Abstract:
An interchangeable lens video camera system which can stably focus on a main target object under any conditions of the object or the environment. The camera system includes a lens assembly detachably attached to a camera for photoelectrically converting incident light to sense an image and output an image signal. The camera system further includes a zoom lens and a focus lens controlled on the basis of an automatic focus evaluation value and data associated with exposure which are received from the camera while referring to a lens cam data unit which stores locus information of the zoom lens and the focus lens in advance. The interchangeable lens video camera system allows for the reduction of blurring and degradation of image quality.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a video camera system whose lens assemblies are interchangeable. 
     Conventionally, a so-called hill-climbing method is known as the method of an automatic focusing (AF) device used in video apparatuses such as video cameras. The method performs focusing by extracting a high-frequency component from a video signal obtained by an image sensing device such as a CCD and driving a taking lens such that the mountain-like characteristic curve of this high-frequency component is a maximum. 
     This automatic focusing method requires no special focusing optical members and has an advantage in that an object can be accurately focused regardless of whether the distance to the object is long or short. An example in which an automatic focusing method of the above sort is applied to an interchangeable lens video camera will be described below with reference to FIG.  24 . 
     Referring to FIG. 24, in a lens assembly  816 , a variable power lens  802  and a compensating lens  803  are connected by a mechanical cam (not shown). When a zooming operation is manually or electrically performed, the variable power lens  802  and the compensating lens  803  integrally move. 
     These variable power lens  802  and compensating lens  803  are called zoom lenses. 
     In this lens system, a front lens  801  which is closest to an object when the image is taken is a focus lens. The focus lens  801  moves in the direction of an optical axis to perform focusing. 
     An image of light transmitting through these lenses is formed on the image sensing surface of an image sensing device  804  of a camera  817 , photoelectrically converted into an electrical signal, and output as a video signal. 
     This video signal is sampled-and-held by a CDS/AGC circuit  805  constituted by a correlated double sampling circuit and an auto gain control circuit, amplified to a predetermined level, and converted into digital video data by an analog/digital (A/D) converter  806 . The digital video data is input to the process circuit (not shown) of the camera  817  and converted into a standard TV signal. The data is also input to a bandpass filter (to be referred to as a BPF hereinafter)  807 . 
     The BPF  807  extracts a high-frequency component which changes in accordance with the focus state from the video signal. A gate circuit  808  extracts only a video signal corresponding to a portion which is set as a focus detection area in a picture frame. A peak hold circuit  809  holds a peak of the video signal at an interval synchronizing with an integral multiple of a vertical sync signal, thereby generating a focus state evaluation value (to be referred to as an AF evaluation value hereinafter) representing the in-focus degree in the automatic focusing operation. 
     The AF evaluation value is fetched by an AF control microcomputer (to be referred to as a main body AF microcomputer hereinafter)  810  on the camera main body  817  side. The main body AF microcomputer  810  determines the focusing speed, i.e., a focus motor speed in accordance with the in-focus degree and the driving direction of the focus motor along which the AF evaluation value increases. The main body AF microcomputer  810  sends the speed and direction of the focus motor to a lens control microcomputer of the lens assembly  816 . 
     A lens microcomputer  811  controls a focus motor  813  through a motor driver  812  in accordance with an instruction from the main body AF microcomputer  810  to drive the focus lens  801  along the optical axis, thereby performing the focusing operation. 
     The main body AF microcomputer  810  also determines the driving directions and the driving speeds of the variable power lens  802  and the compensating lens  803 , which constitute zoom lenses, in accordance with the operation state of a zoom switch  818 . The main body AF microcomputer  810  transmits these driving directions and driving speeds to a zoom motor driver  814  of the lens assembly  816 . The lens assembly side calculates the driving information of a zoom motor  815  in accordance with the zoom speed and direction information sent from the camera main body side and drives the zoom motor  815  through the motor driver  814 , thereby driving the variable power lens  802  and the compensating lens  803 . 
     The camera main body  817  can be detached from the lens assembly  816  and connected to another lens assembly. This widens the sensing range. 
     In recent popular cameras integrated with video recorders for consumers having the above structure, the front lens is fixed while the focus lens is arranged behind the variable power lens, and the cam for mechanically connecting the compensating lens to the variable power lens is no longer used in order to miniaturize a camera and enable sensing at a close distance such as when an object is just in front of the lens. In these cameras, the locus of movement of the compensating lens is previously stored as lens cam data in a microcomputer, and the compensating lens is driven in accordance with this lens cam data. Also, a focusing operation is performed by using this compensating lens. Lenses of this type, i.e., so-called inner focus type (rear focus type) lenses have become most popular. 
     A zooming operation by such an inner focus type lens will be described below. 
     FIG. 25 is a view schematically showing the arrangement of a general inner focus type lens system. 
     Referring to FIG. 25, reference numeral  901  denotes a fixed first lens group;  902 , a second lens group for performing a zooming operation;  903 , an iris stop;  904 , a fixed third lens group;  905 , a fourth lens group (to be referred to as a focus lens hereinafter) having both a focusing function and a so-called compensator function of compensating for the movement of a focal plane caused by zooming; and  906 , an image sensing device. 
     As is well known, in the lens system as illustrated in FIG. 25, the focus lens  905  has both the compensating function and the focusing function. Accordingly, the position of the focus lens  905  for focusing an image on the image sensing surface of the image sensing device  906  changes in accordance with the object distance even at the same focal length. 
     FIG. 26 shows the result of continuous plotting of the position of the focus lens  905  for focusing an image on the image sensing surface while the distance between the focus lens  905  and the object is changed at different focal lengths. 
     During the zooming operation, one of the loci shown in FIG. 26 is selected in accordance with the object distance, and the focus lens  905  is moved to trace that focus. This allows a zooming operation free from a blur. 
     In a conventional front lens focus type lens system, compensating lens is provided independently of a variable power lens, and the variable power lens and the compensating lens are coupled by a mechanical cam ring. A manual zoom knob, for example, is formed on this cam, and the focal length is manually changed. Even if the knob is moved as fast as possible, the cam rotates to trace the movement of the knob, and the variable power lens and the compensating lens move along a cam groove for holding the cam. Therefore, no blur is caused by the above operation as long as the focus lens is focused on an object. 
     In controlling the inner focus type lens system, however, a plurality of pieces of locus information shown in FIG. 26 are stored in some format (the locus itself or a function of a lens position as a variable). In general, one of the loci is selected in accordance with the positions of the focus lens and the variable power lens, and a zooming operation is performed while tracing the selected locus. 
     FIG. 28 is a graph for explaining one invented locus tracing method. In FIG. 28, reference symbols Z 0 , Z 1 , Z 2 , . . . , Z 6  denote the positions of the variable power lens; and a 0 , a 1 , a 2 , . . . , a 6  and b 0 , b 1 , b 2 , . . . , b 6 , representative loci stored in the microcomputer. 
     Also, p 0 , p 1 , p 2 , . . . , p 6  denote loci calculated on the basis of the above two loci. This locus calculation is done by the following equation: 
     
       
           p ( n +1)=| p ( n )− a ( n )|/| b ( n )− a ( n ) |*| b ( n +1)− a ( n +1)|+ a ( n +1)   (1) 
       
     
     In equation (1), if, for example, the focus lens is at p 0  in FIG. 28, the ratio at which p 0  internally divides a line segment b 0 -a 0  is calculated, and the point at which a line segment b 1 -a 1  is internally divided by this ratio is given as p 1 . The focus lens moving speed for holding the in-focus state can be known from this positional difference (p 1 −p 0 ) and the time required for the variable power lens to move from Z 0  to Z 1 . 
     An operation when there is no such limitation that the stop position of the variable power lens must be on a boundary having the previously stored representative locus data will be described below. 
     FIG. 29 is a graph for explaining an interpolation method along the direction of the variable power lens position. FIG. 29 extracts a part of FIG. 28, and the position of the variable power lens is not limited to the previously stored positions, so that the variable power lens can take any arbitrary position. 
     In FIG. 29, the ordinate indicates the focus lens position, and the abscissa indicates the variable power lens position. The representative locus positions (the focus lens positions with respect to the variable power lens positions) stored in the microcomputer are represented as follows for various object distances with respect to variable power lens positions Z 0 , Z 1 , . . . , Zk−1, Zk, . . . , Zn: 
     a 0 , a 1 , . . . , ak−1, ak, . . . , an 
     b 0 , b 1 , . . . , bk−1, bk, . . . , bn 
     If the variable power lens position is Zx not on a zoom boundary and the focus lens position is Px, ax and bx are calculated as follows: 
     
       
           ax=ak −( Zk−Zx )*( ak−ak −1)/( Zk−Zk −1)  (2) 
       
     
     
       
           bx=bk −( Zk−Zx )*( bk−bk −1)/( Zk−Zk −1)  (3) 
       
     
     That is, ax and bx can be calculated by internally dividing data having the same object distance of the four stored representative locus data (ak, ak−1, bk, and bk−1 in FIG. 29) by the internal ratio obtained from the current variable power lens position and the two zoom boundary positions (e.g., Zk and Zk−1 in FIG. 29) on the two sides of the current variable power lens position. 
     In this case, pk and pk−1 can be calculated, as shown in equation (1), by internally dividing data having the same focal length of the four stored representative data (ak, ak−1, bk, and bk−1 in FIG. 29) by the internal ratio obtained from ax, px, and bx. 
     When zooming is performed from wide to telephoto, the focus lens moving speed for holding the in-focus state can be known from the positional difference between the focus position pk to be traced and the current focus position px and the time required for the variable power lens to move from Zx to Zk. 
     When zooming is performed from telephoto to wide, the focus lens moving speed for holding the in-focus state can be known from the positional difference between the focus position pk−1 to be traced and the current focus position px and the time required for the variable power lens to move from Zx to Zk−1. The locus tracing method as described above is invented. 
     When AF control is performed, it is necessary to trace the locus while maintaining the in-focus state. When the variable power lens moves in a direction from telephoto to wide, the diverged loci converge as can be seen from FIG.  26 . Therefore, the in-focus state can be maintained by the above locus tracing method. 
     In a direction from wide to telephoto, however, a locus which the focus lens in the point of convergence is to trace is unknown. Consequently, the in-focus state cannot be maintained by the locus tracing method as above. 
     FIGS. 30A and 30B are graphs for explaining one locus tracing method invented to solve the above problem. In each of FIGS. 30A and 30B, the abscissa indicates the position of a variable power lens. In FIG. 30A, the ordinate indicates the level of a high-frequency component (sharpness signal) of a video signal as an AF evaluation signal. In FIG. 30B, the ordinate indicates the position of a focus lens. 
     Assume that in FIG. 30B, a focusing cam locus is a locus  604  when a zooming operation is performed for a certain object. 
     Assume also that a tracing speed with respect to a locus indicated by lens cam data closer to a wide side than a zoom position  606  (Z 14 ) is positive (the focus lens is moved to the closest focusing distance), and that a tracing speed with respect to a locus indicated by lens cam data when the focus lens is moved in the direction of infinity on a telephoto side from the position  606  is negative. 
     When the focus lens traces the locus  604  while being kept in the in-focus state, the magnitude of the sharpness signal is as indicated by  601  in FIG.  30 A. It is generally known that a zoom lens kept in the in-focus state has an almost fixed sharpness signal level. 
     Assume that in FIG. 30B, a focus lens moving speed for tracing the focusing locus  604  during a zooming operation is Vf 0 . When an actual focus lens moving speed is Vf and a zooming operation is performed by increasing or decreasing Vf with respect to Vf 0  for tracing the locus  604 , the resulting locus is zigzagged as indicated by reference numeral  605 . 
     Consequently, the sharpness signal level so changes as to form peaks and valleys as indicated by reference numeral  603 . The magnitude of the level  603  is a maximum at positions where the loci  604  and  605  intersect (at even-numbered points of Z 0 , Z 1 , . . . , Z 16 ) and is a minimum at odd-numbered points where the moving direction vectors of the locus  605  are switched. 
     Reference numeral  602  denotes a minimum value of the level  603 . When a level TH 1  of the value  602  is set and the moving direction vectors of the locus  605  are switched every time the magnitude of the level  603  equals the level TH 1 , the focus lens moving direction after switching can be set in a direction in which the movement approaches the in-focus locus  604 . 
     That is, each time an image is blurred by the difference between the sharpness signal levels  601  and  602  (TH 1 ), the moving direction and speed of the focus lens are so controlled as to decrease the blur. Consequently, a zooming operation by which a degree (amount) of blur is suppressed can be performed. 
     The use of the above method is effective even in a zooming operation from wide to telephoto, as shown in FIG. 26, in which converged loci diverge. That is, even if the in-focus speed Vf 0  is unknown, the switching operation is repeated as indicated by  605  (in accordance with a change in the sharpness signal level) while the focus lens moving speed Vf is controlled with respect to the tracing speed (calculated by using p(n+1) obtained from equation (1)) explained in FIG.  28 . As a consequence, it is possible to select an in-focus cam locus by which the sharpness signal level is not decreased below the level  602  (TH 1 ), i.e., a predetermined amount or more of blur is not produced. 
     Assuming a positive compensating speed is Vf+ and a negative compensating speed is Vf−, the focus lens moving speed Vf is determined by 
     
       
           Vf=Vf   0 + Vf +  (4) 
       
     
     
       
           Vf   0 + vf −  (5) 
       
     
     In order that no deviation is produced when the tracing locus is selected by the above method of zooming operation, the compensating speeds Vf+ and Vf− are so determined that the internal angle of the two vectors of Vf obtained by equations (4) and (5) is divided into two equal parts by the direction vector of Vf 0 . 
     FIG. 31 is a table showing table data of locus information stored in the microcomputer. FIG. 31 shows in-focus lens position data A (n,v) which changes depending on the zoom lens position at different object distances. The object distance changes in the column direction of a variable n, and the zoom lens position (focal length) changes in the row direction of a variable v. 
