Besides the ubiquitous T-ring, originally designed for a line of cheap telephoto lenses, there are other adapters for putting one kind of lens onto another kind of camera body.
Obviously, any adapter has to contain a receptacle to fit the lens and a flange to fit the camera body. The trouble is, all of this takes up space, placing the lens farther from the film or sensor than it was designed to be. There are three ways to deal with this problem:
• Add a glass element (typically a x 1.2 Barlow lens) so that the lens will still focus correctly. This is a common practice but degrades the image quality.
• Leave out the glass element and give up the ability to focus on infinity. The adapter is now a short extension tube and the lens can only take close-ups.
• If the camera body is shallower than the one the lens was designed for, then make the adapter exactly thick enough to take up the difference. In this case, the lens focuses perfectly, without a glass element.
The third kind of adapter is the only one that interests us, and it's only possible if the camera body is shallower, front to back, than the body the lens was designed for. In that case, the adapter makes up the difference. For example, a Nikon lens can fit onto a Canon EOS body with an adapter 2.5 mm thick.
Olympus Four Thirds System DSLRs are much smaller than 35-mm film cameras, almost any film SLR lens should be adaptable to fit them. The Canon EOS is one of the shallowest full-size SLR bodies, and adapters exist to put Nikon, Contax-Yashica, Olympus, M42, and even Exakta lenses on it. The one thing you can't do is put older Canon FD lenses on an EOS body, because the FD body was even shallower.
Nikon bodies generally can't take non-Nikon lenses because the Nikon body is one of the deepest in the business. Only the Leicaflex is deeper.
One curious zero-thickness adapter does exist. Pentax's adapter to put screw-mount lenses on K-mount cameras simply wraps around the screw-mount threads, taking advantage of the K-mount's larger diameter. More commonly, though, screw-mount-to-K-mount adapters are about 1 mm thick and do not preserve infinity focus.
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