A good introductionary guide to lens adapters, by Ian Plant, at Pop Photo.

These guides don't come often. Ian has written an extensive introduction and tutorial on the how and what of lens adapters, and has also included a rundown of his favorite lenses: 

"These days, there seems to be no shortage of lens adapters available on the market—some good, some not so good. There are a number of reputable manufacturers making adapters, including Adaptimax, Bower, CameraQuest, Fotodiox, Novoflex, Vello, Voigtländer, and others. You can find cheap adapters on places such as eBay, many of which are quite good—but you can also find many adapters that will be inadequate for critical use.

Do your homework before buying an adapter to make sure you are getting the quality and features you need. Consult the websites of adapter makers to ensure you’re getting the functionality you expect. Most important, make sure your adapter will allow you to focus throughout the entire native focus range of the lens, especially infinity focus (good adapters won’t affect focus at all).

Adaptable Camera Systems
Although most camera systems are adaptable up to a point, not all systems are created equal when it comes to adapting alternative lenses for use. Nikon cameras are among the least adaptable. Canon DSLRs are arguably the most adaptable because of a generously sized lensmount, and over the years many people have successfully adapted their Canon DSLRs to a broad range of alternative lenses. Among the ones I’ve used on my Canon bodies are several Zeiss lenses from the old Contax line of film cameras. Most other popular camera systems fall somewhere between the Nikon and Canon systems in terms of adaptability, and Sony, Fujifilm, Olympus, Panasonic, Pentax, and Leica users will find many adapter options available."

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