     In this case, n=0 represents an object distance in the direction of infinity. As the variable n becomes large, the object distance changes to the closest focusing distance, and n=m represents an object distance of 1 cm. 
     On the other hand, v=0 represents a zoom lens position at the wide end. As the variable v becomes large, the focal length increases. Additionally, v=s represents a zoom lens position at the telephoto end. 
     Therefore, table data of one column corresponds to one cam locus. Locus information shown in FIG. 31 is prepared as zoom tracking data on the basis of an optical design value. With an actual lens, a locus corresponding to the design value cannot be obtained because of, e.g., an error in focal length of each lens group. 
     More specifically, to execute the locus tracing operation free from a blur as described above, the coordinate axes of an actual lens must match those of the table data. 
     An actual video camera performs an adjustment operation to determine the telephoto and wide ends of the variable power lens in data stored in advance. 
     A focusing adjustment method is conventionally performed, in which the operation stroke of a variable power lens from the telephoto end to the wide end is kept to be the design value. The in-focus position difference (balance) between the focus lens at the telephoto end and that at the wide end within an adjustment distance (e.g., ∞) is also set to be the design value, thereby defining the telephoto and wide ends. This adjustment method will be referred to as “fixed stroke adjustment”. 
     Another focusing adjustment method is known, in which the difference (balance) between the in-focus position of a focus lens at the telephoto end and that at the wide end is set to be a design value. In addition, a variable power lens position is obtained, at which the uppermost position of the focus lens at the middle (intermediate focal length) on the map as shown in FIGS. 26 and 27 and the moving amount of the focus lens from the telephoto end equal the design values, and defining the telephoto and wide ends of the variable power lens. This method will be referred to as “telephoto-middle tracking adjustment”. “Fixed stroke adjustment” and “telephoto-middle tracking adjustment” performed using a lens group having an error in a direction of increasing the position of the focus lens when the telephoto end position and the wide end position are set at not the design values but the intermediate focal length will be described below with reference to FIG.  27 . 
     In FIG. 27, the abscissa indicates the position of a variable power lens (i.e., a focal length), and the ordinate indicates the position of a focus lens. A locus Sb corresponds to a design locus. An actual focus lens exhibits a locus Sa. At this distance (e.g., ∞), the difference between the in-focus position of the focus lens at a telephoto end T and that at a wide end W is zero. 
     If the locus corresponds to the design value, and telephoto-middle tracking adjustment is to be performed, a point {circle around (1)} on the map is a start point for adjustment. The focus lens is lowered downward in FIG. 27 by a design moving amount A of the focus lens. This position is indicated by {circle around (2)}. From this state, the variable power lens is moved to obtain an in-focus position {circle around (5)} which is defined as a variable power lens position Tb at the telephoto end. 
     In this example, the difference between the in-focus position of the focus lens at the wide end and that at the telephoto end is zero, as described above. Therefore, the variable power lens is moved in a similar manner, and an in-focus position {circle around (6)} is defined as a variable power lens position Wb at the wide end. 
     When telephoto-middle tracking adjustment is to be performed for a lens having the locus Sa with an error, the focus lens is lowered from a start point {circle around (1)}′ for adjustment downward in FIG. 27 by the design value A, thereby obtaining a position {circle around (2)}′. 
     In a similar manner, the variable power lens is moved to an in-focus position. Consequently, a telephoto end Ta can be determined at a position {circle around (1)}, and a wide end Wa can be determined at a position {circle around (1)}. In this case, variations in focal length are generated. However, since the error of the locus Sa can be absorbed during zooming, a zooming operation free from a blur can be realized. 
     In fixed stroke adjustment, the stroke and the balance are adjusted to be predetermined values regardless of whether the locus Sb corresponding to the design value is exhibited or the locus Sa with an error is obtained. In both the cases, the telephoto end position is {circle around (1)}, and the wide end position is {circle around (1)}, so no variations in focal length are generated. However, the error of the locus Sa cannot be completely absorbed, and the locus Sb is traced during the zooming operation, resulting in a blur corresponding to the error. 
     However, the camera system as described above has a function of controlling automatic focusing in the camera main body, and its lens assemblies are interchangeable. When the response for automatic focusing or the like is determined to be optimum for a specific lens, another lens may not exhibit optimum performance. Hence, it is difficult to set optimum performance for all attachable lenses. 
     A technique of transmitting a focus signal necessary to execute focusing from the camera main body to the lens assembly while the function of controlling automatic focusing is assigned to the lens assembly side has been proposed. 
     In this case, a means for determining the size of an extraction area where a focus signal is extracted from a video signal is arranged on the lens assembly side such that the optimum response for automatic focusing for all connectable lenses can be determined. The size information is transferred to the main body side, and an appropriate size is set in correspondence with the focal length of each lens, thereby optimizing the focus signal level obtained from the camera main body. 
     Assume that the extraction area is fixed with respect to the frame size regardless of the types of lenses. For a wide angle lens, various objects are present in the area, so that the focus signal level tends to be high. For a high-luminance object, the signal obtained by an image sensing device is saturated, so focusing can hardly be appropriately performed. For a telephoto lens, an object image is enlarged, so that the focus signal level tends to be low. For a low-luminance object, resultant AF characteristics do not exhibit a desired result. 
     However, in a camera whose lenses are interchangeable and whose function of controlling automatic focusing as in the prior art is arranged in the lens assembly, the image sensing state on the main body side cannot be recognized by the automatic focusing means in the lens assembly, resulting in the following problems. 
     {circle around (1)} When a sensing operation is performed using an illumination equipment such as a home fluorescent lamp using discharge as a light source, discharge repeatedly occurs or stops depending on the frequency of the AC power supply of the light source, i.e., a so-called flicker is generated, so the output level of the image sensing signal sometimes periodically changes. However, the presence/absence of a flicker cannot be recognized on the lens assembly side. During focusing, it can hardly be determined whether the change in AF evaluation value is caused by the movement of the focusing lens or by a flicker, so the in-focus direction may be erroneously set. 
     When, to eliminate the influence of a flicker, the timing for driving the lens or fetching the AF evaluation value is always synchronized with the flicker period, the AF response becomes slow. 
     {circle around (2)} When a low-luminance object is to be taken, the image sensing signal is amplified by AGC. At this time, noise is also amplified, and many noise components are contained in the AF evaluation value. The amplification amount is unknown on the lens assembly side, so an erroneous operation is caused by the influence of noise in reactivation determination for a focusing operation or determination of a hill-climbing direction, often resulting in a blur. 
     {circle around (3)} In a sensing operation using a so-called program mode in which the iris stop, the shutter, AGC, and the like are automatically adjusted to realize effective sensing, and an optimum sensing state is realized, the exposure state changes depending on a change in mode. However, the change in mode cannot be recognized on the lens assembly side. 
     When the program mode changes, the AF evaluation value also changes to result in an erroneous AF operation. Particularly, when the mode changes in an in-focus state to forcibly open the iris stop for a photographic effect, the depth of field becomes small. However, when the field angle is wide, or when a high-luminance object is to be taken, an overexposure state is set, and the image sensing signal level may exceed the dynamic range of the image sensing device. At this time, the AF evaluation value does not change before and after the mode change. Therefore, an out-of-focus state is easily generated because of the decreased depth of field. 
     The lens assembly itself drives the iris stop in accordance with a control command from the camera main body, so that the iris stop state can be recognized. However, it cannot be determined whether the iris stop state optimizes exposure or aims a photographic effect. 
     If the iris stop state changes, the focusing operation may be reactivated to eliminate the above disadvantages. However, if the reactivation operation is performed every time the iris stop state changes, the AF operation is performed restlessly. {circle around (4)} In sensing using a so-called slow shutter, i.e., when the charge accumulation time in the image sensing device is prolonged to an integral multiple of the normal accumulation time, and an image sensing signal is intermittently read out, the focus signal sent from the camera main body is not updated for a time corresponding to the read period. However, the read period is unknown on the lens assembly side. Since the focus signal does not change for a predetermined time, erroneous determination of an in-focus state is made, or the hill-climbing direction is erroneously determined. {circle around (5)} In sensing using an enlargement function such as electronic zooming, the enlargement magnification and the position of enlargement in a picture frame cannot be recognized on the lens assembly side. In some cases, the focus signal extraction area becomes larger than the enlarged area. At this time, focusing is sometimes performed with respect to an object-outside-the monitor. 
     When the picture frame is enlarged, even a blur within the depth of field becomes visible. Therefore, a blur generated by a fine driving operation such as a wobbling operation which is performed to determine an in-focus direction becomes visible. 
     Additionally, the design value A necessary for the focusing operation must be set in correspondence with each interchangeable lens. 
     When a new lens assembly is developed, an old camera main body may not perform sufficient control. 
     A rear focus lens has a lot of complex cam loci, and the lens must accurately trace these loci. For this reason, the positions of the zoom lens and the focus lens must be accurately detected. For this purpose, a technique of performing feedback loop control using an encoder for position detection is available. However, a highly precise encoder is expensive and also requires a space. 
     A technique has been proposed instead in which the lens is driven by a stepping motor, and a moving amount of the stepping motor from a reference position is detected by counting supplied step pulses. According to this technique, the stepping motor is controlled by the microcomputer. Therefore, only by increasing/decreasing the counter value in the microcomputer, the function of an encoder can be realized, though it is open-loop control. 
     However, at the start time, an initialization operation must be performed to temporarily drive the lens to the reference position and reset the counter. If the power supply is turned off, and the microcomputer is reset, the contents in the counter are cleared, so that the control information including the absolute positions of the variable power lens and the compensating lens also returns to an initial value. 
     Therefore, even when a focusing operation is completed before the power supply is turned off, a deviation from the in-focus state is generated at the time of repowering. 
     In addition, when a zooming operation is performed in this state, a cam locus different from that before turning off the power supply is traced because the absolute lens position information changes. For this reason, the focusing operation must be performed again every time the power supply is turned on. 
     To manage the compensating lens and the variable power lens with a microcomputer, the positions of the focus lens and the variable power lens must always be recognized as absolute positions. Therefore, when the power supply is turned on, the initialization operation must be performed. When the power supply is turned off, a post-processing operation-must be-performed. 
     When the power supply is turned on, the focus lens or the variable power lens is moved to the infinite end or the wide end as a predetermined position (reset position), and the absolute position is recognized by the lens microcomputer such that the position matches P( 0 , 0 ) in FIG.  26 . 
     This is the initialization operation for the focus lens or the variable power lens. To perform the initialization operation at a high speed, the position of the focus lens or the variable power lens is stored in the microcomputer as post-processing at the time of turning off the power supply, and the focus lens or the variable power lens is moved close to the reset position. At the time of repowering, the initialization operation is performed, and then, the focus lens or the variable power lens is moved again to the position stored in the lens microcomputer. With this operation, sensing can be started in the same situation as before turning off the power supply. 
     However, when the power supply circuit of the lens is immediately turned on/off in a manner interlocked with the ON/OFF operation of the power which is supplied from the camera main body, when a video signal is output simultaneously with the ON operation of the power supply of the camera main body, or when the operating members arranged on the lens side are enabled simultaneously with the ON operation of the power supply of the camera main body, sensing is started before the lens initialization operation is completed, resulting in a blur in image or a degradation in image quality. In addition, if the power supply is turned off before lens post-processing is completed, control is confused at the time of repowering, and a long time is required to restore a normal state. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the above problems, and has as its object to provide an interchangeable lens video camera system which can stably focus on a main target object under any conditions of the object or the environment. 
     According to the present invention, there are provided a video camera system, and a camera and a lens assembly, which constitute the system, as will be described below. 
     That is, there is provided a camera detachably having a lens assembly including a lens for forming an image of an object and lens control means for controlling the lens, comprising: 
     image sensing means for converting the image of the object into an image signal and outputting the image signal; and 
     control means for generating information associated with an image sensing state of the object on the basis of the image signal obtained by the image sensing means and transmitting the information to a lens assembly. 
     There is also provided a lens assembly detachably attached to a camera having image sensing means for photoelectrically converting incident light to sense an image and outputting an image signal, comprising: 
     a lens for forming an image of an object; 
     memory means which stores locus information of the lens in advance; and 
     control means for receiving information associated with an image sensing state of the object from the camera and controlling the lens on the basis of the locus information and evaluation information representing a focus state of the image signal included in the information associated with the image sensing state. 
     Preferably, the memory means stores design position information of the variable power lens and the focus lens and the control means further comprises adjusting means for adjusting an operation of the focus lens on the basis of the position information to compensate for a movement of an in-focus point caused by the zooming operation of the variable power lens. 
     For example, the adjusting means-adjusts an operation stroke of the variable power lens to change a telephoto end position and a wide end position, and calculates a position of the variable power lens, at which an in-focus position of the focus lens and a moving amount of the focus lens from the telephoto end position equal those of the design position information, thereby changing the telephoto end position and the wide end position. 
     There is also provided a camera detachably having a lens assembly, comprising: 
     image sensing means for photoelectrically converting incident light to sense an image and transmitting an image signal to the lens assembly. 
     There is also provided a lens assembly detachably attached to a camera having image sensing means for sensing an image of an object and outputting an image signal, comprising: 
     a variable power lens for performing a zooming operation; 
     a focus lens for performing a focusing operation and compensating for a movement of an in-focus point caused by the zooming operation of the variable power lens; 
     memory means which stores position information of the variable power lens and the focus lens; 
     focus detection means for receiving the image signal and extracting, from the image signal, evaluation information which changes in accordance with a focus state; and 
     control means for controlling the variable power lens and the focus lens on the basis of the position information stored in the memory means and the evaluation information obtained by the focus detection means. 
     Preferably, the image signal is normalized in accordance with the focus state. 
