Canon EF Lens Mount - Versatility

Versatility

Its large diameter and relatively short flange focal distance of 44.0 mm allow the EF lens mount to work with adapters. It is possible to mount lenses using the Nikon F mount, Olympus OM, Leica R and universal M42 lens mounts (among others) by the use of a mechanical adapter.

Lenses for the earlier Canon FD lens mount are not usable for general photography on an EF or EF-S mount camera, unless adapters with optical elements are used because they are made for a flange focal distance of only 42.0 mm. Infinity focus would be lost with an adapter which lacks optical elements. The Canon FD-EOS adapter is rare and is only usable with certain FD telephoto lenses. With a manual connection, the aperture and focus controls of the lens cannot be controlled or read from the camera; the lens must be focused manually. Since the only possible metering is through-the-lens, the lens must be manually stopped down to accurately meter at anything less than full aperture. (This is called stop-down metering.)

For other lens types, an adapter would act as an extension tube, causing the lens to lose the ability to focus to infinity. Alternatively, the lens adapters would include optical elements and act as weak teleconverters, as well as possibly losing optical quality.