predetermined memory area. 
     A step  112  retracts the focus detecting diaphragm  65 , and a step  113  retracts the focus detecting light shading board  67 . After the steps  112  and  113 , there is restored the initial state shown in FIG.  7 A. 
     A step  114  is a “defocus amount calculation” subroutine, which calculates the defocus amount of the photographing lens, based on the image signals L 1 , R 2  and L 3 , as will be explained later in more details. After the step  114 , the “focus detection” subroutine is terminated in a step  115 . 
     The image accumulating operations in the aforementioned steps  104 ,  107  and  110  may be executed with the same accumulation time and the same gain to equalize the output levels of the image signals, thereby increasing the correlation in the correlation calculation and obtaining a high and stable accuracy of the detection. 
     In the following there will be explained the details of the “defocus amount calculation” subroutine, with reference to a flow chart shown in FIG.  22 . 
     When the “defocus amount calculation” subroutine is called in the step  114  in FIG. 21, the sequence proceeds, through a step  201 , to a step  202 . 
     A step  202  calculates the phase difference δ 12  of the image signals L 1  and R 2 , then a step  203  similarly calculates the phase difference δ 23  of the image signals R 2  and L 3 , and a step  204  calculates the final phase difference δ not containing the detection error resulting from the constant-speed hand vibration component, according to the following calculation: 
     
       
         δ=(δ 12 +δ 23 )/2  (6) 
       
     
     A next step  205  calculates the defocus amount DF of the photographing lens, based on the phase difference δ determined in the foregoing step  204 , the sensitivity K of the focus detection system, determined by the focus detecting diaphragm and the photographing optical system, and the pixel pitch P of the CCD, according to the following formula: 
     
       
           DF=δ·K·P   (7) 
       
     
     After the step  205 , the present subroutine is terminated by a step  206 . 
     In the following there will be explained another case in which the interval of the image signal storages varies because of the variation of the driving time of the focus detecting light shading board  67 . The following explanation will be concentrated on the differences in the focus detecting method, as the configuration of the camera and the control sequence therefor are same as those explained in the foregoing. 
     FIG. 23 shows a flow chart of the “focus detection” subroutine. When it is called in the step  008  in FIG. 20, the sequence proceeds to a step  302  through a step  301 . 
     Steps  302  and  303  drive the focus detecting diaphragm  65  and the focus detecting light shading board  67  to a state shown in FIG. 7B as in the steps  102  and  103  in FIG. 20, thereby preparing for the focus detecting operation. 
     A next step  304  memorizes the starting time of the image accumulating operation for the image signal L 1 , by storing the count TIMER of a self-running timer of the system control unit in a RAM memory area T 1 . A next step  305  executes the accumulation of the image signal L 1 , and a step  306  executes the read-out thereof. 
     Then, a step  307  moves the light shading board  67  to a state shown in FIG. 7C, and a next step  308  memorizes the starting time of the accumulation of the image signal R 2 , by storing the timer count TIMER in a memory area T 2 . Then steps  309 ,  310  execute the accumulation and the read-out of the image signal R 2 . 
     Then, a step  311  moves the light shading board  67  to a state shown in FIG. 7B, and a next step  312  memorizes the starting time of the accumulation of the image signal L 3 , by storing the timer count TIMER in a memory area T 3 . Then steps  313 ,  314  execute the accumulation and the read-out of the image signal L 3 . 
     Steps  315  and  316  retract the focus detecting diaphragm  65  and the focus detecting light shading board  67  to a state shown in FIG. 7A, and a step  317  calculates the defocus amount. Then a step  318  terminates the present subroutine. 
     Now the calculation method for the defocus amount will be explained with reference to FIG. 24, which shows a “defocus amount calculation” subroutine to be used in case the interval of the image signal storages varies. When this subroutine is called in the step  317  in FIG. 23, the sequence proceeds to a step  402  through a step  401 . 
     A step  402  calculates the interval T 12  of the starts of accumulations of the image signals L 1  and R 2 , and a step  403  similarly calculates the interval T 23  of the starts of accumulations of the image signals R 2  and L 3 . Then a step  404  calculates the phase difference δ 12  of the image signals L 1  and R 2  by correlation calculation, and a step  405  similarly calculates the phase difference δ 23  of the image signals R 2  and L 3 . 
     A step  406  calculates the final phase difference δ by eliminating the detection error resulting from the constant-speed movement of the optical image based on the phase differences δ 12 , δ 23  and the intervals T 12 , T 23 , according to the following formula:              δ   =           T   23     ·     δ   12       +       T   12     ·     δ   23             T   12     +     T   23                 (   8   )                                
     A next step  407  calculates the defocus amount DF of the photographing lens, based on the phase difference δ determined in the foregoing step  406 , the sensitivity K of the focus detection system, and the pixel pitch P of the CCD. After this step, the present subroutine is terminated by a step  408 . 
     In case the interval of the start of the charge accumulating operations varies, there are required a memory for measuring the interval and a complex calculation, as explained above. Consequently it is desirable to maintain the interval to be constant, whereby the calculation process can be simplified and executed within a shorter time, with a reduced memory capacity. 
     More specifically, if the driving time for the focus detecting light shading board  67  fluctuates to a certain extent but if the upper limit for such driving time is known, the start of the charge accumulating operation can wait until the lapse of such upper limit time even after the driving of the light shading board is completed, as long as the driving time is sufficiently short. It is, therefore, possible to maintain a constant interval of the starts of the accumulating operations, in such case, by employing the same accumulating time, thereby easily simplifying the calculation process. Also, the presence of a timer for stabilizing the interval of the starts of the charge accumulating operations provides an advantage of dispensing with the means for detecting the completion of driving of the focus detecting light shading board  67 . 
     Second Embodiment 
     In a second embodiment of the present invention, four image signals are stored time-sequentially, in order to eliminate the detection error, caused by a constant-acceleration movement of the optical image resulting from the hand vibration or from the movement of the object. 
     FIG. 25 shows the principle of elimination, in the present embodiment, of the detection error resulting from the hand vibration, wherein the object image is assumed to move from left to right at a constant acceleration caused by the hand vibration, and it is also assumed that the image signals are stored at a constant interval. 
     L 1 , R 2 , L 3  and R 4  indicate the actually stored image signals, and R 1 , L 2 , R 3  and L 4  indicate the other image signals if they are stored at the same timings. 
     The object image moves by a distance δ m1  in the period from the storage of the image signal L 1  to that of the image signal R 2 , and the object image moves by a distance δ m2  in the period from the storage of the image signal R 2  to that of the image signal L 3 , with a relationship δ m1 &lt;δ m2 . Therefore, as in the foregoing second embodiment, the phase difference δ 1  determined from the image signals L 1 , R 2  and L 3  becomes smaller than the true phase difference δ 11  by δ d1 , as indicated by: 
     
