Metabones, a company that specializes in camera adapters and related accessories, along with optics development company Caldwell Photographic, have recently announced an adapter called the Speed Booster. Sitting between a mirrorless camera and an SLR lens, it claims to increase maximum aperture by a stop and increase MTF (Modulation Transfer Function is often used by lens makers to describe lens performance).
As explained by a whitepaper (PDF) from Brian Caldwell (of Caldwell Photographic) and Wilfried Bittner (of WB Design), the Speed Booster is possible thanks to the short flange back distance (that is, the distance from the lens flange to the sensor plane) of mirrorless cameras. With a 0.71x magnification, the Speed Booster reduces the focal length of an SLR lens when its mounted to a mirrorless camera, and this is the main contributor to the claimed MTF improvement. Other specs include a maximum input and output apertures of F1.26 and F0.9 respectively. Thus, if you were to use a Canon EF 85mm F1.2 L II lens on a Sony NEX camera, you’ll get a 59mm F0.9 lens. The benefits of the Speed Booster focal reducer are obvious: You now have the opportunity to use a faster shutter speed (rather than increasing the ISO sensitivity, which introduces more noise) and achieve a shallower depth of field. At the same time, you get an increased field of view.
Currently, you can order ALPA lens to Fuji X, ALPA lens to Sony NEX, Canon EF lens to Sony NEX, Leica R lens to Fuji X, and Leica R lens to Sony NEX Speed Boosters directly on the Metabones website. Prices range from US$399 to US$599. Other lens/mount combinations are expected to be added in the future, but no exact time frame is given.