Photographs on film Photos for prints are usually made by a negative-positive process. In a black and white photo, the picture is made up from millions of tiny grains of blackened silver salts. The picture is darkest where there are most grains. In a negative, light areas in the scene are dark on film. In a positive, light areas in the scene are light in the picture.
Colour photos use three layers of silver salts, sensitive to red, blue or green light.
These layers are converted to coloured dyes during processing. When the picture is taken, light subtly alters some of the film grains. Processing To reveal the negatives, the film is developed in chemicals in a lightproof tank to change the altered film grains into silver. More chemicals dissolve away the unaltered grains to fix the image. Enlarging To make a positive print, each negative is placed in an enlarger.