       
         δ 1 =(δ 12 +δ 23 )/2=δ 11 −δ d1   (9) 
       
     
     This error δ d1  results from the variation in the speed, induced by the acceleration. 
     Also the phase difference δ 2  determined from the image signals R 2 , L 3  and R 4  as in the foregoing second embodiment, wherein δ m2 &lt;δ m3 , becomes larger than the true phase difference δ 11  by δ d2  as indicated by: 
      δ 2 =(δ 23 +δ 34 )/2=δ 11 +δ d2   (10) 
     In case of the movement with a constant acceleration, there stands a relation: 
     
       
         δ d1 =δ d2   (11) 
       
     
     the phase difference δ after the elimination of the detection errors δ d1 , δ d2  is given by:                    δ   =         (       δ   1     +     δ   2       )     /   2     =                  (       δ   11     -     δ   d1     +     δ   11     +     δ   d2       )     /   2                   =                δ   11                   (   12   )                              =       (       δ   12     +     2        δ   23       +     δ   34       )     /   4             (   13   )                                
     In this manner it is rendered possible to eliminate the detection errors caused by the constant-acceleration movement, utilizing the phase differences δ 12 , δ 23  and δ 34  determined from the four image signals L 1 , R 2 , L 3  and R 4 , thereby significantly improving the precision of the focus detection. 
     In the following there will be explained a calculation method for the phase difference δ in case the interval of image signal storages fluctuates. 
     For an interval T 12  from the storage of the image signal L 1  to that of the image signal R 2 , an interval T 23  from the storage of the image signal R 2  to that of the image signal L 3  and an interval T 34  from the storage of the image signal L 3  to that of the image signal R 4 , the phase differences δ 1 , δ 2  are given by the following formulas as in the second embodiment:                δ   1     =           T   23     ·     δ   12       +       T   12     ·     δ   23             T   12     +     T   23                 (   14   )                                              δ   2     =           T   34     ·     δ   23       +       T   23     ·     δ   34             T   23     +     T   34                 (   15   )                                 
     Also the phase difference δ is given by:              δ   =           (       T   23     +     T   34       )     ·     δ   1       +       (       T   12     +     T   23       )     ·     δ   2             T   12     -     2        T   23       +     T   34                 (   16   )               =                 T   23     ·     δ   12       +       T   12     ·     δ   23             T   12     +     T   23              (       T   23     +     T   34       )       +             T   34     ·     δ   23       +       T   23     ·     δ   34             T   23     +     T   34              (       T   12     +     T   23       )             T   12     +     2        T   23       +     T   34                 (   17   )                                
     Assuming that the time required for moving the focus detecting light shading board  67  is constant and that the charge accumulation time also remains constant within a single focus detecting operation, there stands a relation: 
     
       
           T   12   =T   34   (18) 
       
     
     so that the foregoing formula can be transformed as:              δ   =           T   23          (       δ   12     +     δ   23       )       +     2   ·     T   12     ·     δ   23           2        (       T   12     +     T   23       )                 (   19   )                                
     In this manner the calculation can be significantly simplified by giving a regularity to the intervals of the image signal storages. 
     The configuration of the camera and principal operations thereof in the present second embodiment are the same as those in the foregoing first embodiment and will not, therefore, be explained. In the following there will be given an explanation on the differences, from the foregoing first embodiment, in the focus detecting operation. 
     FIG. 26 shows a flow chart of a “focus detection” subroutine in case the interval of the image signal storages always varies. When this subroutine is called, the sequence proceeds, through a step  501 , to a step  502 . 
     Steps  502  and  503  drive the focus detecting diaphragm  65  and the focus detecting light shading board  67  to a state shown in FIG. 7B, thereby preparing for the focus detecting operation. 
     A next step  504  memorizes the starting time of the charge accumulating operation for the image signal L 1 , by storing the count TIMER of a self-running timer of the system control unit in a RAM memory area T 1 . A next step  505  executes the accumulation of the image signal L 1 , and a step  506  executes the read-out thereof. 
     Then a step  507  moves the light shading board  67  to a state shown in FIG.  7 C. 
     A next step  508  memorizes the starting time of the accumulation of the image signal R 2 , by storing the timer count TIMER in a memory area T 2 . Then a step  509  executes the accumulation of the image signal R 2 , and a step  510  executes the read-out of the image signal R 2 . 
     Then a step  511  moves the light shading board  67  to a state shown in FIG. 7B, then a step  512  memorizes the starting time of the accumulation of the image signal L 3 , by storing the timer count TIMER in a memory area T 3 , a step  513  executes the accumulation of the image signal L 3 , and a step  514  executes the read-out of the image signal L 3 . 
     Then a step  515  moves the light shading board  67  to a state shown in FIG. 7C, then a step  516  memorizes the starting time of the accumulation of the image signal R 4 , by storing the timer count TIMER in a memory area T 4 , a step  517  executes the accumulation of the image signal R 4 , and a step  518  executes the read-out of the image signal R 4 . 
     Then steps  519 ,  520  drive the focus detecting diaphragm  65  and the focus detecting light shading board  67  to a state shown in FIG. 7A. A next step  521  calculates the defocus amount, and a step  522  terminates the present subroutine. 
     Now, the calculation method for the defocus amount will be explained in detail with reference to FIG. 27, which shows a “defocus amount calculation” subroutine. When this subroutine is called in the step  521  in FIG. 26, the sequence proceeds to a step  602  through a step  601 . 
     A step  602  calculates the interval T 12  of the starts of accumulations of the image signals L 1  and R 2 , then a step  603  similarly calculates the interval T 23  of the starts of accumulations of the image signals R 2  and L 3 , and a step  604  calculates the interval T 34  of the starts of accumulations of the image signals L 3  and R 4 . 
     Then a step  605  calculates the phase difference δ 12  of the image signals L 1  and R 2  by correlation calculation, then a step  606  similarly calculates the phase difference δ 23  of the image signals R 2  and L 3 , and a step  607  calculates the phase difference δ 34  of the image signals L 3  and R 4 . 
     Steps  608 ,  609  calculate the phase differences δ d1 , δ d2  by eliminating the error of the constant-speed component, and a step  610  calculates the phase difference δ by eliminating the error of the constant-acceleration component. A next step  611  calculates the defocus amount DF of the photographing lens, based on the phase difference δ after the elimination of the error of the constant-acceleration component, the sensitivity K of the focus detection system, and the pixel pitch P of the CCD. Thereafter, the present subroutine is terminated by a step  612 . 
     In the foregoing, there has been explained the principle of focus detection, in case the interval of the starts of charge accumulating operations varies. 
     In the following there will be explained a case in which the driving time required for the focus detecting light shading board  67  is different in the different driving directions but is substantially the same in the same driving direction, namely in a situation where the intervals of the starts of the charge accumulating operations have certain regularity in such a manner that the interval from the start of accumulation of an image signal L to that of accumulation of an image signal R is constant within a single focus detecting operation and also the interval from the start of accumulation of an image signal R to that of accumulation of an image signal L is constant within a single focus detecting operation but, with a single focus detecting operation, the interval from the start of accumulation of an image signal L to that of accumulation of an image signal R is different from the interval from the start of accumulation of an image signal R to that of accumulation of an image signal L, as represented by: 
     