     There is also provided a lens assembly detachably attached to a camera having image sensing means for sensing an image of an object and outputting an image signal, comprising: 
     a variable power lens for performing a zooming operation; 
     a focus lens for performing a focusing operation and compensating for a movement of an in-focus point caused by the zooming operation of the variable power lens; 
     first memory means for storing position information of the variable power lens and the focus lens; 
     control means for controlling the variable power lens and the focus lens; and 
     second memory means for storing current position information of the variable power lens and/or the focus lens, 
     wherein the control means determines, upon turning on a power supply of the lens assembly, whether the camera to which the lens assembly is mounted is the same as that in a previous operation, and if the camera is the same as that in the previous operation, the control means restores an operation state of the variable power lens and/or the focus lens at the time of turning off the power supply on the basis of the current position information stored in the memory means. 
     Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1 is a block diagram showing the arrangement of an interchangeable lens video camera system according to the first embodiment of the present invention; 
     FIG. 2 is a block diagram showing the detailed internal arrangement of an AF signal processing circuit  113  and an AE signal processing circuit  131  in a camera signal processing circuit  112  and a main body microcomputer  114  according to the first embodiment of the present invention; 
     FIGS. 3A and 3B are views for explaining electronic zooming and control of a distance measurement frame according to the electronic zooming operation in the first embodiment of the present invention; 
     FIGS. 4A to  4 C are views for explaining a photometry area setting operation in the first embodiment of the present invention; 
     FIGS. 5A to  5 C are graphs for explaining variations in image sensing signal level caused by a flicker in the first embodiment of the present invention; 
     FIG. 6 is a flow chart for explaining AF control by the microcomputer  114  in the camera main body in the first embodiment of the present invention; 
     FIG. 7 is a graph for explaining a wobbling operation for determining a focus lens driving direction in the AF operation of the first embodiment of the present invention; 
     FIGS. 8A to  8 C are graphs for explaining the wobbling operation considering a countermeasure to a flicker in the first embodiment of the present invention; 
     FIG. 9 is a flow chart for explaining processing performed when a program mode changes during the AF operation in the first embodiment of the present invention; 
     FIG. 10 is a block diagram showing the arrangement of an interchangeable lens video camera system according to the second embodiment of the present invention; 
     FIG. 11 is a block diagram showing the internal arrangement of an AF signal processing circuit on a camera main body side in the second embodiment of the present invention; 
     FIG. 12 is a view for explaining an operation and timing of extracting various focus evaluation values in the second embodiment of the present invention; 
     FIG. 13 is a flow chart for explaining an AF operation according to the second embodiment of the present invention; 
     FIG. 14 is a flow chart for explaining an in-focus state adjustment operation according to the second embodiment of the present invention; 
     FIG. 15 is a block diagram showing the arrangement of an interchangeable lens video camera system according to the third embodiment of the present invention; 
     FIG. 16 is a block diagram showing the internal arrangement of an AF signal processing circuit on a lens assembly side in the third embodiment of the present invention; 
     FIG. 17 is a block diagram showing the arrangement of an interchangeable lens video camera system as a modification of the third embodiment of the present invention; 
     FIG.  18 . is a block diagram showing the arrangement of an interchangeable lens video camera system according to the fourth embodiment of the present invention; 
     FIG. 19 is a flow chart for explaining an operation in the lens microcomputer of the interchangeable lens video camera according to the fourth embodiment of the present invention; 
     FIG. 20 is a flow chart for explaining an operation in the lens microcomputer of an interchangeable lens video camera as the first modification of the fourth embodiment of the present invention; 
     FIG. 21 is a block diagram showing the arrangement of an interchangeable lens video camera system as the second modification of the fourth embodiment of the present invention; 
     FIG. 22 is a block diagram showing the arrangement of an interchangeable lens video camera system according to the fifth embodiment of the present invention; 
     FIG. 23 is a time chart showing the sequence of turning on/off the power supply in the fifth embodiment of the present invention; 
     FIG. 24 is a block diagram showing the typical arrangement of a conventional interchangeable lens video camera; 
     FIG. 25 is a view showing the basic arrangement of an inner focus type lens system; 
     FIG. 26 is a graph showing focus lens moving loci (lens cam data) for correcting the position of a focal plane which is displaced in accordance with the zooming operation of a variable power lens to maintain an in-focus state; 
     FIG. 27 is a graph for explaining an adjustment operation for correcting an error between a can locus stored in the lens cam data and an actual lens position; 
     FIG. 28 is a graph for explaining calculation for interpolating a cam locus which is not stored from a plurality of cam locus information stored in the lens cam data; 
     FIG. 29 is a graph for explaining calculation for interpolating a cam locus which is not stored from a plurality of cam locus information stored in the lens cam data; 
     FIGS. 30A and 30B are graphs for explaining an algorithm for causing a focus lens to trace a locus; and 
     FIG. 31 is a table for explaining the internal structure of the lens cam data. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail in accordance with the accompanying drawings. 
     First Embodiment 
     FIG. 1 is a block diagram showing the arrangement of an embodiment of the present invention. A lens assembly  127  can be separated from a camera main body  128  to constitute a so-called interchangeable lens system. 
     Light from an object passes through a fixed first lens group  101 , a second lens group  102  (to be referred to as a variable power lens hereinafter) for performing a zooming operation, an iris stop  103 , a fixed third lens group  104 , and a fourth lens group (to be referred to as a focus lens hereinafter)  105  having both a focusing function and a compensator function of compensating for the movement of a focal plane caused by the zooming operation, forms an image on the image sensing surfaces of an image sensing device  106  such as a CCD for taking a red component in the three primary colors of red (R), green (G), and blue (B), an image sensing device  107  such as a CCD for taking a green component, and an image sensing device  108  such as a CCD for taking a blue component, and is photoelectrically converted. Image sensing signals corresponding to the respective color components, i.e., the red, green, and blue components are output. 
     The image sensing signals of the respective color components output from the image sensing devices are amplified to their optimum levels by amplifiers  109 ,  110 , and  111 , respectively, input to a camera signal processing circuit  112 , and converted into a standard TV signal. The image sensing signals are also input to an AWB (Auto White Balance) signal processing circuit  130 , an AE (Auto Exposure) signal processing circuit  131 , an AF (Auto Focus) signal processing circuit  113 , and a flicker detection circuit  115  in the camera signal processing circuit  112 . 
     Color difference signals SAWB generated by the AWB signal processing circuit  130  are supplied to an AWB/exposure control unit  135  in a microcomputer  114  for controlling the camera main body  128 . The amplifiers  109 ,  110 , and  111  are controlled such that the color difference signals become zero, so that white balance control is performed. At the same time, the control information is sent to a microcomputer  116  in the lens assembly  127  as color temperature information. 
     A photometry signal SAE generated by the AE signal processing circuit  131  is sent to the AWB/exposure control unit  135  and used for exposure control. At the same time, a photometry area control command for performing photometry only in a partial area of a frame is sent to the AE signal processing circuit  131 . 
     The AWB/exposure control unit  135  also performs exposure control. The AWB/exposure control unit  135  drives a CCD driving circuit  136  such that a photometry signal is set in a desired exposure state, and sends the accumulation times of the image sensing devices  106 ,  107 , and  108 , the gains of the amplifiers  109 ,  110 , and  111 , or an iris stop driving command to an iris stop control unit  120  of the lens microcomputer  116 , thereby performing feedback control of the amount of light passing through the iris stop  103 . 
     Control of the iris stop  103  is performed in the following manner. The iris stop control unit  120  sends a signal to an iris driver  124  in accordance with an iris stop driving command which is sent from the camera main body  128  to drive an IG (Iris Garvano) meter  123 . The state of the driven iris stop is detected by an encoder  129 . An output signal from the encoder is transferred to the AWB/exposure control unit  135  in the microcomputer  114  through the iris stop control unit  120 , thereby controlling the iris stop  103 . 
     The AWB/exposure control unit  135  also controls a program mode while placing an importance on exposure control. When a photographer operates a program mode switching unit  138  and selects a mode, the AWB/exposure control unit  135  controls parameters including an iris stop mechanism, an amplifier for AGC or the like, and an electronic shutter in accordance with the selected mode, thereby realizing an optimum exposure state for an object or sensing situation. 
     The AWB/exposure control unit  135  also controls a slow shutter function for taking a low-luminance object. The slow shutter function is a function of controlling, in accordance with the slow shutter speed selected by a slow shutter switching unit  139 , the CCD driving circuit  136  to prolong the charge accumulation times of the image sensing devices  106 ,  107 , and  108  and intermittently extracting an image sensing signal while synchronizing the read period with the charge accumulation time (for an electronic shutter, the accumulation time changes though the read period does not change). 
     The readout intermittent image sensing signal is received by a field memory  132  through the camera signal processing circuit  112 . The AWB/exposure control unit  135  controls a memory control/interpolation circuit  133  to transfer stored video information to the camera signal processing circuit  112 , thereby compensating for video information of fields which are omitted for the read period. 
     The AWB/exposure control unit  135  performs the above-described exposure control, program mode control, and slow shutter control and also sends electronic shutter information as exposure information, amplification factor information of AGC or the like, iris stop control information, selected program mode information, or read period information in slow shutter control to the lens microcomputer  116  in the lens assembly. 
     An AF evaluation value generated by the AF signal processing circuit  113  is transferred to the lens microcomputer  116  through the microcomputer  114 . The information of a distance measurement area in a frame, which is determined as a frame for measuring a distance to an object by a distance measurement frame size control unit  142  in the lens microcomputer  116  and sent to the AF signal processing circuit  113  through the main body microcomputer  114 . 
     The distance measurement frame size control unit  142  determines a distance measurement area having an optimum size to obtain AF performance in accordance with the focal length of the lens assembly  127  to be mounted. The reason why the size of the distance measurement frame is determined in the lens assembly has been described above. 
     The main body microcomputer  114  reads out the state of a zoom switch unit  137  (a unit for outputting a voltage corresponding to a resistance value which changes in accordance with the operation of a rotary operation member: when the output voltage is A/D-converted, the direction and amount of rotation of the operation member can be obtained as digital signals) and the state of an AF switch  141 , and sends the states of the switches to the lens microcomputer  116 . 
     Upon receiving the information from the main body microcomputer  114 , which represents that the AF switch  141  is OFF (manual focus mode), and the zoom switch unit  137  is depressed, the lens microcomputer  116  sends a control signal to a zoom motor driver  122  while referring to lens cam data  119  by operating an AF/computer zoom control program  117  such that the lens is driven in a direction corresponding to the depressed state of the switch, i.e., to the telephoto side or the wide side. With this operation, the variable power lens  102  is driven through a zoom motor  121  so that a zooming operation is performed. A control signal is also sent to a focus motor driver  126 . With this operation, the focus lens  105  is driven through a focus motor  125  so that a shift of the focal position caused by the zooming operation is compensated for. 
     When the AF switch  141  is ON (auto mode), and the zoom switch unit  137  is depressed, the in-focus state must be maintained for both the zooming operation and a change in object distance. The lens microcomputer  116  performs the zooming operation while referring to an AF evaluation value signal sent from the microcomputer  114  by operating the AF/computer zoom control program  117 , and maintaining a position where the AF evaluation value is maximized. 
     When the AF switch  141  is ON, and the zoom switch unit  137  is not depressed, the AF/computer zoom control program  117  sends a signal to the focus motor driver  126  to drive the focus lens  105  through the focus motor  125  that the AF evaluation value signal sent from the main body microcomputer  114  is maximized, thereby performing an automatic focusing operation. 
     A flicker signal SFL generated by the flicker detection circuit  115  in the camera signal processing circuit  112  in the camera main body  128  is sent to the main body microcomputer  114 . The presence/absence of a flicker is determined, and flicker presence/absence information is sent to the lens microcomputer  116 . 
     The flicker signal SFL will be described with reference to FIGS. 5A to  5 C. FIGS. 5A to  5 C are graphs showing a flicker observed when the frequency of an AC power supply is 50 Hz, and the output signal of the video camera is based on the NTSC standard, i.e., the vertical sync frequency is 60 Hz, and a change in output from an image sensing device. 
     FIG. 5A shows a change in absolute voltage of an AC power supply with respect to time. The AC power supply waveform is a sine wave. Therefore, for the absolute voltage, the waveform of the positive portion of the sine wave is repeated at a period of 100 Hz. 
     FIG. 5B shows the discharge repeating phenomenon of a fluorescent lamp. A fluorescent lamp starts discharge when the absolute value of the power supply voltage exceeds a predetermined value, i.e., VTH in FIG. 5A, and stops discharge when the absolute voltage is smaller than the value VTH. Therefore, the light-emitting amount changes at a period of 100 Hz, as shown in FIG.  5 B. 
     FIG. 5C shows a change in charge amount accumulated in an image sensing device every 1V (vertical scanning period). The image sensing device repeats charge accumulation every 1V, i.e., at a period of 60 Hz. 
     For a period V 1  shown in FIG. 5C, the fluorescent lamp performs the discharge operation almost twice. However, for a period V 2 , the discharge operation is performed one and ⅔ times. For a period V 3 , the discharge operation is performed one and ⅓ times. Since the light amount changes in this manner, the charge accumulation amount also changes as shown in FIG.  5 C. 
     The flicker detection circuit  115  shown in FIG. 1 may detect a change in image sensing signal level as shown in FIG. 5C or extract a component of 20 Hz corresponding to the light amount change period shown in FIG. 5C by using a bandpass filter or the like. 
     If a flicker signal is defined as the former, the main body microcomputer  114  detects the signal change period to determine the presence/absence of a flicker. If a flicker signal is defined as the latter, i.e., the level signal of a specific frequency component, the main body microcomputer  114  determines whether the level of the flicker signal is equal to or higher than a predetermined level, thereby determining the presence/absence of a flicker. 
     A video signal processed by the camera signal processing circuit  112  shown in FIG. 1 is stored in the field memory  132 . The memory control/interpolation circuit  133  controls the memory to read out the stored image, and outputs an enlargement signal obtained by enlarging the image along the vertical and horizontal directions while performing interpolation between the scanning lines and between pixels. 
     The enlargement signal read out from the field memory  132  under the control of the memory control/interpolation circuit  133  is subjected to color processing by the camera signal processing circuit  112  again and converted into a standard TV signal. 
