Fog is any visible deposit or density in the negative or print not forming part of the photographic image. Several kinds of fog can be distinguished:

1. Optical fog, caused by the action of unwanted light, e.g. light leaks in camera body, loading film in direct sunlight.

2. Development fog, caused by excessive silver deposit during processing.

3. Chemical fog, caused by unwanted reactions in the processing solutions.


1978, Focal Encyclopedia of Photography, Desk Edition Reprint, Focal Press, London, New York