       
           T   12   =T   34   , T   12   ≠T   23   (20) 
       
     
     according to the form of the foregoing embodiments. 
     FIG. 28 is a flow chart showing the focus detecting subroutine in case the above-explained regularity is present in the intervals of the starts of the charge accumulating operations. This flow chart will not be explained further as it is the same as the flow chart shown in FIG. 26, except that the start time T 4  for the accumulation of the image signal R 4  is unnecessary and is not, therefore, measured. 
     When a “defocus amount calculation” subroutine is called in a step  720 , the sequence proceeds to a step  802  through a step  801 . 
     A step  802  calculates the interval T 12  of the starts of accumulations of the image signals L 1  and R 2 , and a step  803  calculates the interval T 23  of the starts of accumulations of the image signals R 2  and L 3 . 
     Then steps  804  to  806  calculate the phase differences δ 12 , δ 23  and δ 34  similarly to the steps  605  to  607  in FIG. 34, then a step  807  calculates the phase difference δ, and a step  808  calculates the defocus amount DF of the photographing lens, based on the phase difference δ, the sensitivity K of the focus detection system, and the pixel pitch P of the CCD. Thereafter, the present subroutine is terminated by a step  809 . 
     In the foregoing, there has been explained the focus detecting operation, in case the intervals of the starts of charge accumulations have a certain regularity. Such regularity in the intervals allows the device to simplify the calculation of the phase difference δ in comparison with the case of absence of such regularity, with advantages of a reduced memory capacity and a reduced calculation time. 
     In the following there will be explained the focus detecting operation in case the intervals of the starts of the charge accumulations are constant within a single focus detecting operation. 
     FIG. 30 is a flow chart of the focus detection subroutine in case the intervals of the starts of the charge accumulations are constant. This flow chart will not be explained in detail, since it is similar to that shown in FIG. 26, except that the accumulation starting times are not read because the measurement of the intervals is not necessary. In other parts, the driving of the focus detecting diaphragm  65  and the focus detecting light shading board  67  and the accumulation and read-out of the image signals L 1 , R 2 , L 3  and R 4  are executed in a similar manner as in the flow chart shown in FIG.  26 . 
     When a “defocus amount calculation” subroutine is called in a step  917 , the sequence proceeds, through a step  1001 , to a step  1002  to execute the “defocus amount calculation” subroutine. 
     Steps  1002  to  1004  calculate the phase differences δ 12 , δ 23  and δ 34  by correlation calculations similar to those in the steps  605  to  607  in FIG. 26, then a step  1005  calculates the phase difference δ by eliminating the error caused by the constant-acceleration movement, and a step  1006  calculates the defocus amount DF, based on the phase difference δ, the sensitivity K of the focus detection system, and the pixel pitch P of the CCD. Thereafter, the present subroutine is terminated by a step  1007 . 
     In the foregoing, there has been explained the focus detecting operation in case the intervals of the starts of charge accumulations are constant. The calculation of the phase difference δ not including the error resulting from the constant-acceleration movement can be further simplified, with the advantages of reducing the memory capacity and the calculation time and dispensing with the necessity of memorizing the starting times of the charge accumulations. 
     The intervals of the starts of image accumulations can be given a regularity or made constant by providing a stabilizing timer which starts the image accumulating operation after the lapse of a predetermined time even if the driving of the focus detecting light shading board  67  is completed within a short time. The expiration time of such stabilizing timer need only be longer than the maximum driving time. Also the presence of such stabilizing timer allows the device to dispense with detection means for detecting the completion of driving of the light shading board  67 . 
     Third Embodiment 
     In a third embodiment of the present invention, five image signals are time-sequentially stored, and the movement of the optical image, caused by the hand vibration or the movement of the object, is approximated by a second-order function thereby reducing the error in focus detection resulting from such movement. 
     FIG. 32 is a chart showing the error reducing method of the present third embodiment, wherein the object position y is represented in the ordinate, as a function of time t in the abscissa. The camera stores image signal L 1 , R 2 , L 3 , R 4  and L 5  in a time-sequential manner, while alternately switching the pupils. It is assumed that the movement of the object image in a period from the storage of the image signal L 1  to that of the image signal L 5  can be approximated by a second-order function. There is determined a second-order function: 
     
       
         
           y=At+Bt 
           2 
         
       
     
     passing through the positions of the image signals L 1 , L 3  and L 5 , then the positions of imaginary image functions L 2 , L 4  are determined by an interpolation utilizing such second-order function, and the average of the phase difference between L 2  and R 2  and that between L 4  and R 4  is adopted as the final target phase difference δ. 
     This calculation method will be explained in the following. 
     Assuming that the image signal L 1  has a position y 1  and a time t 1 , the image signal L 3  has y 3  and t 3  and the image signal L 5  has y 5  and t 5 , and for an initial condition of y 1 =0 and t 1 =0, the second-order function: 
     
       
         
           y=At+Bt 
           2 
         
       
     
     can be rewritten as: 
     
       
           y   3   =At   3   +Bt   3   2   (21) 
       
     
     
       
           y   5   =At   5   +Bt   5   2   (22) 
       