     The memory control/interpolation circuit  133  performs control in accordance with the enlargement magnification information from an electronic zoom control unit  134  in the main body microcomputer  114 . The electronic zoom enlargement magnification information from the electronic zoom control unit  134  is sent to the lens microcomputer  116 . 
     The distance measurement frame size control unit  142  in the lens microcomputer  116  changes the size of the distance measurement frame on the basis of the enlargement magnification information sent from the main body microcomputer  114  (to be described later in detail with reference to FIG.  3 ). The size information is sent to the AF signal processing circuit  113  through the main body microcomputer  114 . 
     The AF signal processing circuit  113  and the AE signal processing circuit  131  will be described below in detail with reference to FIG.  2 . The image sensing device outputs of red (R), green (G), and blue (B), which are amplified to optimum levels by the amplifiers  109 ,  110 , and  111 , respectively, are converted into digital signals by A/D converters  206 ,  207 , and  208 , respectively, and sent to the camera signal processing circuit  112 . These signals are appropriately amplified by amplifiers  209 ,  210 , and  211 , respectively, and added by an adder  212  to generate a luminance signal S 5 . 
     The luminance signal S 5  is input to a bandpass filter  213 , and only a high-frequency component whose signal level changes in accordance with the focus state is extracted. Only the signal of scanning lines in a specific image area (area in the distance measurement frame) in a picture frame is gates by a gate circuit  214 , and the peak value is held by a peak hold circuit  215 . Upon completion of gate processing in one field, a peak value S 6  of a focus signal is transferred to the lens microcomputer  116  through the main body microcomputer  114 , so that the peak hold circuit  215  is initialized. 
     ON/OFF control of the gate circuit  214  is performed by a gate timing generation circuit  222  and a gate pulse control circuit  216 . On the basis of information S 10  from the distance measurement frame size control unit  142  in the lens microcomputer  116 , the main body microcomputer  114  determines an extraction start position CR 1  and an end position IR 1  of a distance measurement frame as indicated by reference numeral  303  in FIG.  3 A. ON/OFF control of the gate circuit is performed on the basis of information S 12 . 
     The luminance signal S 5  is also input to the AE signal processing circuit  131 . The luminance signal S 5  input to the AE signal processing circuit  131  is divided into an averaged overall light reading signal S 7   a  obtained by detection of the entire video area, as shown in FIG. 4A, and a center-weighted light reading signal S 7   b  obtained by detection of only the central portion of the video area, as shown in FIG.  4 B. These signals are weighted by weighting circuits  217  and  219 , respectively, added by an adder  221 , and sent as a photometry evaluation value S 8  to an exposure control arithmetic unit  231  in the AWB/exposure control unit  135 . Control of ON/OFF timing or weighting ratio of a gate circuit  218  for performing center-weighted light reading is performed on the basis of information from the exposure control arithmetic unit  231 . 
     An exposure control operation will be described below using an example of exposure control in a program mode. Control parameters for determining exposure include parameters of the iris stop mechanism, AGC, and the electronic shutter. Data with these parameters set in units of program modes in accordance with an object or sensing situation are prepared as look-up tables (LUTS) in the AWB/exposure control unit  135 . There are LUT  1  ( 227 ) corresponding to program mode  1 , LUT  2  ( 228 ) corresponding to program mode  2 , LUT  3  ( 229 ) corresponding to program mode  3 , and LUT  4  ( 230 ) corresponding to program mode  4 . 
     The AWB/exposure control unit  135  reads out the data of a look-up table corresponding to the program mode set by the program mode switch unit  138  into an LUT data control unit  226  and controls the parameters on the basis of the data, thereby enabling the program mode. 
     When the object moves at a high speed, an electronic shutter control unit  224  controls the image sensing device (CCD) driving circuit  136  such that the electronic shutter for controlling the accumulation time of an image sensing device is set at a high speed with priority. With this processing, a sensing mode excellent in dynamic resolution, i.e., a so-called “sport mode” can be set. 
     When an iris stop control unit  225  transfers an iris stop driving command to the lens microcomputer  116  to set the iris stop mechanism to the open side with priority, and exposure control is performed on the basis of the remaining parameters, the depth of field becomes small. With this processing, an effect of vignetting the background is obtained. That is, a so-called “portrait mode” suitable for taking a person or the like can be set. 
     In this manner, a sensing operation optimum for the sensing situation can be realized. 
     When the AE signal processing circuit  131  controls the photometry distribution by setting the detection area or detection position of the video signal for exposure control set by a gate pulse control circuit  220 , a more optimum sensing operation can be performed. 
     For example, so-called averaged overall light reading in which the entire video area is detected, as shown in FIG. 4A, and exposure control is performed such that the detection signal reaches a predetermined level, or center-weighted light reading in which only the central portion of the video area is detected, as shown in FIG. 4B, and exposure control is performed such that the detection signal reaches a predetermined level can be performed. 
     In the AE signal processing circuit  131 , the detection data of the overall light reading area and the detection data of the center-weighted light reading area are weighted by the weighting circuits  217  and  219 , respectively. Exposure control is performed on the basis of the detection data obtained by adding the above data at a predetermined ratio. With this processing, exposure control based on photometry which combines averaged overall light reading and center-weighted light reading can be performed. 
     When the weighting ratio is changed for each program mode in accordance with the object or sensing situation, more optimum exposure control can be performed using the advantages of the two photometry techniques. 
     For example, for an object illuminated with a spot light with a dark background, or for a backlighted object, weighting of center-weighted light reading is increased to adjust the ratio to averaged overall light reading. With this processing, proper exposure control can be performed for not only the main object but also an object such as the background. 
     The picture frame is divided, as shown in FIG. 4C, and video detection is performed in each area. The area of the detection data used for exposure control is limited, or weighting is changed in units of program modes in accordance with the object or sensing situation. With this processing, fine exposure control can be realized. 
     An example of automatic focusing control in a lens assembly will be described below with reference to FIG.  6 . The flow chart of FIG. 6 shows an algorithm for the automatic focusing operation of the AF/computer zoom control program  117 , which is performed when the lens microcomputer  116  in the lens assembly does not perform a zooming operation. 
     Referring to FIG. 6, AF control processing is started in step S 601 . In step S 602 , the above-described wobbling operation for determining a hill-climbing direction is performed. The wobbling operation will be described below with reference to FIG.  7 . 
     FIG. 7 is a graph showing a change in characteristic curve  701  of an AF evaluation value which is obtained when the focus lens is moved relative to a certain object from the infinity side to the closest focusing distance. The abscissa indicates the position of the focus lens, and the ordinate indicates an AF evaluation value level. 
     An in-focus point is indicated by reference numeral  702 , where the AF evaluation value level is maximized (an in-focus lens position is indicated by reference numeral  708 ). The focus lens position is controlled such that the AF evaluation value level is always maximized. 
     In the wobbling operation, the focus lens is finely vibrated, and it is determined from the variation in signal level whether the in-focus point is present in the direction of the closest focusing distance or on the direction of infinity with respect to the current focus lens position. 
     In the wobbling operation, the AF evaluation value is fetched while finely driving the focus lens, thereby determining whether the current state is an in-focus state or a blurred state (if there is a blur, it is determined whether the focus point deviates from the in-focus state in the direction of infinity or in the direction of the closest focusing distance). 
     For example, when the current focus position is on the infinity side with respect to the in-focus point (e.g., at a position indicated by reference numeral  709  in FIG.  7 ), a wobbling operation is executed to finely drive the lens from the direction of infinity (the focus lens position is moved as indicated by reference numeral  703 : the time axis is set from the upper side to the lower side with respect to the sheet surface). A change in AF evaluation value level observed at that time is indicated by reference numeral  704 . 
     When the focus lens position is on the closest focusing distance side with respect to the in-focus point (e.g., at a position indicated by reference numeral  710  in FIG.  7 ), the lens is finely driven as indicated by reference numeral  705 . A change in AF evaluation value level is indicated by reference numeral  706 . 
     The phase of the change in AF evaluation value level indicated by reference numeral  704  opposes that indicated by reference numeral  706 . By determining this phase, the side on which the focus lens is positioned with respect to the in-focus point, i.e., the direction to which the focus lens must be moved can be known. 
     When the lens is finely driven at the peak of the mountain-like characteristic curve  701  of the AF evaluation value ( 711 ), a resultant change in AF evaluation value level ( 712 ) has a small amplitude and a different shape, so that a blur or an in-focus state can be detected. 
     In the wobbling operation near the in-focus point, a blur is visible to the photographer depending on the driving amplitude amount (a in FIG. 7) of fine drive of the focus lens. Therefore, a minimum amplitude amount for obtaining a sufficient evaluation value must be set. 
     Near the base of the mountain-like characteristic curve  701 , even when the focus lens is finely driven, the amplitude of the AF evaluation value may not be sufficiently obtained in some cases, so the direction cannot be determined. Therefore, the lens driving amplitude is preferably set to be relatively large. 
     In an actual wobbling operation, instead of driving the lens along a sine wave, as indicated by reference numerals  703 ,  711 , and  705 , the focus lens at, e.g., the position  709  is driven by the distance ain the direction of infinity, and the AF evaluation value is fetched (the evaluation value level corresponds to a point  714 ). Thereafter, the lens is driven by  2   a  to the closest focusing distance indicated by reference numeral  715 , and an evaluation value is fetched at a position  715  (the level corresponds to a point.  716 ). The level difference is defined as a driving direction evaluation value. When the driving direction evaluation value has an absolute value amount larger than a noise amount, the hill-climbing direction is determined in accordance with the sign of the driving direction evaluation value. 
     With a wobbling operation near the in-focus point, i.e., at the position  702 , the level of the obtained driving direction evaluation value may be insufficient. However, since the differential amount between the evaluation value before the start of the wobbling operation and the AF evaluation value obtained during the wobbling operation can be detected, and the evaluation value level at this time is high, it can be determined whether the lens is positioned at the in-focus point (since the evaluation value level is high, the influence of the noise component is minimized, so that the above-described significant signal change amount can be made smaller than that at the base of the mountain). 
     Referring back to the flow chart of FIG. 6, it is determined in step S 603  from the result of the wobbling operation in step S 602  whether the current sensing state is an in-focus state or a blurred state. If it is determined that an in-focus state is set, the focus lens is stopped, and the flow advances to a reactivation monitor processing routine starting from step S 609 . 
     If it is determined in step S 603  that an out-of-focus state is set, a wobbling operation is performed in step S 604  to determine the direction of the in-focus point, and hill-climbing processing is executed in the direction of the determination result (step S 605 ). 
     In step S 606 , it is determined whether the peak of the in-focus point, i.e., the in-focus evaluation signal is passed. If NO in step S 606 , hill-climbing processing is continued. If YES in step S 606 , the focus lens is returned to the peak (steps S 607  and S 608 ). 
     During this hill-climbing operation, the hill-climbing speed is controlled in accordance with the shape of the mountain while always monitoring the shape (the lens is driven at a high speed near the base of the mountain, though the driving speed is gradually decreased toward the peak). 
     When the focus lens is returning to the peak, the object sometimes changes due to panning or the like. Therefore, when the focus lens arrives at the peak, the flow returns to step S 602  to determine whether the focus lens is properly present at the peak, i.e., in-focus point, so that the wobbling operation is performed again. 
     If it is determined in step S 603  that an in-focus state is set, the flow advances to the reactivation monitoring routine starting from step S 609 . In step S 609 , the AF evaluation value level in the in-focus state is stored. 
     In step S 610 , reactivation determination is performed. 
     This processing will be described in detail with reference to FIG.  7 . As shown in FIG. 7, assume that the focus lens is at the position  708 , and the AF evaluation level at that time is indicated by reference numeral  702 . This level  702  corresponds to the AF evaluation value level stored in step S 609 . 
     Assume that the evaluation value level is lowered from  702  to  707  due to a change in object or the like. Whether reactivation is to be executed is determined in the following manner. 
     When the evaluation value level changes from the level  702  by a reactivation determination threshold value β or more shown in FIG. 7, it is determined that a deviation from the in-focus state is generated, and reactivation is executed. If the change amount of the evaluation value is smaller than the reactivation determination threshold value β, it is determined that reactivation is not executed. 
     Referring back to the flow chart of FIG. 6, the determination result in step S 610  in FIG. 6 is determined in step S 611 . If reactivation is not executed, the focus lens is stopped (step S 612 ), and the flow returns to step S 610  to perform reactivation monitoring again. 
     If it is determined that reactivation is to be executed, the flow returns to step S 602 . The wobbling operation is performed again to determine the focus lens moving direction. By repeating these operations, the focus lens is operated such that the in-focus state is always maintained. 
     In the loop of the automatic focusing operation, the AF evaluation value is normally generated in synchronism with the vertical sync signal period. The AF control routine is also performed in synchronism with the vertical sync signal period accordingly. 
     The reason for this is that the latest focus signal information can be effectively used to increase the AF response. 
     The algorithm of the focusing operation by a specific lens has been described above. For other lenses, the degree of speed control, the wobbling amplitude amount, or parameters used for in-focus determination/reactivation determination can be optimized in accordance with the characteristics of the individual lenses. Consequently, under various conditions of the object or environment, a stable AF operation for a main target object can be realized. 
     A characteristic feature of the present invention, i.e., a technique of using sensing state information transferred from the main body microcomputer  114  to the lens assembly  127  side for AF control will be described below. 
     First, in accordance with flicker presence/absence information (if the camera main body has no flicker detection circuit, the presence/absence of a flicker can be recognized from color temperature information and electronic shutter information), the lens driving timing and the AF evaluation value receiving timing are changed to eliminate the influence of the flicker, thereby preventing an erroneous AF operation. 
     Processing of preventing an erroneous AF operation will be described with reference to FIGS. 8A to  8 C by using, as an example, a wobbling operation performed in step S 602  in FIG.  6 . FIG. 8A corresponds to FIG. 5C with its time axis extended and shows a periodical change in level caused by a flicker. 
