1.  Field of the Invention
The present invention relates to improvements in an image pickup apparatus such as a video camera, a still camera or a silver-halide camera, an image reproducing apparatus for reproducing an image recorded by such an image pickup apparatus, and a video system formed by a combination of these apparatus. 
2.  Description of the Related Art
Conventionally, in an image pickup apparatus, the distortion of a photographic image occurs due to the distortional aberration of each individual photographic lens and a distortion which cannot be completely eliminated in every stage of the design of the photographic lens remains in an image as a residual distortional aberration.  As one method for electronically eliminating the residual distortional aberration, it is known that an electron beam is made to scan in distorted form on the basis of the distortional aberration of a photographic lens in an image pickup apparatus using an image pickup tube such as the Plumbicon. 
Japanese Laid-Open Patent Application No.  Hei 2-252375 discloses another proposal to obtain image data from an image pickup device such as a CCD and correct the image data in the form of a digital signal. 
However, the above-described conventional example has a number of problems.  For example, since correction of image data is performed with the center (optical axis) of an image plane selected as a datum, on the basis of the focal length of the photographic lens, i.e., as shown in FIGS.  25(a) and 25(b), no image data present in the peripheral portion of the image plane is employed in the case of correction of a negative (barrel form) distortion, as shown in FIG.  26(a).  In contrast, in the case of correction of a positive (pincushion form) distortion, image data present in the peripheral portion of the image plane is lost as shown in FIG.  26(b). 
Another distortional-aberration correcting method has been proposed with respect to a camera provided with a solid-state image pickup device.  According to this method, pixel signals are read out from the solid-state image pickup device in accordance with the geometrical deformation of an image which occurs in a photographic lens, and the read-out pixel signals are subjected to interpolation so that the distortional aberration of the image is corrected.  However, in a case where a zoom lens is used as the objective lens, if correction of the distortional aberration of the zoom lens is to be performed at each photographing position thereof by using signal processing, an enormous amount of computing processing and an extremely long computing time as well as a large-scale signal processing circuit are needed for the signal processing.  For this reason, although the utilization of this proposal is considered in an extremely limited field, such as a broadcasting station, which needs high-quality images, the aforesaid proposal has not currently been popular. 
An art of carrying out electronic zooming by performing signal processing of a signal obtained from an image pickup device, such as a CCD, has also heretofore been known. 
According to the known art, since it is necessary to position an image of an object to be electronically zoomed, in an electronic-zoom frame provided in the center of an image plane, the electronic zooming is not very useful in photographing a moving object to which the electronic zooming seems able to be advantageously applied.  This is because it is necessary to move the entire image pickup apparatus in accordance with the movement of the object so that an image of the moving object can be positioned in the electronic-zoom frame at all times, with the result that an image shake easily occurs due to the movement of the entire image pickup apparatus. 
For example, such an image pickup apparatus having an electronic-zoom function is arranged to perform coordinate transformation so as to enlarge an original image which is not electronically zoomed, as shown in the left-hand portion of FIG.  56, to a size equivalent to an electronic-zoom frame in which an object selected as a target is to be positioned.  By performing the coordinate transformation, the image pickup apparatus can provide an enlarged image such as that shown in the right-hand portion of FIG.  56. 
However, the above-described conventional example has the following problem.  Since a photographer cannot obtain image information outside of the electronic-zoom frame while the electronic-zoom function is operating, the photographer encounters the problem that an object, particularly a moving object, occasionally leaves the image plane or that another foreign object suddenly enters the electronic-zoom frame and appears in the image plane.  For this reason, it is difficult for the photographer to fully utilize the electronic-zoom function. 
As is known in the art, the so-called electronic zoom is intended to achieve an apparent increase in the telephoto effect of an optical system when the optical system is set to its telephoto end, and to attain a large zoom effect without increasing the size of the optical system. 
By sequentially executing the electronic zoom and optical zoom (or an optical zooming operation) capable of varying the focal length of the optical system, it is possible to achieve an effect equivalent to the zoom range of a zoom lens having a zoom ratio equal to a multiplication of the zoom ratio of the optical zoom and that of the electronic zoom. 
For example, if the optical system has focal lengths of 8-80 mm and the electronic has a zoom ratio of 2 times, it is possible to obtain a lens having focal lengths equivalent to 8-160 mm. 
However, in such an electronic zoom, although it is possible to enlarge an image, it is impossible to enlarge an angle of view.  Accordingly, although it is possible to achieve "an increase in the telephoto effect", it is impossible to realize "an increase in a wide-angle effect". 
Although a lens system may be beforehand designed to have a wide angle of view, there is the problem that the lens system becomes large. 
A plurality of means for shifting the focal length of the lens system toward its wide-angle side have recently been proposed in light of the problem. 
One well-known method for shifting the focal length of the lens system toward the wide-angle side is disclosed in, for example, Japanese Laid-Open Patent Application No.  Sho 59-204817.  In this method, an afocal converter made up of lenses having negative and positive refractive powers is disposed forward of the optical system so that the wide-angle effect of the lens system can be increased. 
However, since the afocal converter is made up of two or three lenses, there is the problem that if the afocal converter is disposed forward of the optical system, the entire lens system increases in size. 
According to another known method, a negative lens is inserted forward of a principal optical system, and the close-up photography capability of the principal optical system is utilized to achieve an increase in the wide-angle effect of the lens system. 
However, if the focal length of the lens system is discontinuously converted from its wide-angle end toward a far wider-angle side, there is the problem that no photography can be performed at any angle of view between the wide-angle end and the far wider-angle side.