     
     From these two equations, the coefficients A and B can be determined as follows:              A   =           y   5          t   3   2       -       y   3          t   5   2             t   3            t   5          (       t   3     -     t   5       )                   (   23   )               B   =           y   3          t   5       -       y   5          t   3             t   3            t   5          (       t   3     -     t   5       )                   (   24   )                                
     Also the positions y 2 , y 4  of the imaginary image signals L 2 , L 4  can be given as follows:                y   2     =       At   2     +     Bt   2   2               (   25   )                          =           (         y   5          t   3   2       -       y   3          t   5   2         )          t   2           t   3            t   5          (       t   3     -     t   5       )           +         (         y   3          t   5       -       y   5          t   3         )          t   2   2           t   3            t   5          (       t   3     -     t   5       )                     (   26   )                 y   4     =       At   4     +     Bt   4   2               (   27   )                          =           (         y   5          t   3   2       -       y   3          t   5   2         )          t   4           t   3            t   5          (       t   3     -     t   5       )           +         (         y   3          t   5       -       y   5          t   3         )          t   4   2           t   3            t   5          (       t   3     -     t   5       )                     (   28   )                                
     Thus, the phase difference δ 22  between the imaginary image signal L 2  and the image signal R 2  is given by:                δ   22     =       y   2     -     y   2   ′               (   29   )                           =           (         y   5          t   3   2       -       y   3          t   5   2         )          t   2           t   3            t   5          (       t   3     -     t   5       )           +         (         y   3          t   5       -       y   5          t   3         )          t   2   2           t   3            t   5          (       t   3     -     t   5       )           -     y   2               (   30   )                                
     Also, the phase difference δ 44  between the imaginary image signal L 4  and the image signal R 4  is given by:                δ   44     =       y   4     -     y   4   ′               (   31   )                           =           (         y   5          t   3   2       -       y   3          t   5   2         )          t   4           t   3            t   5          (       t   3     -     t   5       )           +         (         y   3          t   5       -       y   5          t   3         )          t   4   2           t   3            t   5          (       t   3     -     t   5       )           -     y   4               (   32   )                                
     since 
     
       
           t   1 =0 , y   b   = 0     (33) 
       
     
     there can be determined: 
     
       
           t   2   =T   12   , y   2 ′=−δ 12   (34) 
       
     
     
       
           t   3   =T   12   +T   23   , y   3 =−δ 12 +δ 23   (35) 
       
     
     
       
           t   4   =T   12   +T   23   +T   34   , y   4 ′=−δ 12 +δ 23 −δ 34   (36) 
       
     
     
       
           t   5   =T   12   +T   23   +T   34   +T   45   , y   5 =−δ 12 +δ 23 −δ 34 +δ 45   (37) 
       
     
     wherein δ 12  indicates the phase difference between the image signals L 1  and R 2 , δ 23  indicates the phase difference between the image signals R 2  and L 3 , δ 34  indicates the phase difference between the image signals L 3  and R 4 , and δ 45  indicates the phase difference between the image signals R 4  and L 5 . 
     By substituting the relations (34) to (37) into the equations (23) and (24), there are obtained:              A   =                 (       -     δ   12       +     δ   23     -     δ   34     +     δ   45       )            (       T   12     +     T   23       )     2       -                 (       -     δ   12       +     δ   23       )            (       T   12     +     T   23     +     T   34     +     T   45       )     2                 (       T   12     +     T   23       )          (       T   12     +     T   23     +     T   34     +     T   45       )          (       -     T   34       -     T   45       )                 (   38   )               B   =                 (       -     δ   12       +     δ   23       )          (       T   12     +     T   23     +     T   34     +     T   45       )       -                 (       -     δ   12       +     δ   23     -     δ   34     +     δ   45       )          (       T   12     +     T   23       )                 (       T   12     +     T   23       )          (       T   12     +     T   23     +     T   34     +     T   45       )          (       -     T   34       -     T   45       )                 (   39   )                                
     Also δ 22  and δ 44  are given by: 
     
       
         δ 22   =A·T   12   +B·T   12   2 +δ 12   (40) 
       
     
     
       
         δ 44   =A· ( T   12   +T   23   +T   34 )+ B ·( T   12   +T   23   +T   34 ) 2 +δ 12 −δ 23 +δ 34   (41) 
       
     
     The finally desired phase difference δ, being the average of δ 22  and δ 44 , is represented by:              δ   =         δ   22     +     δ   44       2             (   42   )                        =               A   ·     (       2   ·     T   12       +     T   23     +     T   34       )       +     B   ·                   (       T   12   2     +       (       T   12     +     T   23     +     T   34       )     2       )     +     2   ·     δ   12       -     δ   23     +     δ   34             2             (   43   )                                
     The foregoing shows the calculation method for the phase difference δ, in case the intervals of the image signal storages constantly vary. 
     In case the time required for moving the focus detecting light shading board  67  in the same direction is always constant and the image accumulation times remains the same during a single focus detecting operation, there are obtained: 
     
       
           T   12   =T   34   , T   23   =T   45   (44) 
       
     
     By substituting these conditions into the equations (38), (39) and (40), there are obtained:              A   =           -   3     ·     δ   12       +     3   ·     δ   23       +     δ   34     -     δ   45         2        (       T   12     +     T   23       )                 (   45   )               B   =         δ   12     -     δ   23     -     δ   34     +     δ   45         2          (       T   12     +     T   23       )     2                 (   46   )               δ   =               A   ·     (       3   ·     T   12       +     T   23       )       +     B   ·                   (       T   12   2     +       (       2   ·     T   12       +     T   23       )     2       )     +     2   ·     δ   12       -     δ   23     +     δ   34             2             (   47   )                                
     The foregoing shows the calculation method for the phase difference δ in case the intervals of the image signal storages have regularity. 
     In the following, there will be explained the calculation method for the phase difference δ in case the intervals of the image signal storages remain constant within a single focus detecting operation. Under such condition, there are obtained: 
     
       
           T   12   =T   23   =T   34   =T   45   (48) 
       
