In a previous post I mentioned adapted lenses as being a legitimate choice for the new Sony a7mk2. In fact, all mirrorless cameras are ideal for adapting lenses from a variety of systems, and, in recent years, a huge interest and demand in older lenses started for the same reason: lack of native lens choice.
Until almost the end of the last decade, film-era manual lenses were considered almost relics of the past in most photographic cycles. DSLR makers had established a vast selection of native autofocus lenses and adapting an older lens was not a particularly popular venture. Sure, Leica and similar high end lenses were always in high demand. After all, Leica never stopped making film cameras, and, even their digital cameras were, and still basically are, manual focusing. But 35mm film SLR lenses could, in general, be obtained in very low prices. Another reason was, DSLRs were and still are virtually useless for manual focusing. Since they lack the split screens of film SLRs, you have to make-do with an approximation of critical focus; i.e. it depends on perfect eyesight and basically learning how to hold your breath.
For their native lenses, DSLRs provide focus confirmation via the electronic connection. To be fair, this is no different than what Contax RX did on film, and, obviously worthless for other system lenses. Only using the Live View provides a viable method of focusing, transforming a DSLR into a glorified non-EVF mirrorless camera, at 2-3 times the weight and bulk.
True interest about adapted lenses was sparkled with the introduction of the first viable interchangeable compact camera system -what we all today call "mirrorless"- the Micro Four Thirds System.
When m43 was introduced, we only had a couple of kit lenses, followed by a few -not to exciting, to be honest- primes. Adapting lenses from other systems was about the only way for the enthusiast in need of various focal lengths; and in this manner a new market flourished, with dozens of adapters on offer. Demand drove prices up, especially soon thereafter, when the NEX, Samsung and Fuji-X mirrorless systems were introduced.
At this point let's clarify that we are talking about manual focusing lenses; even older AF lenses are not able to communicate with the camera except in a handful of cases where an electronics-enabled adapter is used, and, even then, AF is quite slow for practical use. Now, there are a couple of major advantages mirrorless cameras enjoy, when adapting lenses.
First of all, mirrorless have a much shorter flange distance (or register) than DSLRs (and old film SLRs). A vey useful list of flange distances, for a variety of systems, can be found in this link. Having a shorter register, means an adapted lens can focus to infinity, without the use of additional optical elements (which are almost always detrimental to image quality). A simple tube or ring type adaptor is commonly used; it just needs to feature the appropriate screw or bayonet type mount for the lens in use. Contrary to this, modern DSLRs, having to accommodate the mirror assembly, have much longer registers, thus limiting practical options.
Second of all, mirrorless companies gradually introduced a number of very useful and precise manual focusing aids, which almost guarantee pin-point focus accuracy. Today, we have various EVF magnification options, focus peaking -a feature familiar to videographers for quite some time- and even rangefinder-style electronic focusing "patches".
I mentioned videographers above, because, in all fairness, adapted lenses owe their popularity to the emergence of video in photographic cameras, in a major way. In fact most people today adapt lenses mainly for video, since all systems have at least a satisfactory native lens selection for photography, and, additionally, video is not so much dependant on autofocus. That doesn't mean they are not a great creative -and even practical- choice for general photography. There are, of course, pros and cons, as well as better or worse alternatives; and we shall discuss these in part 2.