     FIG. 8C shows a time change in focus position observed when a normal wobbling operation is repeated. As shown in FIG. 8C, in the wobbling operation, the focus lens is driven to the closest focusing distance at a predetermined amplitude. When the focus lens reaches a predetermined focus position  801 , driving is stopped. When the focus lens is set in a still state, charges are accumulated in an image sensing device for a period of 1V. For the next vertical sync signal period, the video signal accumulated for a period V 1  is read out from the image sensing device, thereby obtaining the AF evaluation value at the focus position  801 . 
     The focus lens is driven to a predetermined focus position  802  in the direction of infinity. Similarly, for a period V 4 , charges accumulated for a period V 3  are read out, thereby obtaining the AF evaluation value at the focus position  802 . 
     When the wobbling operation is performed as shown in FIG. 8C, and a flicker is present, the obtained AF evaluation value varies due to the influence of the flicker, so the direction of the in-focus point cannot be properly determined. 
     Only when a flicker is present, the wobbling operation period is synchronized with the flicker period, as shown in FIG.  8 B. By receiving the AF evaluation value used to determine the direction at timings of V 1 , V 4 , V 7 , V 10 , . . . , V 2 , V 5 , V 8 , V 11 , . . . , or V 3 , V 6 , V 9 , V 12  free from a change in light amount, the influence of a flicker is eliminated. 
     In FIG. 8B, the AF evaluation values for the periods V 2  and V 5  are received. However, the present invention is not limited to this. Determination may also be made using a combination V 1 +V 2 , or V 4 +V 5 . 
     In FIG. 8B, the wobbling operation period is represented by 3V. However, to eliminate the influence of a flicker, any period can be used as long as the period is an integral multiple of the period of the video signal output change caused by a flicker. 
     As described above, when a flicker is present, control as shown in FIG. 8B is performed. With this processing, proper direction determination is performed while eliminating the influence of the flicker. When no flicker is detected, the wobbling operation is completed as fast as possible, as shown in FIG. 8C, thereby improving the AF response. 
     Second, amplification factor information of AGC or the like is used. In the wobbling operation shown in FIG. 7, a driving direction evaluation value level higher than a noise level is valid as a condition for direction determination. Since the amplification amount of the noise component changes depending on the amplification factor of AGC, the driving direction evaluation value level to be neglected is also changed in accordance with the amplification factor, thereby preventing an erroneous AF operation. 
     Third, program mode information is used. When the program mode is changed during the hill-climbing operation (steps S 605  and S 606 ) or reactivation determination (steps S 610 , S 611 , and S 612 ) shown in FIG. 6, the exposure state also changes, and the AF evaluation value also changes accordingly, resulting in an erroneous operation. 
     As a means for solving this problem, when the program mode is changed, the flow returns to step S 602  in the flow chart of FIG.  6 . Processing from the wobbling operation is started again, thereby preventing driving in an erroneous direction for generating a blur. 
     FIG. 9 shows the improved part of the algorithm shown in the flow chart of FIG. 6. A detailed description thereof will be omitted. Steps S 1001 , S 1002 , S 1003 , and S 1004  are added to steps S 605  and S 610  in FIG.  6 . If a change in program mode is detected (step S 1001 ), processing waits for stabilization of the exposure state (steps S 1002  and S 1003 ). Thereafter, processing from the wobbling operation (step S 602 ) is started again. 
     In this series of processing, a wait time counter C in step S 1002  does not exceed a predetermined value CO of the wait time because the RAM (not shown) in the lens microcomputer  116  is cleared by the initialization operation of the lens microcomputer  116 . 
     Fourth, read period information for slow shutter control is used. In slow shutter control, the AF evaluation value cannot be obtained every 1V sync period. For example, when the slow shutter speed is {fraction (1/15)}, the AF evaluation value is obtained only every 4V sync period. If AF control as shown in FIG. 6 is performed assuming that the AF evaluation value is updated every V, an in-focus state is erroneously determined because no difference is present between the evaluation value levels as wobbling results even in an out-of-focus state. For this reason, the AF operation is completed in the out-of-focus state. 
     To prevent such an erroneous operation, assume that, in slow shutter control, the AF evaluation value is updated only at the read period. Mountain shape determination or the reactivation operation during the wobbling or hill-climbing operation is performed in synchronism with the read period, thereby preventing an erroneous operation. 
     Fifth, image enlargement magnification information of electronic zooming is used. This will be described with reference to FIGS. 3A and 3B. Referring to FIGS. 3A and 3B, reference numeral  301  denotes a sensing frame;  302 , a distance measurement area (distance measurement frame) for extracting an AF evaluation value which has been already described above. 
     Assume that an object  304  at a closer position and a distant object  305  are present in the sensing frame. In this embodiment, the AF evaluation value is defined as the peak value of a high-frequency component of a video signal in the distance measurement frame. Therefore, when the rear object  305  has a luminance higher than that of the object  304 , AF control is performed to make the lens focus on the object  305 . 
     Assume that the area  302  is enlarged by electronic zooming or the like, as shown in FIG.  3 B. At this time, the photographer looks at the screen of the monitor indicated by reference numeral  306  in which the object  304  is enlarged, as shown in FIG. 3B (FIG. 3B shows display on the monitor, though FIG. 3A shows the picture frame to be sensed by the image sensing device). 
     If the AF distance measurement frame is kept in the size indicated by reference numeral  302 , the lens may be focused on the object  305  which is not displayed on the monitor. In this case, the image on the monitor at which the photographer looks is kept blurred. 
     To eliminate this disadvantage, the size of the distance measurement frame is changed in accordance with the enlargement magnification information of electronic zooming. In this case, the distance measurement frame in electronic zooming is set as indicated by reference numeral, e.g.,  303 . 
     When the size of the distance measurement frame is changed in accordance with the image magnification information, AF control can be realized while preventing the focus state from shifting from the main object intended by the photographer. 
     In electronic zooming, since the object is enlarged, the AF evaluation value largely changes due to a change in object, the camera operation, or a camera shake. To stabilize the AF performance, the distance measurement frame is preferably set to be as large as possible (in this embodiment, the distance measurement frame is set to equal the enlarged frame size). 
     When an object is enlarged, even a blur within the depth of field sometimes becomes visible. It is preferable therefore to set the fine driving amount a of the focus lens in a wobbling operation or the like to be smaller than that in a normal mode. 
     Changing the size of the distance measurement frame in accordance with the selected program mode is useful for sensing reflecting the intention of the photographer. For example, a portrait mode aims at an effect of vignetting the background. The main object is present at the center of the picture frame and in a bust-up state to some extent. Therefore, it is preferable that only the central portion of the picture frame be set as a distance measurement frame smaller than that in a normal mode. 
     In a landscape mode for sensing a landscape, the upper portion of the picture frame is mainly occupied by the sky. The target object is often present on the lower side of the picture frame, so the focus point sometimes shifts due to the movement of the object. Therefore, a relatively large distance measurement frame is preferably set mainly on the lower side of the picture frame to prevent the object from leaving the distance measurement frame and entering into it because of a camera shake. 
     Typical examples of sensing state information which is transferred from the camera main body to the lens assembly have been described above. However, the present invention is not limited to the above examples. Any information can be transferred to the lens assembly as long as the information represents a sensing state such as a camera signal processing state, i.e., gamma correction or an aperture state. 
     An example in which the AF evaluation value is transferred from the camera main body to the lens assembly has been described above. The present invention can be applied to a system having a lens control means for focusing in the lens assembly. Instead of the AF evaluation value, a video signal itself may be transferred, and the AF evaluation value may be generated in the lens assembly having the AF signal processing circuit  113 . 
     Second Embodiment 
     The second embodiment of the present invention will be described below in detail with reference to the accompanying drawings. In this embodiment, AF control processing of adjusting a shift between an actual lens position and locus data as a design value stored in advance will be described. In addition, the operation of an AF signal processing circuit will be described in more detail than in the first embodiment. FIG. 10 is a block diagram showing the arrangement of an interchangeable lens video camera system according to the second embodiment of the present invention. 
     Referring to FIG. 10, a lens assembly  1127  is detachably attached to a camera main body  1128  to constitute a so-called interchangeable lens system. 
     Light from an object form images on the image sensing surfaces of image sensing devices  1106  to  1108 , e.g., CCDs, in the camera main body through a fixed first lens group  1101 , a second lens group  1102  (to be referred to as a variable power lens hereinafter) for performing a zooming operation, an iris stop  1103 , a fixed third lens group  1104 , and a fourth lens group  1105  (to be referred to as a focus lens hereinafter) in the lens assembly  1127 . The fourth lens  1105  has both a focusing function and a function of compensating for the movement of a focal plane caused by zooming. 
     The image sensing devices in the camera main body  1128  are provided for three primary colors, red (R), green (G), and blue (B), respectively, constituting a so-called three-sensor image sensing system. 
     Images of the three primary colors, red, green, and blue, are formed on the image sensing devices  1106 ,  1107 , and  1108 , respectively. 
     The images formed on the image sensing devices  1106 ,  1107 , and  1108  are photoelectrically converted and amplified to their respective optimum levels by amplifiers  1109 ,  1110 , and  1111 , respectively. These images are then input to a camera signal processing circuit  1112  and converted into a standard TV signal. This signal is output to, e.g., a video recorder (not shown) and also input to an AF signal processing circuit  1113 . 
     A focus evaluation value (AF evaluation value) is generated by the AF signal processing circuit  1113  in accordance with the focus state and is read out at a period which is an integral multiple of a vertical sync signal by a data read program  1115  of a microcomputer  1114  in the camera main body  1128 . The readout AF evaluation value is transferred to a lens microcomputer  1116  on the lens assembly  1127  side. 
     In the camera signal processing circuit  1112 , the levels of luminance signals of the output image sensing signals from the image sensing devices are detected and transferred to the lens microcomputer  1116  in the lens assembly  1127  through the microcomputer  1114 . On the basis of this luminance signal information, an iris driver  1124  is controlled, an IG meter  1123  is driven, and the iris stop  1103  is controlled. 
     The aperture value of the iris stop  1103  is detected by an encoder  1129 , supplied to the lens microcomputer  1116 , and used as depth-of-field information. 
     The microcomputer  1114  of the camera main body  1128  reads out the states of a zoom switch  1130  and an AF switch (when ON, an AF operation is performed; when OFF, a manual focus mode is set)  1131  and transmits the readout states of the switches to the lens microcomputer  1116 . With this operation, a motor driver  1122  is controlled in accordance with the operation state of the zoom switch  1130  to drive a zoom motor  1121  and drive the zoom lens  1102  in the direction operated by the zoom switch, thereby performing a zooming operation. 
     In the lens microcomputer  1116 , an AF program  1117  is operated, and the state of the AF switch  1131  and the AF evaluation value from the microcomputer  1114  are received. When the AF switch  1131  is ON, a motor control program  1118  is operated on the basis of the AF evaluation value to drive a focus motor  1125  through a focus motor driver  1126  and move the focus lens  1105  along the optical axis, thereby performing focusing. 
     An adjustment start switch  1135  is arranged on the camera main body  1128  side to start an adjustment operation on the lens assembly  1127  side to adjust the locus of lens cam data stored in advance and an actual lens driving locus. The operation state of this switch is also transmitted to the lens assembly  1127  side through the microcomputer  1114 . When the adjustment switch  1135  is ON, an adjustment program  1132  corresponding to an adjusting means in the lens microcomputer  1116  (to be described later) is operated to drive the lens group with reference to the AF evaluation value. With this operation, adjustment for the actual lens optical system and lens cam data  1120  as a memory means is performed. 
     An operation which changes depending on the states of the adjustment switch  1135 , the AF switch  1131 , and the zoom switch  1130  will be described below. 
     The adjustment switch  1135  may be arbitrarily manually turned on/off. However, when the adjustment switch  1135  is automatically turned on in a manner interlocked with an operation of mounting the lens assembly in the camera main body, adjustment can be automatically performed every time the lens unit attached to the camera main body is exchanged, resulting in an improvement in operability. In addition, an adjustment operation can be executed while the operator is not conscious of it. Therefore, the lens assembly can always be controlled with optimum characteristics. 
     The adjustment switch  1135  may be mechanically controlled upon coupling a mount, or controlled using software such as initial communication between the lens microcomputer and the main body microcomputer of the camera main body. 
     When the adjustment switch  1135  is OFF, a normal sensing operation as will be described below is executed on the basis of the states of the AF switch  1131  and the zoom switch  1130 . 
     When the AF switch  1131  is OFF (manual focus mode) and the zoom switch  1130  is depressed, a computer zoom program  1119  serving as a zooming control means is operated. In accordance with the information of the zoom direction operated by the zoom switch  1130  and the position information obtained by detecting the positions of the zoom lens and the focus lens from the respective motor driving amounts or by using an encoder, the computer zoom program  1119  specifies the in-focus locus along which the focus lens is to trace during a zooming operation and the trace direction. The computer zoom program  1119  reads out the specified locus and trace direction from the lens cam data  1120  and calculates the compensating speed and direction of the focus lens corresponding to the zooming operation. 
     The calculation result is sent as a signal to the zoom motor driver  1122  to drive the variable power lens  1102  through the zoom motor  1121 . The signal is also sent to the focus motor driver  1126  to drive the focus lens  1105  through the focus motor  1125 , thereby performing a zooming operation. As the lens cam data  1120 , locus data obtained by storing an in-focus can locus representing a change in in-focus position of the focus lens with respect to a change in position of the variable power lens, as shown in FIG. 26, for each object distance is stored in the ROM (not shown) in the lens microcomputer  1116 . With the operation of the computer zoom program  1119 , a lens cam locus to be traced by the focus lens is read out from the lens cam data  1120  in the zooming operation, thereby driving and controlling the focus lens. 
     When the AF switch  1131  is ON, and the zoom switch  1130  is depressed, it is necessary to hold the in-focus state even if the object moves. Accordingly, the computer zoom program  1119  operates to not only perform control on the basis of the lens cam data  1120  stored in the lens microcomputer  1116  as described above but also simultaneously refer to the AF evaluation value signal sent from the main body microcomputer  1114  on the camera side, thereby performing a zooming operation while holding the position at which the AF evaluation value is maximized. 