     
     so that:              A   =           -   3     ·     δ   12       +     3   ·     δ   23       +     δ   34     -     δ   45         4   ·     T   12                 (   49   )               B   =         δ   12     -     δ   23     -     δ   34     +     δ   45         8   ·     T   12   2                 (   50   )               δ   =         4   ·   A   ·     T   12       +     10   ·   B   ·     T   12   2       +     2   ·     δ   12       -     δ   23     +     δ   34       2             (   51   )                        =               4        (         -   3     ·     δ   12       +     3   ·     δ   23       +     δ   34     -     δ   45       )       +                 5        (       δ   12     -     δ   23     -     δ   34     +     δ   45       )       +     4        (       2   ·     δ   12       -     δ   23     +     δ   34       )               8             (   52   )                        =         δ   12     +     3   ·     δ   23       +     3   ·     δ   34       +     δ   45       8             (   53   )                                
     Thus, the calculation formulas can be significantly simplified by giving regularity to the intervals of the image signal storages or selecting such intervals at the same value. 
     The configuration of the camera of the present third embodiment and the main control sequence will not be explained as they are same as those of the foregoing first embodiment, and, in the following, there will be explained the differences from the foregoing first and second embodiments in the focus detecting operation. 
     FIG. 33 is a flow chart showing the focus detection subroutine in case the intervals of the image signal storages vary constantly. When this subroutine is called, the sequence proceeds, through a step  1101 , to a step  1102 . 
     Steps  1102  and  1103  respectively drive the focus detecting diaphragm  65  and the focus detecting light shading board  67  to a state shown in FIG.  7 B. 
     A next step  1104  memorizes the starting time of the charge accumulating operation for the image signal L 1 , by storing the count TIMER of a self-running timer of the system control unit in a RAM memory area T 1 . A next step  1105  executes the accumulation of the image signal L 1 , and a step  1106  executes the read-out thereof. 
     Then, a step  1107  moves the light shading board  67  to the state shown in FIG. 7C. A next step  1108  memorizes the starting time of the accumulation of the image signal R 2 , by storing the timer count TIMER in a memory area T 2 . Then a step  1109  executes the accumulation of the image signal R 2 , and a step  1110  executes the read-out of the image signal R 2 . 
     Then, a step  1111  moves the light shading board  67  to a state shown in FIG. 7B, then a step  1112  memorizes the starting time of the accumulation of the image signal L 3 , by storing the timer count TIMER in a memory area T 3 , a step  1113  executes the accumulation of the image signal L 3 , and a step  1114  executes the read-out of the image signal L 3 . 
     Then, a step  1115  moves the light shading board  67  to a state shown in FIG. 7C, then a step  1116  memorizes the starting time of the accumulation of the image signal R 4 , by storing the timer count TIMER in a memory area T 4 , a step  1117  executes the accumulation of the image signal R 4 , and a step  1118  executes the read-out of the image signal R 4 . 
     Then, a step  1119  moves the light shading board  67  to the state shown in FIG. 7B, then a step  1120  memorizes the starting time of the accumulation of the image signal L 5 , by storing the timer count TIMER in a memory area T 5 , a step  1121  executes the accumulation of the image signal L 5 , and a step  1122  executes the read-out of the image signal L 5 . 
     Then, steps  1123 ,  1124  drive the focus detecting diaphragm  65  and the focus detecting light shading board  67  to a state shown in FIG. 7A. A next step  1125  calculates the defocus amount, and a step  1126  terminates the present subroutine. 
     In the following there will be explained the calculation method for the defocus amount, with reference to FIG. 34, which shows a “defocus amount calculation” subroutine. When this subroutine is called in the step  1125  in FIG. 33, the sequence proceeds to a step  1202  through a step  1201 . 
     Steps  1202  to  1205  calculate the intervals T 12 , T 23 , T 34 , T 45  of the starts of accumulations of the image signals, then steps  1206  to  1209  calculate the phase differences δ 12 , δ 23 , δ 34 , δ 45  of the image signals by correlation calculation. Then a step  1210  calculates the coefficient A of the first-order term of the second-order function, and a step  1211  calculates the coefficient B of the second-order term. A next step  1212  calculates the phase difference δ, by the approximation with the second-order function, and a step  1213  calculates the defocus amount DF of the photographing lens, based on the phase difference δ, the sensitivity K of the focus detection system, and the pixel pitch P of the CCD. Thereafter, the present subroutine is terminated by a step  1214 . 
     In the foregoing there has been explained the focus detecting operation in case the intervals of the start times of the image accumulations vary. 
     In the following there will be explained a case in which the driving time required for the focus detecting light shading board  67  is different in the different driving directions but is constant in the same driving direction, namely in a situation where the intervals of the starts of the image accumulating operations have certain regularity in such a manner the intervals T 12 , T 34  from the start of accumulation of an image signal L to that of accumulation of an image signal R are same and also the intervals T 23 , T 45  from the start of accumulation of an image signal R to that of accumulation of an image signal L are the same but the interval from the start of accumulation of an image signal L to that of accumulation of an image signal R is different from the interval from the start of accumulation of an image signal R to that of accumulation of an image signal L, as represented by: 
     
       
         T 12   =T   34   , T   23   =T   45   , T   12   ≠T   23   (54) 
       
     
     FIG. 35 is a flow chart showing the focus detecting subroutine in case the above-explained regularity is present in the intervals of the starts of the charge accumulating operations. This flow chart will not be explained further as it is the same as the flow chart shown in FIG. 33, except that the start times T 4 , T 5  for the accumulation of the image signals R 4 , R 5  are unnecessary and are not, therefore, measured. 
     When the “defocus amount calculation” subroutine is called in a step  1324 , the sequence proceeds to a step  1402  through a step  1401 . 
     A step  1402  calculates the interval T 12  of the starts of accumulations of the image signals L 1  and R 2 , and a step  1403  calculates the interval T 23  of the starts of accumulations of the image signals R 2  and L 3 . Then steps  1404  to  1407  calculate the phase differences δ 12 , δ 23 , δ 34  and δ 45  similarly to the steps  1206  to  1209  in FIG. 34, then a step  1408  calculates the coefficient A of the first-order term of the second-order function, and a step  1409  calculates the coefficient B of the second-order term. 
     A step  1410  calculates the phase difference δ determined by the approximation with the second-order function, and a step  1411  calculates the defocus amount DF of the photographing lens, based on the phase difference δ, the sensitivity K of the focus detection system, and the pixel pitch P of the CCD. Thereafter, the present subroutine is terminated by a step  1412 . 
     In the foregoing, there has been explained the focus detecting operation, in case the intervals of the starts of image accumulations have a certain regularity. Such regularity in the intervals allows simplification of the calculation of the phase difference δ in comparison with the case of absence of such regularity, with advantages of a reduced memory capacity and a reduced calculation time. 
     In the following there will be explained the focus detecting operation in case the intervals of the starts of the image accumulations are constant within a single focus detecting operation. 
     FIG. 37 is a flow chart of the focus detection subroutine in case the intervals of the starts of the charge accumulations are constant. This flow chart will not be explained in detail, since it is similar to that shown in FIG. 33, except that the accumulation starting times are not read because the measurement of the intervals is not necessary. In other parts, the driving of the focus detecting diaphragm  65  and the focus detecting light shading board  67  and the accumulation and read-out of the image signals L 1 , R 2 , L 3 , R 4  and L 5  are executed in a similar manner as in the flow chart shown in FIG.  33 . 
     When a “defocus amount calculation” subroutine is called in a step  1520 , the sequence proceeds, through a step  1601 , to a step  1602  to execute the “defocus amount calculation” subroutine. 
     Steps  1602  to  1605  calculate the phase differences δ 12 , δ 23 , δ 34  and δ 45  by correlation calculations similar to those in the steps  1206  to  1209  in FIG. 34, then a step  1607  calculates the phase difference δ corrected by approximation with the second-order function. Then a step  1607  calculates the defocus amount DF, based on the phase difference δ, the sensitivity K of the focus detection system, and the pixel pitch P of the CCD. Thereafter, the present subroutine is terminated by a step  1608 . 
     In the foregoing, there has been explained the focus detecting operation in case the intervals of the starts of image accumulations are constant. The calculation of the phase difference δ can be further simplified by the approximation with the second-order function, with the advantages of significantly reducing the memory capacity and the calculation time. The memory and the calculation time are further reduced since memorizing of the starting times of the image accumulations is unnecessary. 
     In case the driving time of the focus detecting light shading board  67  fluctuates by a certain level, there may be provided a stabilizing timer which starts the image accumulating operation after the lapse of a predetermined time even if the driving of the focus detecting light shading board  67  is completed within a short time, thereby easily giving regularity to the intervals of the start times of the image accumulations or maintaining such intervals constant. The expiration time of such stabilizing timer need only be longer than the maximum driving time. Also the presence of such a stabilizing timer allows the device to dispense with detection means for detecting the completion of driving of the light shading board  67 . 
     The foregoing embodiments utilize 3 to 5 image signals for calculating the phase difference, corrected for the object movement on the CCD, resulting from the movement of the object or from the hand vibration. For example, with a constant interval of the image accumulations, there may be utilized 6 image signals L 1 , R 2 , L 3 , R 4 , L 5  and R 6  to determine the phase differences δ 12 , δ 23 , δ 34 , δ 45  and δ 56  from which the phase difference δ can be calculated according to the following equation: 
     
