Sans Mirror  

June 2012: added information on CPU chips on legacy lenses to increase compatibility
July 2022: updated comments, as many things have changed

Mirrorless cameras tend to have smaller-than-film sensors and shorter than DSLR/SLR distances from lens mount to film plane. This combination makes them strong candidates for adapting existing interchangeable lenses from almost any 35mm or DSLR lens mount (Canon EF to Sony FE shown above). Virtually every system has the manufacturer providing at least one lens adapter for their mirrorless cameras for their legacy DSLR/SLR lenses, and dozens upon dozens of specialty adapters have appeared (see, for example, this page full of just third-party adapters for the Nikon Z-mount).

While that's good news, beware, as there's potential bad news lurking, too. As I noted in a FAQ response, we have a number of things to watch for:

1. Automatic features (focus, VR, etc.) don't always work. Exceptions: the Canon EOS adapter for the EOS M, the Sony Alpha mount adapter for the NEX, the Nikon FT1 F-mount adapter for the Nikon 1, plus a few third party adapters from Fringer, Sigma, Megadap, and others)
2. You may be restricted to certain exposure modes (typically manual exposure mode) and features on the camera.
3. You may have to set something on the camera (e.g. "Photograph without lens" setting on some models).
4. Manual focus lenses are still manual focus. Some cameras have useful focus aids to help (magnification, peaking), while others have none. Note: we've now exceptions where an adapter uses physical positioning to recreate autofocus for an otherwise manual focus lens.
5. You have to be aware of the crop factor: a 50mm lens on APS-C mirrorless cameras is a telephoto, not a normal lens.
6. The lenses may be a lot bigger than the camera, especially since they were designed to be used on a deeper camera with a larger "sensor" area in the first place; the adapter sticks out from the front of the camera's lens mount, too.
7. Not all older lenses perform great on newer digital cameras. Optical design has changed and improved considerably in both the DSLR and mirrorless eras.
8. Some lenses may need special correction. The Fujifilm X cameras with Leica M-mount lenses are a good example. Fortunately, Fujifilm built correction features into the most recent cameras and a way to trigger them quickly from the adapter itself.

That said, quite a few users are enjoying their older lenses on their new mirrorless cameras. Look especially for adapters that convey information to the camera via electronic connections, otherwise things like EXIF data won't reflect the focal length or aperture of the lens. 

Here's a quick set of advice for each of the primary mounts (I'll continue updating it as I get more experience):

• m4/3 -- The m4/3 mount is amongst the most flexible, as we have not only Panasonic and Olympus providing sets of adapters for various lenses, but we also have Novoflex, Voigtlander, and many others providing adapters for virtually any lens mount out there. Even C mount lenses have been successfully adapted to m4/3 (though primarily those designed for 16mm movie cameras and 1"+ sensors). All of the Panasonic and Olympus/OMDS models will work with an adapter out front, though you may have to select something in the menu system to allow the camera to operate without an automated lens out front. Light falloff can be pronounced with smaller lenses (especially C mount and Leica S/M mount lenses, and especially wide angle lenses), but some people like that vignetting effect. Panasonic and Olympus/OMDS cameras have some basic manual focus aids that are helpful, though none that are as helpful as what Sony has provided (customizable peaking), in my experience. The 2x crop of the m4/3 bodies means that wide angle options aren't especially abundant or useful. Even a 15mm Leica lens becomes the equivalent of 30mm on the m4/3 bodies. (That's one reason why some have explored the C mount lenses, where we can find 6, 7, 8mm focal lengths that would be really wide on m4/3.) Frankly, I'm less inclined to use adapted lenses on my m4/3 cameras these days because we have over dozens of very good m4/3 native mount lens options these days. The hassle of adapted lenses (typically manual focus, more trial and error exposure setting, crop factor, size of the lenses) just doesn't appeal to me when we now have two full sets of zooms and a full set of small primes available. I tend to dip into adapted lenses on m4/3 only when something isn't available in the m4/3 mount. 
  
  
• Nikon Z— Perhaps the most flexible mount, as long as automated communications aren't needed. See this list of adapters. Nikon's own FTZ Adapter is excellent at working with legacy F-mount lenses, with the primary exception being that screw-drive autofocus is not supported (older autofocus lenses that pre-date AF-S). We also have a Fringer adapter that can let you use Canon EF lenses with autofocus, and the Megadap adapter that does the same for Sony FE lenses. Both work well with most lenses I've tried.
  