     That is, the driving speed and direction of the focus lens  1105  are calculated by adding the information of the compensating speed and direction of the focus lens obtained by the computer zoom program  1119  in accordance with the zooming operation to the information of the driving speed and direction of the focus lens based on the out-of-focus information output with the operation of the AF program  1117 . The driving speed and direction thus calculated are supplied to the focus motor driver  1126 . 
     When the AF switch  1131  is ON, and the zoom switch  1130  is not depressed, the AF program  1117  in the lens microcomputer  1116  receives the AF evaluation value transmitted from the microcomputer  1114 . On the basis of this AF evaluation value, the motor control program  1118  is operated. The focus motor  1125  is driven by the focus motor driver  1126 , and a signal is sent to the focus motor driver  1126  to drive the focus lens  1105  through the focus motor  1125  such that the AF evaluation value is maximized, thereby performing an automatic focusing operation. 
     The aperture value of the iris stop  1103  is detected by the encoder  1129 , supplied to the lens microcomputer  1116 , and used as the depth-of-field information to compensate for, e.g., the speed of the focus lens. 
     The AF signal processing circuit  1113  in the camera signal processing circuit  1112  will be described below with reference to FIG.  11 . The image sensing device outputs of red (R), green (G), and blue (B) are amplified to their respective optimum levels by the amplifiers  1109 ,  1110 , and  1111  and supplied to the AF signal processing circuit  1113 . The output signals are converted into digital signals by A/D converters  1206 ,  1207 , and  1208  and supplied to the camera signal processing circuit  1112 . At the same time, these digital signals are amplified to their respective optimum levels by amplifiers  1209 ,  1210 , and  1211  and added by an adder  1208 , generating an automatic focusing luminance signal S 15 . 
     The luminance signal S 15  is input to a gamma circuit  1213  and gamma-converted in accordance with a preset gamma curve, forming a signal S 16  whose low-luminance component is increased and high-luminance component is decreased. The gamma-converted signal S 16  is applied to a low-pass filter (to be referred to as an LPF hereinafter) with a high cut-off frequency, i.e., a TE-LPF  1214 , and to an FE-LPF  1215  which is an LPF with a low cut-off frequency. The TE-LPF  1214  and the FE-LPF  1215  extract low-frequency components on the basis of the respective filter characteristics determined by the main body microcomputer  1114  via a microcomputer interface  1253 . Consequently, the TE-LPF  1214  generates an output signal S 17 , and the FE-LPF  1215  generates an output signal S 18 . 
     A line E/O signal is generated by the microcomputer  1114  to identify whether the horizontal line is an even-numbered line or an odd-numbered line. On the basis of this signal, the signals S 17  and S 18  are selectively switched by a switch  1216  and applied to a high-pass filter (to be referred to as an HPF hereinafter)  1217 . 
     That is, the signal S 17  is supplied to the HPF  1217  when the horizontal line is an even-numbered line, and the signal S 18  is supplied to the HPF  1217  when the horizontal line is an odd-numbered line. 
     The HPF  1217  extracts only a high-frequency component in accordance with filter characteristics determined for even- and odd-numbered lines by the main body microcomputer  1114  via the microcomputer interface  1253 . An absolute value circuit  1218  obtains an absolute value of the extracted signal to generate a positive signal S 19 . That is, the signal S 19  alternately indicates the levels of high-frequency components extracted by the filter having different filter characteristics for even- and odd-numbered lines. Consequently, different frequency components can be obtained by scanning one picture frame. 
     In accordance with an instruction supplied by the microcomputer  1114  via the microcomputer interface  1253 , a frame generating circuit  1254  generates gate signals L, C, and R for forming focus control gate frames L, C, and R, respectively, at positions in the image sensing surface as shown in FIG.  12 . Timings at which various kinds of information are fetched in the AF signal processing circuit  1113  will be described below with reference to FIG. 12 which shows the layout of focus detection areas in the image sensing surface. 
     FIG. 12 is a view for explaining the operations and timings of extraction of various focus evaluation values in the second embodiment of the present invention. Referring to FIG. 12, the outside frame is an effective image sensing surface of the outputs from the image sensing devices  1106 ,  1107 , and  1108 . 
     Three divided inside frames are focus detection gate frames. The left frame L, the central frame C, and the right frame R are formed in accordance with the frame L generating gate signal, the frame C generating gate signal, and the frame R generating gate signal, respectively, from the frame generating circuit  1254 . 
     At the start positions of these frames L, C, and R, reset signals are output for the frames L, C, and R to generate initialization (reset) signals LR 1 , CR 1 , and RR 1 , respectively, thereby resetting integrating circuits  1232  to  1237  and peak hold circuits  1219  to  1221 ,  1225  to  1227 , and  1247  to  1249 . 
     Also, when the focus detection area consisting of the frames L, C, and R is completely scanned, a data transfer signal IR 1  is generated to transfer the integral values of the integrating circuits and the peak hold values of the peak hold circuits to their respective buffers. 
     Referring to FIG. 12, the scan of an even-numbered field is indicated by solid lines, and the scan of an odd-numbered field is indicated by dotted lines. In both the even- and odd-numbered fields, the TE-LPF output is selected on an even-numbered line, and the FE-LPF output is selected on an odd-numbered line. 
     An automatic focusing operation performed by the microcomputer by using a TE/FE peak evaluation value, a TE line peak integral evaluation value, an FE line peak integral evaluation value, a Y signal peak evaluation value, and a Max-Min evaluation value in each frame will be described below. Note that these evaluation values are transmitted to the microcomputer  1116  in the lens assembly and the microcomputer  1116  performs actual control. 
     The signal S 19  is supplied to the peak hold circuits  1225 ,  1226 , and  1227  for detecting signal peak values in the left, central, and right frames (to be referred to as frames L, C, and R hereinafter) in the image sensing surface. These peak hold circuits detect the peak values of high-frequency components in their respective frames. The signal S 19  is also supplied to a line peak hold circuit  1231  to detect the peak value of each horizontal line. 
     The peak hold circuit  1225  receives the output gate signal L for forming the frame L from the frame generating circuit  1254 , the signal S 19 , and the Line E/O signal. As shown in FIG. 12, the peak hold circuit  1225  is initialized in the upper left corner, i.e., LR 1 , which is the start position of the focusing frame L. The peak hold circuit  1225  holds a peak value of the signal S 19  in the frame L of either an even- or odd-numbered line designated by the microcomputer  1114  via the microcomputer interface  1253 . In the lower right corner IR 1 , i.e., when the entire focusing area is completely scanned, the peak hold value in the frame L is transferred to an area buffer  1228  to generate a TE/FE peak evaluation value. 
     Likewise, the peak hold circuit  1226  receives the output frame C signal from the frame generating circuit  1254 , the Line E/O signal, and the signal S 19 . As in FIG. 12, the peak hold circuit  1226  is initialized in the upper left corner, i.e., CR 1 , which is the start position of the focusing frame C. The peak hold circuit  1226  holds a peak value of the signal S 19  in the frame C of either an even- or odd-numbered line designated by the microcomputer  1114  via the microcomputer interface  1253 . In IR 1 , i.e., when the overall focusing area is completely scanned, the peak hold value in the frame C is transferred to an area buffer  1229  to generate a TE/FE peak evaluation value. 
     Similarly, the peak hold circuit  1227  receives the output frame R signal from the frame generating circuit  1254 , the Line E/O signal, and the signal S 19 . As in FIG. 12, the peak hold circuit  1227  is initialized in the upper left corner, i.e., RR 1 , which is the start position of the focusing frame R. The peak hold circuit  1227  holds a peak value of the signal S 19  in the frame R of either an even- or odd-numbered line designated by the microcomputer  1114  via the microcomputer interface  1253 . In IR 1 , i.e., when the overall focusing area is completely scanned, the peak hold value in the frame R is transferred to a buffer  1230  to generate a TE/FE peak evaluation value. 
     The line peak hold circuit  1231  receives the signal S 19  and the output gate signals for generating the frames L, C, and R from the frame generating circuit  1254 . The line peak hold circuit  1231  is initialized at the start point in the horizontal direction of each frame and holds a peak value of each line in the horizontal line of the signal S 19  in each frame. 
     The integrating circuits  1232 ,  1233 ,  1234 ,  1235 ,  1236 , and  1237  receive the output from the line peak hold circuit  1231  and the Line E/O signal which identifies whether the horizontal line is an even- or odd-numbered line. The integrating circuits  1232  and  1235  receive the frame L generating gate signal supplied from the frame generating circuit  1254 . The integrating circuits  1233  and  1236  receive the frame C generating gate signal supplied from the frame generating circuit  1254 . The integrating circuits  1234  and  1237  receive the frame R generating gate signal supplied from the frame generating circuit  1254 . 
     The integrating circuit  1232  is initialized in the upper left corner, i.e., LR 1 , which is the start position of the focusing frame L. The integrating circuit  1232  adds the output from the line peak hold circuit  1231  to an internal register immediately before the end of an even-numbered line in each frame. In IR 1 , the integrating circuit  1232  transfers the peak hold value to an area buffer  1238  to generate a TE line peak integral evaluation value. 
     The integrating circuit  1233  is initialized in the upper left corner, i.e., CR 1 , which is the start position of the focusing frame C. The integrating circuit  1233  adds the output from the line peak hold circuit  1231  to an internal register immediately before the end of an even-numbered line in each frame. In IR 1 , the integrating circuit  1233  transfers the peak hold value to a buffer  1239  to generate a TE line peak integral evaluation value. 
     The integrating circuit  1234  is initialized in the upper left corner, i.e., RR 1 , which is the start position of the focusing frame R. The integrating circuit  1234  adds the output from the line peak hold circuit  1231  to an internal register immediately before the end of an even-numbered line in each frame. In IR 1 , the integrating circuit  1234  transfers the peak hold value to an area buffer  1240  to generate a TE line peak integral evaluation value. 
     The integrating circuits  1235 ,  1236 , and  1237  perform the same operations as the integrating circuits  1232 ,  1233 , and  1234 , respectively, except that the integrating circuits  1235 ,  1236 , and  1237  perform addition of odd-numbered line data, instead of performing addition of even-numbered line data such as done by the integrating circuits  1232 ,  1233 , and  1234 . The integrating circuits  1235 ,  1236 , and  1237  transfer the results to area buffers  1241 ,  1242 , and  1243 , respectively, generating FE line peak integral evaluation values. 
     The signal S 17  is input to the peak hold circuits  1219 ,  1220 , and  1221 , a line maximum value hold circuit  1244 , and a line minimum value hold circuit  1245 . 
     The peak hold circuit  1219  receives the frame L generating gate signal supplied from the frame generating circuit  1254 . The peak hold circuit  1219  is initialized in the upper left corner, i.e., LR 1 , which is the start position of the frame L, and holds a peak value of the signal S 17  in each frame. In IR 1 , the peak hold circuit  1219  transfers the peak hold result to the buffer  1222  to generate a peak evaluation value of a luminance level (to be referred to as a Y signal hereinafter). 
     Analogously, the peak hold circuit  1220  receives the frame C generating gate signal supplied from the frame generating circuit  1254 . The peak hold circuit  1220  is initialized in the upper left corner, i.e., CR 1 , which is the start position of the frame C, and holds a peak value of the signal S 17  in each frame. In IR 1 , the peak hold circuit  1220  transfers the peak hold result to the buffer  1223  to generate a Y signal peak evaluation value. 
     Likewise, the peak hold circuit  1221  receives the frame R generating gate signal from the frame generating circuit  1254 . The peak hold circuit  1221  is initialized in the upper left corner, i.e., RR 1 , which is the start position of the frame R, and holds the peak value of the signal S 17  in each frame. In IR 1 , the peak hold circuit  1221  transfers the peak hold result to the buffer  1224  to generate a Y signal peak evaluation value. 
     The line maximum value hold circuit  1244  and the line minimum value hold circuit  1245  receive the frame L, C, and R generating gate signals supplied from the frame generating circuit  1254 . The line maximum value hold circuit  1244  and the line minimum value hold circuit  1245  are initialized at the start point in the horizontal direction in each frame and hold the maximum value and the minimum value, respectively, of the Y signal on one horizontal line of the signal S 17  in each frame. 
     The maximum and the minimum values of the Y signal held by the line maximum value hold circuit  1244  and the line minimum value hold circuit  1245  are input to a subtracter  1246 . The subtracter  1246  calculates a (maximum value-minimum value) signal, i.e., a signal S 20  which indicates the contrast, and inputs the signal to the peak hold circuits  1247 ,  1248 , and  1249 . 
     The peak hold circuit  1247  is applied with the frame L generating gate signal from the frame generating circuit  1254 . The peak hold circuit  1247  is initialized in the upper left corner, i.e., LR 1 , which is the start position of the frame L, and holds a peak value of the signal S 20  in each frame. In IR 1 ., the peak hold circuit  1247  transfers the peak hold result to a buffer  1250  to generate a Max-Min evaluation value. 
     Similarly, the peak hold circuit  1248  receives the frame C generating gate signal from the frame generating circuit  1254 . The peak hold circuit  1248  is initialized in the upper left corner, i.e., CR 1 , which is the start position of the frame C, and holds a peak value of the signal S 20  in each frame. In IR 1 , the peak hold circuit  1248  transfers the peak hold result to a buffer  1251  to generate a Max-Min evaluation value. 
     Analogously, the peak hold circuit  1249  is applied with the frame R generating gate signal from the frame generating circuit  1254 . The peak hold circuit  1249  is initialized in the upper left corner, i.e., RR 1 , which is the start position of the frame R, and holds a peak value of the signal S 20  in each frame. In IR 1 , the peak hold circuit  1249  transfers the peak hold result to a buffer  1252  to generate a Max-Min evaluation value. 