       
         δ=(δ 12 +3δ 23 +4δ 34 +3δ 45 +δ 56 )/12  (55) 
       
     
     Similarly there may be employed 7 image signals L 1 , R 2 , L 3 , R 4 , L 5 , R 6  and L 7  to determine the phase differences δ 12 , δ 23 , δ 34 , δ 45 , δ 56  and δ 67 , from which the phase difference δ can be calculated according to the following equation: 
     
       
         δ=(δ 12 +3δ 23 +4δ 34 +4δ 45 +3δ 56 +δ 67 )/16  (56) 
       
     
     Fourth Embodiment 
     A fourth embodiment of the present invention selects the number of the image signals and the calculation method therefor, according to the focal length of the photographing lens and the intervals of the storages of the image signals. 
     FIG. 39 is a simulated chart showing the relationship between the interval of the image signal storages and the focal length of the photographing lens, for which the detection error caused by the hand vibration becomes equal to the permissible error (permissible focal length), in various countermeasures for reducing the influence of the hand vibration. 
     In the following it is assumed that the image signals L 1  and R 2  have a phase difference δ 12 , the image signals L 3  and R 2  have a phase difference δ 23 , the image signals L 3  and R 4  have a phase difference δ 34 , the image signals L 5  and R 4  have a phase difference δ 45 , and the intervals of the image signal storages are constant. 
     In the following: 
     Non-countermeasure: the phase difference δ to be used in the defocus amount calculation is δ=δ 12   
     Countermeasure 1: the phase difference δ to be used in the defocus amount calculation is δ=(δ 12 +δ 23 )/2 
     Countermeasure 2: the phase difference δ to be used in the defocus amount calculation is represented by: 
     
       
         δ=(δ 12 +2·δ 23 +δ 34 )/4 
       
     
     Countermeasure 3: the phase difference δ to be used in the defocus amount calculation is represented by: 
     
       
         δ=(δ 12 +3·δ 23 +3δ 34 +δ 45 )/8 
       
     
     Countermeasure 4: the phase difference δ to be used in the defocus amount calculation is represented by: 
     
