1. A method of determining light exposure comprising the steps of: reading information in relation to a color original image and dividing said color original image into a plurality of picture elements to analyze respective ones of the picture elements with respect to three colors of light for photometry, said information in relation to said color original image being any one of size information representing a size of a main image and size related information for finding a size of said main image; generating a histogram in relation to hue value depending upon data obtained from said photometry and dividing said histogram into portions each containing a crest of said histogram; deciding to which of said crests said respective picture elements of said color original image belong so as to distribute each of said picture elements into a corresponding cluster, and analyzing said color original image for each cluster; selecting a region of said color original image, which corresponds to said information, as a feature image region; and determining said light exposure for copy material depending upon photometric image data obtained from said feature image region.