1. an imaging apparatus comprising: a lens; an imaging device that detects light focused by the lens at a plurality of detection points; and an image processing unit that generates image data for displaying an image on a display screen having a predetermined area, on the basis of image signals detected at the detection points of the imaging device, wherein the image processing unit performs image processing to enlarge a section, which is a portion of the image captured by the imaging device, and displays the enlarged image on the display screen, and in the image processing, an enlargement ratio of a field angle varies depending on the place in the section, wherein, in the image processing, the enlargement ratio of the field angle in an X-axis direction corresponding to a horizontal axis of the display screen, and the enlargement ratio of the field angle in a Y-axis direction that is orthogonal to the X-axis direction are changed on the basis of a predetermined change function, and wherein, in the image processing, the following processes are performed on each pixel Pi of the display screen, and a process of making the pixels Pi correspond to image signals of the detection points of the imaging device is performed: (1) a process of substituting the X-axis coordinate of the pixel Pi on the display screen into the change function to calculate a field angle αx in the X-axis direction corresponding to the coordinate and substituting the Y-axis coordinate of the pixel Pi on the display screen into the change function to calculate a field angle (αy in the Y-axis direction corresponding to the coordinate; (2) a process of calculating the positions rx and ry of an image point of a display image on the basis of a focal length f corresponding to the enlargement ratio of the display image, the field angle αx, and the field angle αy; (3) a process of calculating a zenith angle θi and an azimuth angle Φi of a spatial point to be projected onto the pixel Pi on the basis of the positions rx and ry; (4) a process of making the pixel Pi correspond to the image signal detected at any one of the detection points of the imaging device on the basis of the zenith angle θi, the azimuth angle Φi, and a projection polynomial of the lens; and (5) a process of performing the processes (1) to (4) on all the pixels of the display screen.