       
         δ=(δ 12 +2δ 23 +2δ 34 +δ 45 )/6 
       
     
     and the results are simulated under the condition that the intervals of the image signal storages are constant. 
     This chart indicates that the permissible focal length becomes larger for a shorter interval of the storage of the image signals and for a larger number of the image signals employed in a single calculation. 
     However, an increase in the number of the image signals employed in the calculation improves the precision of detection but increases the time required for storing the image signal, thus prolonging the time required for focus detection and deteriorating the operability. 
     The present embodiment is, therefore, designed to minimize the time required for the focus detection by selecting a minimum necessary number of the image signals required for securing the necessary precision and a matching calculation method, according to the focal length of the actually employed photographing lens, the image accumulation time and the driving time of the focus detecting light shading board  67 . 
     Referring to FIG. 39, the countermeasure 4 is more effective, in comparison with the countermeasure 3, when the interval of the image signal storages is longer than 15 msec, but is less effective in case the interval does not exceed 15 msec. For this reason, the countermeasure 4 or 3 is employed respectively in case the storage interval is longer or shorter than 15 msec. 
     The configuration of the camera of the fourth embodiment and the principal functions thereof are the same as those of the foregoing first embodiment and will not, therefore, be explained, and, in the following there will be explained the focus detecting operation. 
     FIG. 40 shows a flow chart of a “focus detection” subroutine. When it is called, the sequence proceeds to a step  1702  through a step  1701 . 
     A step  1702  calculates the accumulation time TC and the gain at the focus detection, based on the object luminance and the aperture value of the focus detecting diaphragm  65  obtained in a light metering subroutine of the step  005  shown in FIG. 20, then a step  1703  reads the drive time TD for the focus detecting light shading board  67  for pupil switching, from the data stored in the ROM, and a step  1704  calculates the interval of the image signal storages as the sum of TC and TD. 
     A step  1705  detects the focal length FA of the photographing lens, by reading the zoom encoder of the zoom lens. A step  1706  calculates the number NA of the image signals required for a single focus detection, from the interval TA of the image signal storages and the focal length FA of the photographing lens. This calculation method will be explained later in more details. 
     Steps  1707  and  1708  respectively drive the focus detecting diaphragm  65  and the focus detecting light shading board  67  to the state shown in FIG.  7 B. 
     A step  1709  discriminates whether the number NA calculated in the foregoing step  1706  is 2, and, if NA=2, the sequence proceeds to a step  1713 , but, if not, the sequence proceeds to a step  1710 . The step  1710  discriminates if NA=3, and, if so, the sequence proceeds to a step  1714 , but, if not, the sequence proceeds to a step  1711 . The step  1711  discriminates whether NA=4, and, if so, the sequence proceeds to a step  1715 , but, if NA≠4, NA has to be 5 and the sequence proceeds to a step  1712 . 
     A step  1713  is an “image signal storage  2 ” subroutine for storage of two image signals; a step  1714  is an “image signal storage  3 ” subroutine for storage of three image signals; a step  1715  is an “image signal storage  2 ” subroutine for storage of two image signals; and a step  1712  is an “image signal storage  5 ” subroutine for storage of five two image signals. 
     The subroutines in the steps  1712  to  1715  will be explained later in more details. 
     After any of the subroutines in the steps  1712  to  1715 , the sequence proceeds to steps  1716 ,  1717  for respectively driving the focus detecting diaphragm  65  and the focus detecting light shading board  67  to the state shown in FIG.  7 A. 
     A next step  1718  calculates the defocus amount of the photographing lens by a “defocus amount calculation” subroutine, and a step  1719  terminates this subroutine. The details of the “defocus amount calculation” subroutine will be explained later. 
     FIGS. 41 and 42 are flow charts of the “image signal number NA calculating” subroutine. When this subroutine is called in the step  1706  shown in FIG. 40, the sequence proceeds to a step  1802  through a step  1801 . 
     A step  1802  discriminates whether the interval TA of the image signal storages is shorter than 5 msec, and, if TA&lt;5 msec, the sequence proceeds to a step  1803 , but, if not, the sequence proceeds to a step  1810 . The step  1810  discriminates whether the interval TA of the image signal storages is shorter than 10 msec, and, if TA&lt;10 msec, the sequence proceeds to a step  1811 , but, if not, the sequence proceeds to a step  1818 . The step  1818  discriminates whether the interval TA of the image signal storages is shorter than 15 msec, and, if TA&lt;15 msec, the sequence proceeds to a step  1819 , but, if not, the sequence proceeds to a step  1824 . The step  1824  discriminates whether the interval TA of the image signal storages is shorter than 20 msec, and, if TA&lt;20 msec, the sequence proceeds to a step  1825 , but, if not, the sequence proceeds to a step  1830 . 
     Assuming that the upper limit value of the accumulation time TC is 20 msec and the driving time of the focus detecting light shading board  67  is 3 msec, the sequence proceeds to the step  1803  in case of TA&lt;5 msec, to the step  1811  in case of 5 msec ≦TA&lt;10 msec, to the step  1819  in case of 10 msec≦TA&lt;15 msec, to the step  1825  in case of 15 msec≦TA&lt;20 msec, or to the step  1830  in case of 20 msec≦TA&lt;23 msec. 
     The step  1803  discriminates whether the focal length FA of the photographing lens is smaller than 11 mm, and, if FA&lt;11 mm, the sequence proceeds to a step  1807 , but, if not, the sequence proceeds to a step  1804 . The step  1804  discriminates whether FA is smaller than 83 mm, and, if FA&lt;83 mm, the sequence proceeds to a step  1808  for entering 3 as NA, but, if not, the sequence proceeds to a step  1805 . The step  1805  discriminates whether FA is smaller than 207 mm, and, if FA&lt;207 mm, the sequence proceeds to a step  1809  for entering 4 as NA, but, if not, the sequence proceeds to a step  1806  for entering 5 as NA. 
     In this manner the steps  1803  to  1809  select NA=2 if the focal length FA&lt;11 mm; NA=3 if 11 mm≦FA&lt;83 mm; NA=4 if 83 mm≦FA&lt;207 mm; or NA=5 if FA≧207 mm. 
     Similarly steps  1811  to  1817  select NA=2 if FA&lt;6 mm; NA=3 if 6 mm≦FA&lt;23 mm; NA=4 if 23 mm≦FA&lt;77 mm; or NA=5 if FA≧77 mm. 
     Similarly steps  1819  to  1823  select NA=3 if FA&lt;11 mm; NA=4 if 11 mm≦FA&lt;26 mm; or NA=5 if FA≧26 mm. 
     Similarly steps  1825  to  1829  select NA=3 if FA&lt;6 mm; NA=4 if 6 mm≦FA&lt;12 mm; or NA=5 if FA≧12 mm. Also steps  1830  to  1832  select NA=4 if FA&lt;10 mm; or NA=5 if FA≧10 mm. 
     After the number NA of the image signals to be employed in a single focus detection is determined from the interval TA of the image signal storages and the focal length FA of the photographing lens, a step  1833  terminates this subroutine. 
     FIG. 43 shows a flow chart of the “image signal storage  2 ” subroutine for storage two image signals L 1  and R 2 . When this subroutine is called in the step  1713  shown in FIG. 40, the sequence proceeds to a step  1902  through a step  1901 . 
     A step  1902  executes accumulation of the image signal L 1 , and a next step  1903  executes read-out thereof. A step  1904  drives the focus detecting light shading board  67  to the state shown in FIG. 7C, then a step  1905  executes accumulation of the image signal R 2 , a step  1906  executes read-out thereof, and a step  1907  terminates this subroutine. 
     FIG. 44 shows a flow chart of the “image signal storage  3 ” subroutine for storage three image signals L 1 , R 2  and L 3 . In the present case, it is assumed that the interval of the image signal storages is constant, namely the drive time TD of the focus detecting light shading board  67  and the accumulation time TC are constant. Consequently the measurement of the interval is unnecessary and is not, therefore, conducted. 
     When the “image signal storage  3 ” subroutine is called in the step  1714  shown in FIG. 40, the sequence proceeds to a step  2002  through a step  2001 . 
     A step  2002  executes accumulation of the image signal L 1 , and a next step  2003  executes read-out thereof. A step  2004  drives the focus detecting light shading board  67  to the state shown in FIG. 7C, and steps  2005  and  2006  execute accumulation and read-out of the image signal R 2 . A step  2007  drives again the focus detecting light shading board  67  to the state shown in FIG. 7B, and steps  2008  and  2009  execute accumulation and read-out of the image signal L 3 . Then a step  2010  terminates this subroutine. 
     FIG. 45 shows a flow chart of the “image signal storage  4 ” subroutine for storage of four image signals L 1 , R 2 , L 3  and R 4 . Also in the present case, it is assumed that the interval of the image signal storages is constant, so that the measurement of the interval is not conducted. When the “image signal storage  4 ” subroutine is called in the step  1715  shown in FIG. 40, the sequence proceeds to a step  2102  through a step  2101 . 