     In IR 1 , i.e., when the entire focusing area consisting of the frames L, C, and R is completely scanned, the data in these frames are transferred to the buffers  1222 ,  1223 ,  1224 ,  1228 ,  1229 ,  1230 ,  1238 ,  1239 ,  1240 ,  1241 ,  1242 ,  1243 ,  1250 ,  1251 , and  1252 . Simultaneously, the frame generating circuit  1254  sends an interrupt signal to the microcomputer  1114  and transfers the data, which are transferred to these buffers, to the microcomputer  1114 . 
     That is, upon receiving the interrupt signal, the microcomputer  1114  reads out the data (focus evaluation values) from the buffers  1222 ,  1223 ,  1224 ,  1228 ,  1229 ,  1230 ,  1238 ,  1239 ,  1240 ,  1241 ,  1242 ,  1243 ,  1250 ,  1251 , and  1252  via the microcomputer interface  1253  before the succeeding scan of the frames L, C, and R is completed and the data are transferred to these buffers. As will be described later, the microcomputer  1114  transfers the data to the microcomputer  1116  in synchronism with a vertical sync signal. 
     The microcomputer  1116  of the lens assembly  1127  detects the focus state by performing calculations by using these transferred focus evaluation values. The microcomputer  1116  then calculates, e.g., the driving speed and the driving direction of the focus motor  1125  and controls driving of the focus motor  1125 , thereby driving the focusing lens  1105 . 
     The characteristics and applications of the above evaluation values will be described below. 
     The TE/FE peak evaluation value represents an in-focus degree and is a peak hold value. Therefore, this evaluation value is less influenced by a camera shake and comparatively less depends upon the state of an object. For these reasons, this evaluation value is optimum for in-focus degree determination and reactivation determination. 
     The TE line peak integral evaluation value and the FE line peak integral evaluation value also represent an in-focus degree. However, these evaluation values are optimum for direction determination since they have little noise and are stable as a result of integration. 
     Of the above peak evaluation values and line peak integral evaluation values, each TE evaluation value is formed by extracting higher frequencies and hence is optimum as an evaluation value near the in-focus point. In contrast, each FE evaluation value is optimum when an image is largely blurred in a position very far from the in-focus point. Accordingly, by adding these signals or selectively switching the signals in accordance with the TE level, it is possible to perform AF over a wide dynamic range from the state in which an image is largely blurred to the vicinity of the in-focus point. 
     The Y signal peak evaluation value and the Max-Min evaluation value do not depend much upon the in-focus degree but upon the state of an object. Therefore, these evaluation values are optimum to check the change or movement of an object in order to reliably perform in-focus degree determination, reactivation determination, and direction determination. These values are also used in normalization for removing the influence of a change in brightness. 
     More specifically, the Y signal peak evaluation value is used to check whether the object is a high-luminance object or a low-luminance object. The Max-Min evaluation value is used to check whether the contrast is high or low. Furthermore, optimum AF control can be performed by predicting and compensating for the peak values, i.e., the magnitudes of peaks, on the characteristic curves of the TE/FE peak evaluation value, the TE line peak integral evaluation value, and the FE line peak integral evaluation value. 
     These evaluation values are transferred from the camera main body  1128  to the lens assembly  1127  and supplied to the lens microcomputer  1116  of the lens assembly  1127 , and the automatic focusing operation is performed. 
     The algorithm of an automatic focusing operation performed by the lens microcomputer  1116  of the lens assembly  1127  will be described below with reference to FIG.  13 . 
     When the processing is started, the microcomputer  1116  activates the AF operation in step S 1301 , and the flow advances to step S 1302 . In step S 1302 , the microcomputer  1116  checks the distance from the in-focus point by comparing the level of the TE or FE peak with a predetermined threshold, and performs speed control. 
     If the TE level is low, i.e., if the current focus point is far from the in-focus point and therefore the image is predicted to be largely blurred, the microcomputer  1116  performs hill-climbing control for the focus lens by controlling the direction of the lens by primarily using the FE line peak integral evaluation value. When the TE level rises to a certain degree near the peak of the characteristic curve, the microcomputer  1116  performs hill-climbing control for the focus lens by using the TE line peak integral evaluation value. In this way, the microcomputer  1116  so performs control that the in-focus point can be accurately detected. 
     If the lens comes close to the in-focus point, the flow advances to step S 1303 , and the microcomputer  1116  determines the peak of the characteristic curve by using the absolute value of the TE or FE peak evaluation value or a change in the TE line peak integral evaluation value. If the microcomputer  1116  determines that the level of the evaluation value is highest at the peak, i.e., the in-focus point, the microcomputer  1116  stops the focus lens in step S 1304  and advances to reactivation waiting in step S 1305 . 
     In reactivation waiting, if the microcomputer  1116  detects that the level of the TE or FE peak evaluation value decreases by a predetermined level or more from the peak value obtained when the in-focus point is detected, the microcomputer  1116  reactivates the operation in step S 1306 . 
     In the loop of the automatic focusing operation as described above, the speed of the focus lens is controlled by using the TE/FE peak. The level of the absolute value for determining the peak of the characteristic curve and the change in the TE line peak integral evaluation value are determined by predicting the height of the hill by checking the object by using the Y peak evaluation value or the Max-Min evaluation value. The AF operation can always be performed by repeating the above processing. 
     An in-focus state adjustment operation performed when the adjustment switch  1135  is ON will be described below. 
     FIG. 14 shows the algorithm of an adjustment operation performed by the adjustment program  1132  in the lens microcomputer  1116  when focusing as a characteristic feature of the present invention is performed. 
     Processing is started in step S 1401 . In step S 1402 , the position of the variable power lens on the optical axis is set at a zoom position (position {circle around (1)} in FIG. 27) corresponding to the vicinity of the peak of the locus of the focus lens. 
     In step S 1403 , the focus lens  1105  is moved by the focus motor to perform focusing. 
     The object distance is set as an adjustment distance (∞). An object, e.g., a chart is arranged for adjustment, and the adjustment distance is set. 
     In step S 1404 , it is checked whether the lens is at an in-focus position. The focus lens  1105  is moved until an in-focus state is set. 
     In actual focusing, the AF program  1117  shown in FIG. 27 is operated to detect the focus lens position at which the AF evaluation value is maximized, thereby detecting the in-focus position. 
     If it is confirmed in step S 1404  that the lens is at the in-focus position, the focus lens is lowered by A on the basis of the design value of this lens in step S 1405  (“lower” means that the lens is moved to the lower side of FIG.  27 : in fact, the zoom lens is extended to the object side or retracted to the image plane side depending on its zoom type). 
     In step S 1406 , the variable power lens in this state is driven to a telephoto side T. It is determined in step S 1407  whether an in-focus state is set. 
     When movement of the variable power lens is completed, and an in-focus state is detected at that position, the position of the variable power lens corresponds to the variable power lens position at the telephoto end. 
     In step S 1408 , the position of the zoom encoder in that state is stored in Vta as a value for defining the position of the telephoto end. 
     In step S 1409 , the focus lens is moved along the optical axis by a balance amount corresponding to the difference between the in-focus position of the focus lens at the telephoto end and that at the wide end within the adjustment distance. 
     However, if this balance is zero, as in FIG. 27, the focus lens need not be moved. Subsequently, in steps S 1410  and S 1411 , the variable power lens is moved as in determination of the telephoto end, thereby determining the reference position of the variable power lens on the wide side. 
     In step S 1412 , the position of the zoom encoder, which corresponds to the position of the variable power lens, is stored in Vwa as the position of the variable power lens with a focus reference value. In step S 1413 , this in-focus position is set as the reference position of the focus lens. In step S 1414 , the adjustment operation is ended. 
     As described above, Vwa, Vta, and the focus lens reference position, which are obtained with the adjustment operation in FIG.  14 ,.respectively correspond to the wide end v=0, the telephoto end v=s, and in-focus position data A 00  in the direction of infinity of the wide end (=A 0 s: in this embodiment, the balance difference between the wide end and the telephoto end is zero), as shown in FIG.  31 . 
     By matching the coordinate axes of the actual lens position with those of the locus table data as design data stored in advance, zooming free from a blur is realized. In addition, when the lens assembly  1127  incorporates the program shown in FIG. 14, an interchangeable lens system to which not only a front focus type lens but also lenses of various types including an inner focus type lens assembly are connectable can be realized. 
     Third Embodiment 
     The third embodiment will be described below. 
     FIG. 15 is a block diagram showing the arrangement of the third embodiment of the present invention. The basic arrangement is the same as that in the second embodiment. Hence, a detailed description thereof will be omitted (the same reference numerals as in the second embodiment denote the same elements in the third embodiment), and only different portions will be described below. In this embodiment, a camera main body  1127  transfers not an AF evaluation value but a video signal to a lens assembly  1127 . On the basis of an AF evaluation value generated in the lens assembly  1127 , a lens focusing or AF/zooming operation is realized. 
     Object images formed on image sensing devices  1106 ,  1107 , and  1108  are photoelectrically converted and amplified to their respective optimum levels by amplifiers  1209 ,  1210 , and  1211 , input to a camera signal processing circuit  1112 , and converted into a standard TV signal. At the same time, a video signal S 13  obtained by mixing R, G, and B signals without gamma conversion is output and input to a video signal normalizing circuit  1601 . 
     When all cameras take the same object, the video signal normalizing circuit  1601  normalizes the video signal to have the same level, so that a normalized video signal S 14  is output. 
     The normalized video signal S 14  is sent from the camera main body  1128  to the lens assembly  1127  through a lens mount. The lens assembly  1127  inputs the normalized video signal S 14  from the camera main body  1128  to an AF signal processing circuit  1602 . 
     An AF evaluation value generated by the AF signal processing circuit  1602  is read out with the operation of a data read program  1603  in a lens microcomputer  1116 B. 
     A main body microcomputer  1114 B reads out the states of a zoom switch  1130 , an AF switch  1131 , and an adjustment start switch  1135  and sends the states of the switches to the lens microcomputer  1116 B, thereby performing the same control as in the above-described second embodiment. 
     The AF signal processing circuit  1602  has an arrangement shown in FIG.  16 . The normalized video signal S 14  received from the camera main body  1128  is converted into a digital signal by an A/D converter  1701  to generate an automatic focusing luminance signal S 15 . 
     The signal S 15  is input to a gamma circuit  1213  and subjected to the same processing as in the second embodiment, which has been described with reference to FIG. 11, to generate an AF evaluation value. 
     In this embodiment, the normalized video signal S 14  is an analog signal which is converted into a digital video signal by an AF signal processing circuit  1113 . However, the digital signal output from the camera signal processing circuit  1112  may be normalized and, without conversion, transferred from the camera main body  1128  to the lens assembly  1127 . If adjustment of an in-focus state is unnecessary, processing by the adjustment start switch  1135  of the camera main body  1128  and associated processing by a microcomputer  1605  may be omitted. In addition, the adjustment program for the lens assembly  1127  may be omitted to realize a system configuration shown in FIG.  17 . In this case, the AF signal processing circuit has an arrangement shown in FIG. 16, as a matter of course. 
     Fourth Embodiment 
     The fourth embodiment of the present invention will be described below. 
     FIG. 18 is a block diagram of an interchangeable lens video camera system according to the fourth embodiment of the present invention. The basic arrangement is the same as that in the second embodiment except that the adjustment start switch  1135  and the adjustment program  1132  shown in FIG. 10 are omitted. Hence, a detailed description thereof will be omitted (the same reference numeral as in the second embodiment denote the same elements in the fourth embodiment), and only different portions will be described below. 
     In this embodiment, a lens microcomputer  1116 D in a lens assembly  1127  has a lens data memory unit  1133  which is backed up by a memory holding power supply  1135 . An application of a lens locus stored in the lens data memory unit  1133  will be described below. 
     Assume that the power supply of the system with the arrangement in FIG. 18 is turned off, and the system is repowered. At this time, to trace an in-focus locus which has been previously traced, representative locus data traced by a focus lens, the internal ratio, the position of a focus lens  1105 , and the position of a variable power lens  1102  before turning off the power supply must be kept stored in the lens microcomputer  1116 D or reproduced. 
     An algorithm for reproducing data at the time of turning on the power supply will be described below with reference to FIG.  19 . 
     When the power supply is turned on in step S 1901 , the lens microcomputer  1116 D refers to the backed-up memory in the lens microcomputer to determine whether the lens unit is detached/attached from/to the camera main body. 
     If it is determined in step S 1902  from the state before turning off the power supply that the lens unit has been exchanged, the flow advances to step S 1903  to confirm whether lens data is stored in the lens data memory unit  1133  in the lens microcomputer  1116 D. 
     This confirmation is also made to determine whether the memory holding power supply  1135  of the lens data memory unit  1133  in the lens microcomputer  1116 D has been normally operated after the power-OFF. 
     If NO in step S 1903 , the flow advances to step S 1905 . If YES in step S 1903 , the flow advances to step S 1904 , and locus data including the position data of the focus lens and the variable power lens, the representative locus to be used, and the internal ratio is read out from the lens data memory unit  1133  into a focus control program  1117  and a computer zoom program  1119  of the lens microcomputer  1116 D. On the basis of these backup data, the positions of the focus lens and the variable power lens and the locus to be traced are determined. The positions of the focus lens and the variable power lens and the control state are returned to those before the power supply is turned off, and the flow advances to step S 1906 . 
     If NO in step S 1902 , or if NO in step S 1903 , the lens positions and locus are initialized in the lens assembly in step S 1905  to set the focus lens and the variable power lens to their initial positions, and the flow advances to step S 1906 . 
     When the power-ON sequence is completed, and a normal operation is started, in step S 1906 , the current lens positions and locus in the lens microcomputer  1116 D are written in the lens data memory unit  1133  at a predetermined period (e.g., a period which is an integral multiple of the vertical sync signal of a video signal) such that the data can be stored even when the power supply is turned off. 
     With this arrangement, when a detaching or exchange operation of the lens assembly is performed before or after the power supply is turned off, the initialization operation for the focus lens and the variable power lens is performed upon repowering. If the detaching or exchange operation of the lens assembly is not performed before or after the power supply is turned off, the state before the power supply is turned off is read out from the lens data memory unit, so that the state before the power supply is turned off can be restored. 
