The lens mount is more than just a mechanism for securing the lens to the camera. It's an interface between two high-tech devices that allows information and functionality to flow back and forth to give you an image that's properly exposed and sharp. In general, the more up to date the camera and lens are, the more information flow there is. This is particularly beneficial for electronic functions like autofocus and auto exposure. But if you don't mind stepping out of the comfort zone created by modern electronics, you can tap into an almost unlimited choice of lenses through the use of an adapter.
There has been a trend toward classic glass in the past few years—older lenses that create a particular look. Many of these film-era lenses were available on eBay at almost giveaway prices until recently, but no more. That's because there are so many adapters available now that the user of a brand-new Canon EOS 5D Mark III DSLR can fit a lens made for a circa-1970s Nikon F2 body to the Canon. That's an extreme example. More common and increasingly popular camera/lens brand mismatches take advantage of a particularly good lens and make it available to any camera body. One of the most popular examples is the use of the Nikon AF-S 14-24mm ƒ/2.8G ED lens, which has been embraced by many non-Nikon photographers for its overall excellence.
In our article on large-sensor mirrorless cameras in this issue of DPP ("Time To Lose The Mirror"), we highlight another trend where adapters are playing a role. As photographers slim down their outfits for travel or for street shooting and landscape photography, many are turning to mirrorless bodies and outfitting those bodies with relatively small manual-focus lenses. The use of Leica, Voigtländer and Zeiss M-mount lenses on Sony A7-series cameras gives you a full-frame image sensor, optical excellence and a very small overall package. That can be a small price to pay for the loss of auto-focus and electronic aperture control.
Mirrorless cameras, in particular, have helped the adapter market to grow because mirrorless bodies have a shorter flange-back distance compared to a DSLR with a mirror box. The Canon EOS mount has a short flange-back distance compared to other DSLRs, which makes it possible to use various lens/adapter combinations on EOS cameras.
The flange-back distance is the distance between the lens mount and the image plane. If the lens is designed for a shorter distance than that of the camera body, the lens won't focus out to infinity (although you can use it for close-up photography). There are adapters that contain glass elements to allow such lenses to focus out to infinity, but these also act as low-power teleconverters (they increase the focal length somewhat and reduce the effective aperture a bit), and the added elements can reduce sharpness and contrast. If the adapter isn't thick enough, the flange-back distance will be too short, and the lens will focus beyond infinity.
Motion shooters also have found adapters to be invaluable as they put their kits together. When the Panasonic LUMIX DMC-GH4 came out, motion shooters were immediately intrigued by the possibility of having 4K capture in the small body. Panasonic's Micro Four Thirds cameras have been popular filmmaking cameras for years. The sensor isn't as large as a full-frame or an APS-C sensor, but it's much larger than the typical video camera sensor, and Panasonic has made a concerted effort to promote their philosophy of hybrid shooting—still and motion. Even before the term "hybrid" showed up in their marketing efforts, cinematographers were having their Panasonic LUMIX cameras hacked and rigged with PL mounts to become full-time motion cameras. The Blackmagic Cinema Camera and Pocket Cinema Camera are dedicated motion cameras that have MFT mounts (the Cinema Camera is also available with an EF mount). The biggest challenge for any MFT system is at the wide end of the focal range. Adapter manufacturer Metabones makes a line of Speed Booster adapters specifically to help alleviate the 4/3rds sensor's magnification factor.
All adapters are not created equal. When you place something between the camera and the lens, you're introducing the possibility of light leaks, internal flare, optical misalignment and other potential image-destroying flaws. Precision is important, and precision doesn't come cheap.