     Steps  2102  and  2103  execute accumulation and read-out of the image signal L 1 . A step  2104  drives the focus detecting light shading board  67  to the state shown in FIG. 7C, and steps  2105  and  2106  execute accumulation and read-out of the image signal R 2 . A step  2107  drives again the focus detecting light shading board  67  to the state shown in FIG. 7B, and steps  2108  and  2109  execute accumulation and read-out of the image signal L 3 . A step  2110  drives again the focus detecting light shading board  67  to the state shown in FIG. 7C, and steps  2111  and  2112  execute accumulation and read-out of the image signal R 4 . Then a step  2113  terminates this subroutine. 
     FIG. 46 shows a flow chart of the “image signal storage  5 ” subroutine for storage of five image signals L 1 , R 2 , L 3 , R 4  and L 5 . Also, in the present case, it is assumed that the interval of the image signal storages is constant, so that the measurement of the interval is not conducted. When the “image signal storage  5 ” subroutine is called in the step  1712  shown in FIG. 40, the sequence proceeds to a step  2202  through a step  2201 . 
     Steps  2202  and  2203  execute accumulation and read-out of the image signal L 1 . A step  2204  drives the focus detecting light shading board  67  to the state shown in FIG. 7C, and steps  2205  and  2206  execute accumulation and read-out of the image signal R 2 . A step  2207  drives again the focus detecting light shading board  67  to the state shown in FIG. 7B, and steps  2208  and  2209  execute accumulation and read-out of the image signal L 3 . A step  2210  drives again the focus detecting light shading board  67  to the state shown in FIG. 7C, and steps  2211  and  2212  execute accumulation and read-out of the image signal R 4 . A step  2213  drives again the focus detecting light shading board  67  to the state shown in FIG. 7B, and steps  2214  and  2215  execute accumulation and read-out of the image signal L 5 . Then a step  2216  terminates this subroutine. 
     FIG. 47 shows a “defocus amount calculation” subroutine, which switches the calculation formula according to the photographing conditions, in order to secure the necessary precision of detection. 
     When this subroutine is called in a step  1718  shown in FIG. 40, the sequence proceeds to a step  2302  through a step  2301 . 
     A step  2302  discriminates whether NA is 5, and, if NA=5, a step  2303  executes a calculation process utilizing 5 image signals, but, if NA≠5, the sequence proceeds to a step  2310 . The step  2310  discriminates whether NA is 4, and, if NA=4, a step  2311  executes a calculation process utilizing 4 image signals, but, if NA≠4, the sequence proceeds to a step  2315 . The step  2315  discriminates whether NA is 3, and, if NA=3, a step  2316  executes a calculation process utilizing 3 image signals, but, if NA≠3, the sequence proceeds to a step  2319  for executing a calculation process utilizing 2 image signals. 
     The step  2303  calculates the phase difference δ 12  of the image signals L 1  and R 2  by correlation calculation, then a step  2304  calculates the phase difference δ 23  of the image signals R 2  and L 3 , a step  2305  calculates the phase difference δ 34  of the image signals L 3  and R 4 , and a step  2306  calculates the phase difference δ 45  of the image signals R 4  and L 5 . 
     A next step  2307  discriminates whether the interval TA of the image signal storages is longer than 15 msec, and if TA&gt;15 msec, the sequence proceeds to a step  2308 , but, if not, the sequence proceeds to a step  2309 . This is because, as shown in FIG. 39, the permissible focal length is larger in the countermeasure 4 than in the countermeasure 3 in case the interval of the image signal storages is longer than 15 msec. Thus, in case the interval TA is longer than 15 msec, the sequence proceeds to the step  2308  to execute the calculation according to the countermeasure 4, but, in case the interval TA is shorter than 15 msec, the sequence proceeds to the step  2309  to execute the calculation according to the countermeasure 4, thereby calculating the phase difference δ to be employed in the calculation of the defocus amount. 
     A step  2311  calculates the phase difference δ 12  of the image signals L 1  and R 2 , then a step  2312  calculates the phase difference δ 23  of the image signals R 2  and L 3 , and a step  2313  calculates the phase difference δ 34  of the image signals L 3  and R 4 . A next step  2314  calculates the phase difference δ to be employed in the calculation of the defocus amount, according to the calculation process of the countermeasure 2. 
     A step  2316  calculates the phase difference δ 12  of the image signals L 1  and R 2 , and a step  2317  calculates the phase difference δ 23  of the image signals R 2  and L 3 . A next step  2318  calculates the phase difference δ to be employed in the calculation of the defocus amount, according to the calculation process of the countermeasure 1. 
     A step  2319  calculates the phase difference δ 12  of the image signals L 1  and R 2 , and a step  2320  enters δ 12  as the phase difference δ to be employed in the calculation of the defocus amount. 
     After the calculation of the phase difference δ by the step  2308 ,  2309 ,  2314 ,  2318  or  2320 , the sequence proceeds to a step  2321  for calculating the defocus amount DF according the phase difference δ, the sensitivity K of the focus detecting system and the pixel pitch P of the CCD, and a step  2322  terminates this subroutine. 
     The foregoing embodiment, capable of selecting the minimum number of the image signals and the calculation method capable of securing the necessary precision of detection, according to the photographing conditions (focal length of photographing lens, and interval of image signal storages), can dispense with the storage operation for the unnecessary image signals, thereby reducing the time required for focus detection and also reducing the electric power consumption. 
     The foregoing embodiments employ a CCD as the solid-state image pickup device, and such CCD is not limited to an area sensor but can also be composed of a line sensor. Also, the means for changing the shape of the pupil is composed of the focus detecting diaphragm  65 , but it may also be composed of a diaphragm utilizing physical property, such as a diaphragm composed of an LCD. 
     The entry and retraction of the focus detecting light shading board  67  into and from the photographing optical path are executed by a motor, but an actuator, such as a plunger, may also be used for this purpose. 
     In the foregoing there has been considered a camera with two pupil areas, but there can be provided three or more pupil areas. Also these pupil areas are so arranged that they are in the horizontal direction (together with the direction of lines of the CCD) when the camera is held in the normal position, but such arrangement is not restrictive and the pupil areas may be so arranged as to be in the vertical direction when the camera is held in the normal position. 
     Also, the moving amount detection range, for detecting the moving amount of the object in the vertical direction (perpendicular to the direction of arrangement of the pupil areas), is determined, as explained with reference to FIG. 17, in consideration of the focal length of the photographing lens, the interval of the image signal storages, the image signal accumulating time and the driving time of the focus detecting light shading board  67  (means for changing the pupil shape), but the effect of a certain level can be obtained by considering at least one of these information. Particularly, in case the entry and retraction of the focus detecting light shading board  67  into and from the photographing optical path are executed by a motor as in the foregoing embodiments, the driving time of the light shading board is almost constant and this information is therefore almost negligible. 
     Also, the number of the image signals to be employed for detecting the moving amount of the object in the vertical direction (perpendicular to the direction of arrangement of the pupil areas), is determined, as explained with reference to FIGS. 41 and 42, in consideration of the focal length of the photographing lens, the interval of the image signal storages, the image signal accumulating time and the driving time of the focus detecting light shading board  67  (means for changing the pupil shape), but the effect of a certain level can be obtained by considering at least one of these information. 
     Also, the foregoing embodiments intend to reduce the error in the focus detection, resulting from the hand vibration or from the movement of the object, for example by storage the image signals L 1 , R 2 , L 3 , R 4  and L 5  and utilizing the phase differences between the image signals L 1  and R 2 , between R 2  and L 3 , between L 3  and R 4  and between R 4  and L 5 , but such reduction in the error of focus detection can also be attained by such calculations plural times and taking the average or the weighted average of the obtained results. 
     Furthermore, though the present invention has been explained by its application to a digital camera, it is likewise applicable also to a video camera of a conventional camera utilizing a silver halide-based film.