An autofocus mechanism is a device which allows to change the magnification of an optical system by moving the object, image plane and even the lens, still maintaining the image always at focus.This task can be accomplished by using a simple mechanical device, a sort of an analog computer.Three techniques are reported here: the three hinged rods, the endless belt and the sliding rods.

These mechanical autofocus mechanisms are used in some photographic enlarger and photocopier.