  
• Sony E/FE -- Like Nikon, Sony has supplied an adapter that supports its legacy lenses. Actually, multiple adapters. The first (LA-EA1 and LA-EA3) were just a basic automatic adapter. In my experience, the Alpha mount autofocus primes I've tried with it perform decently in terms of focus speed for acquisition, poorly for tracking. The second batch (LA-EA2 and LA-EA4) includes a phase detect autofocus system and gives near Alpha DSLR performance at the expense of very high battery consumption. If you've got older Sony Alpha lenses (DSLR and SLT versions), you'll probably want one of those adapters. The downside is that these lenses, with adapter, will definitely drawf the camera body, especially with the APS-C cameras. You'll be holding "all lens." The Sony cameras also adapt reasonably well for Leica S/M-mount lenses with third party adapters, though some tend to show corner issues with wide angles that need post processing correction. Sony's "peaking" method of verifying manual focus also works very well, better than all the other aids I've seen on most of the other cameras to date. Coupled with the tilting LCD or EVF on the higher end models, you can frame, focus, and expose with manual focus lenses pretty darned fast, even for street photography. The FE-mount Sonys (A7 series) also work pretty well with most film camera lenses with the right adapter.
  
  
• Fujifilm X — Fujifilm's M-mount adapter allows you to mount a number of M-mount lenses on the camera. Not all such lenses are compatible (see Fujifilm's Compatibility List). But those that are can be corrected for distortion, vignetting, and side-to-side color shift. You'll be focusing these lenses manually, so pay attention to and learn the camera's manual focus capabilities. Frankly, the Fujifilm M-mount adapter has proven to be the best way to use Leica M mount lenses outside a Leica. Yeah, that good. The only real issue is that you've got a 1.5x angle of view factor due to the APS sensor in the X models. This means that there aren't a lot of great truly wide angle options if you go this route. I have a lot of Voigtlander M-mount lenses, and they've all proven to be excellent on the X-Pro1. The other option that a lot of Fujifilm users have taken to is the Fringer adapters for either the Canon EF or Nikon F mounts. These work well with Canon or Nikon lenses, less so for third party lenses in those mounts.
  
  
• Canon EOS M — Canon, like Nikon and Sony, has an all-automatic mount adapter for their DSLR lenses. It supports autofocus, exposure automation, and image stabilization with any EF or EF-S lens. That's the good news. The bad news is that the EOS M's autofocus system is slow even with its own lenses. If you expect DSLR-level experience with the adapter and your DSLR lenses, you'll be disappointed. 
  
  
• Canon EOS RF — Again, Canon has an excellent adapter for EF and EF-S lenses.

The underlying question is this: is it worth it to go the adapter route?

Technically, not usually unless you're adapting a brand's own DSLR lenses to mirrorless. The exceptions I've seen are the Fringer and Megadap adapters that take one autofocus system and adapt it to another. 

Funwise, adapters are absolutely worth it. With perhaps the exception of the manufacturer legacy telephotos (e.g. 4/3 to m4/3, FX/DX to CX, Alpha to E/FE-mount, EF/EF-S mount to EOS M), the primary compelling issue for most people is that they already have some other maker lenses sitting around and they want to see how they perform on their new camera. Great. That can provide endless fun and tinkering.

Some think that used legacy manual focus lenses are inexpensive enough to warrant trying out. I don't dispute that, but the lack of exposure and EXIF automation makes these much more useful for slow or structured shooting, and I'm not 100% convinced that the image quality gains (if any) are worth the hassle.  The one exception I’d make to this is the Fujifilm XF cameras with Leica M-mount lenses. Many of the Fujifilm cameras tend to be slower, deliberate cameras and manual focus fits with its typical user profile. That, plus the fact that Fujifilm spent time figuring out optical corrections for all the Leica lenses is a real plus.

Another usually stated reason is "to provide an option that doesn't exist in the current system" (sometimes in conjunction with the "inexpensive" point). Typically, this means fast aperture, as all the systems now have the basics covered reasonably well. In m4/3, we now have a reasonable set of fast primes and zooms, so I don't judge that m4/3 users really will be able to use this justification for long, if at all. On a Canon EOS M, Samsung NX or Sony NEX, that justification still applies, though. On a Nikon 1, the severe crop factor makes it only interesting for telephoto work.

If you already have some lenses in another mount, the cost of playing with them on your new mirrorless camera isn't terribly high. I've seen adapters anywhere from US$20 (simple C mount for m4/3) to US$350 (sophisticated manufacturer provided legacy adapter). So, for the price of a low-cost lens, you can use your existing lens, which is why so many people are doing it.

Finally, the emergence of the Metabones Speed Booster type of adapter changed the scene a bit, too. A Speed Booster is not just an adapter, but an adapter with optics that attempt to correct for the crop factor. In doing so, they also increase the effective aperture by a stop and often increase the overall sharpness of the adapted lens. Frankly, if you’re interested in using adapters on crop sensor cameras, I’d tend to recommend the Speed Booster approach, even though its more expensive than a plain tube with electronics. The optics in the original Speed Booster were designed by one of the best optical designers in the US, and the results really showed. While there are now many Speed Booster clones coming out of China, none I’ve seen come close to the level of quality the Metabones originals produce. Note that Metabones also makes a full like of simpler adapters that work well, too.