Submitted by Al …
(Adapted from The Encyclopedia of Photography ©1963)
When white light strikes a simple positive lens, the different colours come to a focus at different points. This is what is called chromatic aberration… where the red rays are bent less and have longer focus, and the violet rays are bent more and focus nearer to the lens. Every lens has a different focal length for each colour of the spectrum. The optical focus of a lens is the point at which the yellow-green rays form a sharp image (the point at which the rays from a theoretical single point of light source would cross behind the lens).
The chemical focus is the point at which the extreme violet rays form a sharp image. Most lenses are achromatic and corrected for this. With certain uncorrected lenses, box camera lenses for example, the presence of chemical focus is apt to cause a blurred image outline. The blurring caused by chemical focus may also occur with some enlarger lenses.
To adjust non-corrected lenses for chemical focus, reduce the distance between the lens and the negative by about 1/40th of the focal length for normal scenes, with more reduction for close-up work. On box cameras, the film plane has been shifted slightly to take care of this chemical focus.
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