     Even when the power supply is turned off, the state before turning off the power supply can be reproduced at the time of repowering. The state is not reset every time the power supply is turned on/off, so that the sensing state before the power supply is turned off can be continued. 
     FIG. 20 is a flow chart showing the first modification of the fourth embodiment of the present invention. 
     In the fourth embodiment shown in FIG. 19, when the lens assembly is detached/attached before or after the power supply is turned off, initialization is performed. When the lens assembly is not detached/attached, the state before the power supply is turned off is reproduced on the basis of the data stored in the lens data memory unit. In this modification, identification information for the camera main body connected before the power supply is turned off is stored. If the lens assembly is connected to the same camera main body at the time of repowering, the initialization operation is not performed. The state before the power supply is turned off is reset on the basis of the data stored in the lens data memory unit. If another lens assembly is connected, the initialization operation is performed. 
     As shown in FIG. 18, the arrangement of this modification is the same as that in the fourth embodiment of the present invention, and a detailed description and illustration thereof will be omitted. Only processing of the lens microcomputer  1116 D is shown in the flow chart of FIG.  20 . 
     As a means for identifying whether the camera main body is different from that before the power supply is turned off, identification information such as the number unique to the camera main body (any information unique to the camera main body, such as a serial number, can be used) is received in initial communication between the camera main body and the lens assembly and written in the memory in the lens microcomputer  1116 D. 
     The algorithm for controlling the lens assembly at the time of turning on the power supply in this modification will be described below with reference to FIG.  20 . 
     When the power supply is turned on in step S 2001 , the lens microcomputer  1116 D determines on the basis of the identification information obtained from the camera main body whether the camera main body mounted before turning off the power supply is exchanged with another camera main body. 
     If YES in step S 2002 , the flow advances to step S 2003  to confirm whether lens data is stored in the lens data memory unit  1133  of the lens microcomputer  1116 D. 
     This confirmation is also made to determine whether the memory holding power supply  1135  of the lens data memory unit  1133  of the lens microcomputer  1116 D has been normally operated. 
     If NO in step S 2003 , the flow advances to step S 2005 . If YES in step S 2003 , the flow advances to step S 2004 , and locus data including the position data of the focus lens and the variable power lens, the representative locus used, and the internal ratio are read out from the lens data memory unit  1133  into a focus control program  1117  and a computer zoom program  1119  of the lens microcomputer  1116 D. On the basis of these backup data, the positions of the focus lens and the variable power lens and the locus to be traced are determined. The positions of the focus lens and the variable power lens and the control state are returned to those before the power supply is turned off, and the flow advances to step S 2006 . 
     If NO in step S 2002 , or if NO in step S 2003 , the lens positions and cam locus are initialized in the lens assembly to set the focus lens and the variable power lens at their initial positions, and the flow advances to step S 2006 . 
     When the power ON sequence is completed, and a normal operation is started, in step S 2006 , the current lens positions and locus in the lens microcomputer  1116 D are written in the lens data memory unit  1133  at a predetermined period (e.g., an integer multiple of the vertical sync signal of a video signal) such that the data can be stored even when the power supply is turned off. 
     With this arrangement, when an exchange operation is performed between the lens assembly and the camera main body before or after the power supply is turned off, the initialization operation for the focus lens and the variable power lens is performed upon repowering. If the exchange operation is not performed between the lens assembly and the camera main body before or after the power supply is turned off, the state before the power supply is turned off is read out from the lens data memory unit  1133 , so that the state before the power supply is turned off can be restored. 
     Even when the power supply is turned off, the state before turning off can be reproduced at the time of repowering as long as the combination of the lens assembly and the camera main body is not changed. The state is not reset every time the power supply is turned on/off, so that the sensing state before the power supply is turned off can be continued. 
     As long as the lens assembly is not exchanged, the initialization operation for the lens assembly is not performed regardless of the ON/OFF operation of the power supply. The sensing operation can be continued while the state before turning off the power supply is set as an initial state, resulting in an improvement in operability. In addition, since the ON/OFF operation of the power supply does not affect the sensing state, the power supply can be frequently turned on/off, and a power saving effect can be obtained. 
     FIG. 21 is a block diagram showing the arrangement of the second modification of the fourth embodiment of the present invention. The same reference numeral as in the fourth embodiment denote the same elements in the second modification, and a detailed description thereof will be omitted. 
     In this modification, the data holding power for storing the lens data in the fourth embodiment is supplied from the camera main body. With this arrangement, even when the power supply of the camera main body or the lens assembly is turned off, the lens data memory unit can be backed up as long as the lens assembly is not detached, so the data can be held. 
     In the fourth embodiment and the first modification of the fourth embodiment, a memory holding battery is used to store lens data in the lens microcomputer. Instead, an EEPROM or a nonvolatile memory such as a flash memory may be used. 
     Fifth Embodiment 
     The fifth embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 22 is a block diagram showing an example in which the present invention is applied to an interchangeable lens video camera. AF control and a zooming operation in this embodiment are the same as those in the above-described embodiments. Hence, a detailed description is omitted, and only different portions will be described. 
     Light from an object passes through a fixed first lens group  2101 , a second lens group (to be referred to as a variable power lens hereinafter)  2102  for performing a zooming operation, an iris stop  2103 , a fixed third lens group  2104 , and a fourth lens group (to be referred to as a focus lens hereinafter)  2105  having both a focusing function and a function of compensating for the movement of a focal plane caused by zooming. The red, green, and blue components in the three primary colors form images on the image sensing surfaces of image sensing devices  2106 ,  2107 , and  2108  such as CCDs, respectively. 
     The images of the respective color components, which are formed on the image sensing surfaces of the image sensing devices, are photoelectrically converted, amplified to their respective optimum levels by amplifiers  2109 ,  2110 , and  2111 , input to a camera signal processing circuit  2112 , and converted into a standard TV signal. 
     The video signal output from the camera signal processing circuit  2112  is supplied to a video recorder and an electronic viewfinder (neither are shown) through a switch  2140 , so that recording and monitoring are enabled. 
     By switching the switch  2140 , immediately preceding recorded image information can be reproduced with the video recorder to check the recording state (so-called “rec review”). 
     The luminance signal generated in the camera signal processing circuit  2112  is input to an AF signal processing circuit  2113 . Though not illustrated in FIG. 22, information associated with the luminance signal level is sent from the camera signal processing circuit  2112  to a lens microcomputer  2116  in the lens assembly. On the basis of this information, control for opening/closing the iris stop  2103  and maintaining a predetermined luminance signal level is performed. The aperture opening amount of the iris stop is detected by an encoder  2129  and used as the depth-of-field information for AF control or manual iris stop control. 
     The AF signal processing circuit  2113  detects the high-frequency component in the luminance signal, which changes according to the focus state, as an AF evaluation value. The AF evaluation value is read out by a data read program  2115  in a main body microcomputer  2114  in a camera main body  2128  and transferred to the lens microcomputer  2116 . 
     The microcomputer  2114  loads the information of a power switch  2138  of the camera. When the power switch  2138  is turned on, the main body microcomputer controls a switch  2139  to supply power from a battery (not shown) arranged in the camera main body to a lens assembly  2127  side. 
     In addition to the AF evaluation value, a lens ON/OFF request signal  2142  for performing ON/OFF control on the lens assembly side, a lens key inhibition signal  2145  for inhibiting the operation of operation keys on the lens assembly side, and the like are transmitted from the microcomputer  2114  to the lens assembly side. 
     The camera main body receives, from the lens assembly  2127  side, an image display permission signal  2143  for permitting to supply a video signal output from the camera signal processing circuit  2112  to the electronic viewfinder or the video recorder and display an image, and a lens OFF permission signal  2144  representing that the power supply on the lens side can be turned off, thereby performing control according to the operation state of the lens assembly. 
     The lens microcomputer  2116  loads the states of an AF switch (when ON, an AF operation is performed; when OFF, a manual mode is set)  2131 , a zoom switch  2136  for operating the variable power lens to the telephoto side (T) or the wide side (W) to perform a zooming operation, and a power focus switch  2137  for operating the focus lens to the closest focusing distance or in the direction of infinity when the AF switch is OFF in the manual focus state, so that control according to the operation states of the switches is performed. 
     When the AF switch  2131  is OFF, and the zoom switch  2136  is depressed, the lens microcomputer  2116  sends a signal to a zoom motor driver  2122  such that the variable power lens is driven in the direction operated by a computer zoom program  2119 , i.e., to the telephoto side or the wide side, thereby driving the variable power lens  2102  through a zoom motor  2121 . At the same time, to compensate for the position of the focal plane corresponding to the movement of the variable power lens, the focus motor  2125  is driven through a focus motor driver  2126  on the basis of lens cam data (FIG. 22) stored in the lens microcomputer  2116  in advance to drive the focus lens  2105 . 
     When the AF switch  2131  is ON (AF mode), and the zoom switch  2136  is depressed, it is necessary to hold the in-focus state while compensating for the displacement of the focal plane caused by the zooming operation and a blur generated according to the movement of the lens relative to the object. Accordingly, the computer zoom program  2119  operates to not only perform control on the basis of the lens cam data  2120  stored in the lens microcomputer  2116  in advance but also simultaneously refer to the AF evaluation value signal sent from the main body microcomputer  2114 , thereby performing a zooming operation while holding the position at which the AF evaluation value is maximized. 
     When the AF switch  2131  is ON, and the zoom switch  2136  is not depressed, the AF program  2117  sends a signal to the focus motor driver  2126  such that the AF evaluation value signal transmitted from the main body microcomputer  2114  is maximized to drive the focus lens  2105  through the focus motor  2125 , thereby performing an automatic focusing operation. 
     When the AF switch  2131  is OFF (manual mode), and the zoom switch  2136  is not depressed, a signal is sent to the focus motor driver  2126  to drive the focus lens  2105  in the direction operated by the power focus switch (manual focus switch)  2137 , i.e., to the closest focusing distance or the direction of infinity, thereby performing manual focusing. 
     The sequence from turning on to turning off the camera will be described below with reference to FIGS. 22 and 23. 
     When the power switch  2138  of the camera is turned on at time t 0 , the main body microcomputer  2114  is powered. At time t 1 , the power switch  2139  for supplying a power to the lens assembly is turned on, and at the same time, the lens ON/OFF request signal  2142  goes high. 
     With this operation, the lens microcomputer  2116  initializes the lens assembly (lens reset). At time t 2 , initialization is completed, and the image display permission signal  2143  goes high. Completion of initialization of the lens assembly is transmitted to the camera  2128  accordingly. 
     Upon receiving the image display permission signal  2143 , the main body microcomputer  2114  on the camera side sets the lens key inhibition signal  2145  of low level to high level (the operation keys on the lens assembly side are enabled). At the same time, a video signal output from the camera signal processing circuit  2112  is output to the electronic viewfinder or the video recorder. 
     From time t 0  to time t 2 , the lens microcomputer  2116  inhibits the operations of the AF switches  2131 , the manual focus switch  2137 , and the zoom switch  2136  (the lens key inhibition signal  2145  is set at low level). However, when the lens key inhibition signal  2145  goes high, the AF switch  2131 , the manual focus switch  2137 , and the zoom switch  2136  are enabled. 
     At time t 3 , the rec review signal  2147  goes high. The switch  2140  is switched to the video recorder side, and the immediately preceding recorded video signal output from the reproducing unit is supplied to the viewfinder  2148 . Simultaneously, the lens key inhibition signal  2145  goes low. 
     With this operation, the lens microcomputer  2116  disables the AF switch  2131 , the manual focus switch  2137 , and the zoom switch  2136 . More specifically, during reproduction such as rec review, driving of the lenses on the lens assembly side is inhibited not to change the states of the respective switches. 
     At time t 4 , the rec review signal  2147  goes low, and at the same time, the switch  2140  is switched such that the video signal from the camera signal processing circuit  2112  is supplied to the viewfinder  2148 . The lens key inhibition signal  2145  goes high. With this operation, the lens microcomputer  2116  enables the AF switch  2131 , the manual focus switch  2137 , and the zoom switch  2136 . 
     When the camera power switch  2138  is turned off, the main body microcomputer  2114  sets the lens ON/OFF request signal  2142  to low level at time t 5 . Simultaneously, the lens key inhibition signal  2145  also goes low. 
     With this operation, the lens microcomputer  2116  starts preparation for turning off the power supply in the lens assembly. For example, the focus lens  2105  and the zoom lens  2102  are moved to predetermined positions. Simultaneously, the lens display permission signal goes low to inhibit display of a camera image. In addition, the AF switch  2131 , the manual focus switch  2137 , and the zoom switch  2136  are disabled. 
     That is, before the power supply is turned off, the movable units such as the lenses in the lens assembly are moved to predetermined positions before the power supply is turned off. In addition, an image with poor quality during this operation can be prevented from being displayed on the electronic viewfinder or the video recorder. 
     The lens microcomputer  2116  sets the lens OFF permission signal  2144  to high level at time t 6  at which preparation for turning off the power supply is completed. When the lens OFF permission signal  2114  goes high, the microcomputer  2114  turns off the switch  2139  to stop power supply to the lens assembly side. Thereafter, the power supply of the camera  2128  is turned off. 
     In this embodiment, the AF evaluation value  2141 , the lens ON/OFF request signal  2142 , the display permission signal  2143 , the lens OFF signal  2144 , the lens key inhibition signal  2145 , and the like are transferred between the camera main body  2128  and the lens assembly  2127  through dedicated signal lines. However, bidirectional serial or parallel data communication may be performed between the main body microcomputer  2114  and the lens microcomputer  2116  so that the respective contents are transferred at predetermined positions of data communication. In addition, the above-described embodiments, i.e., in-focus state adjustment processing, mounting of an AF signal processing circuit in the lens assembly, or holding of an in-focus state at the time of repowering may be combined with the processing of this embodiment, as a matter of course. 
     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.