1. An image pickup apparatus comprising: an image pickup device having a plurality of high-sensitivity pixel devices that receive a relatively large amount of light and a plurality of low-sensitivity pixel devices that receive a relatively small amount of light, the high-sensitivity pixel devices and the low-sensitivity pixel devices being arranged in an array; an exposure control unit configured to independently control an exposure period of the high-sensitivity pixel devices and an exposure period of the low-sensitivity pixel devices, the high-sensitivity pixel devices and the low-sensitivity pixel devices being included in the image pickup device; an image generation unit configured to perform image generation processing on the basis of a signal output from the image pickup device, and wherein the image generation unit performs comparison processing of a high-sensitivity pixel evaluation image generated using data output from the high-sensitivity pixel devices and a low-sensitivity pixel evaluation image generated using data output from the low-sensitivity pixel devices by obtaining a difference or ratio between a pixel value of each pixel included in the high-sensitivity pixel evaluation image and a pixel value of a corresponding pixel included in the low-sensitivity pixel evaluation image, discriminates a pixel region composed of pixels each having a small difference between corresponding pixels in the two evaluation images from a pixel region composed of pixels each having a large difference between corresponding pixels in the two evaluation images, and performs different types of image processing for the pixel regions.