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This article is from the Photographic Lenses FAQ, by David Jacobson with numerous contributions by others.

17. If I get glasses (or bifocals) will my focusing be off?

No. The focusing screen is a diffusing plane in the same optical position as the film. If the image is sharp on the focusing screen, it will also be sharp on the film. Many single lens reflex cameras have a split image focusing aid. In effect one half causes the eye to see through the left side of the lens and the other half causes the eye to see through the right side of the lens. Objects appear to be in the same position in both halves only if the image plane coincides with the plane of the focusing screen and thus with film plane, and hence are in sharp focus. All your glasses do is enable you to see the focusing screen or focusing aid better.

 

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photography, lenses, focal length, bellows, focus, pivot