Metabones and Caldwell Photographic announced their collaborative new camera/lens accessory called the Speed Booster. The Speed Booster is an adaptor that mounts between a mirrorless camera and an SLR lens. It promises to increase the aperture by 1 stop and has a focal length multiplier of 0.71x. This means you could attach a Canon 50mm lens by way of the Speed Booster adaptor to a Sony NEX camera (with a 1.5x crop factor this lens would usually become a 75mm lens) and have that lens effectively become 53.25mm. By using this attachment, the company states that you will have a faster f-stop which will give you a shallower depth of field and require a lower ISO setting. In addition, you are receiving almost the same focal length as if you were using that lens on a full frame camera.
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Currently, the Speed Boost adapter has a limited number of camera-to-lens combinations being released in January 2013. As of this release ALPA and Leica R lenses are able to be combined on Fuji X and Sony cameras. An adaptor for Canon lenses and the Sony NEX camera is also available. The company promises that their collection will grow to include ALPA, Contarex C/Y, Leica R, and Nikon F (with aperture control for G lenses) lenses. They will also add Micro Four Thirds mounts to their line-up. The Speed Booster ranges from $399-$599 depending on the adapter needed.
Not looking for a Speed Booster, but want to attach SLR lenses to mirrorless cameras? You're in luck. Metabones also provides less expensive adaptors to accomplish this task.