1.  Field of the Invention
This invention relates to an image pickup apparatus arranged to be capable of varying the size of an image plane. 
2.  Description of Related Art
It has been known that image pickup apparatuses such as video cameras are arranged to include automatic focus adjusting devices called AF devices for short.  The AF device is arranged to detect the sharpness of an image plane from a video signal representative of an object image and to automatically perform a focusing action by controlling the position of a focusing lens in such a way as to maximize the sharpness of the image plane. 
In general, the above sharpness is evaluated by using, as a sharpness signal, a signal which indicates the intensity of a high frequency component of a video signal extracted through a band-pass filter or a signal which indicates a detected intensity of a blurring width of a video signal extracted through a differentiating circuit or the like.  Generally, in shooting an ordinary object, the sharpness signal is small when a focusing lens is out of focus.  The sharpness signal becomes larger accordingly as the focusing state of the focusing lens comes closer to an in-focus state and reaches a maximum value when a perfect in-focus state is obtained.  In controlling the focusing lens, the focusing lens is moved as fast as possible when the sharpness signal is small.  The focusing lens is moved slower accordingly as the sharpness signal increases, so that the focusing lens can be brought to a stop precisely at a peak of a hill-like control curve, where the sharpness signal reaches a maximum value, thereby attaining an in-focus state.  This automatic focusing method is generally called a hill-climbing automatic focusing method (hereinafter referred to as a hill-climbing AF).  The hill-climbing AF permits simplification and reduction in size of cameras and is thus advantageous for compact and light-weight cameras.  On account of this advantage, the hill-climbing AF is now most popularly employed for recent video cameras. 
Meanwhile, a TV set having an image plane of the aspect ratio of 16:9 (hereinafter referred to as the wide TV) has recently been becoming popular to replace the conventional TV set having an image plane of the aspect ratio of 4:3.  The wide TV is provided with a mode of providing the 16:9 image plane by magnifying the conventional 4:3 image plane to 1.33 times horizontally (in the width direction).  To cope with this magnifying mode, some of known video cameras are arranged to record a picked-up image in a state of being vertically (in the height direction) magnified to 1.33 times beforehand at the time of shooting. 
However, since the video camera of such a kind is arranged to perform the AF control and the change-over of the aspect ratio independently of each other, there arises the following problem, which will be explained with reference to FIG.  1. 
In FIG.  1, reference numeral 1 denotes a picked-up image plane obtained on an image pickup element such as a CCD, reference numeral 2 denotes an AF distance measuring frame, reference numeral 3 denotes an object which is in an in-focus state, reference numeral 4 denotes an object which is in an out-of-focus state, and reference numeral 5 denotes a recording image plane obtained by changing the aspect ratio of the picked-up image plane 1.  As shown in FIG.  1, when the aspect ratio is changed by magnifying the picked-up image plane 1 vertically, parts of upper and lower sides of the picked-up image plane 1 are expanded out of the recording image plane 5 and, therefore, disappears therefrom, and the AF distance measuring frame 2 also becomes relatively larger in the vertical direction.  However, since the AF distance measuring frame 2 is left as it is, the focusing lens then tends to remain in focus on the object 3, which is located in the part disappearing from the image plane, so that the object 4 located inside the image plane would not be brought into focus, thereby remaining blurring. 
Further, in performing the automatic exposure (abbreviated to AE) control as well as in performing the AF control, the same problem exists for the same reason, so that an adequate exposure for an object such as the object 4 would fail to be made.