Specialist accessory manufacturer Metabones and optics company Caldwell Photographics have jointly announced the 'Speed Booster', a lens adapter for for mounting SLR lenses on APS-C and Micro Four Thirds mirrorless cameras that reduces the focal length by a factor of 0.71x and increases the maximum aperture by 1 stop. The Speed Booster also promises sharper images compared to using the lens with a simple adapter. The first version will allow use of Canon EF lenses on Sony NEX bodies, and will be available this month from Metabones' web site for US$599. Support for additional lens mounts (including Nikon F) and camera systems (including Fujifilm X and Micro Four Thirds) is also planned.

In a technical white paper on the Metabones web site, designers Brian Caldwell and Wilfried Bittner set out the principles behind the Speed Booster and its practical advantages in some detail. In essence it's the opposite of a teleconverter - rather than increasing the focal length and reducing the aperture, it decreases the focal length and increases the aperture. This also promises sharper images compared to using the same lens with a simple, non-optical adapter, as the visibility of aberrations is reduced.

The wideangle converter is made possible by the short flange (i.e. lens mount to sensor) distance of mirrorless cameras, which allows the addition of focal length reducing optics to the space within the adapter between the lens and camera body (in an SLR, the mirror is in the way). It has the effect of shrinking the lens's image circle, which means that it only works with optics designed for larger formats. All full frame lenses should work on APS-C cameras, but lenses designed for APS-C SLRs will likely vignette, although they should work on Micro Four Thirds bodies.

The Speed Booster uses a 4 element/ 4 group optical system, and works with lenses as fast as F1.2. The additional optics mean that it's actually shorter than a conventional adapter, resulting in a more compact overall system. On APS-C mirrorless cameras it offers an effective crop factor of 1.1x, meaning a lens will behave very similarly to how it does on full frame; on Micro Four Thirds the effective crop factor is 1.4x.

The adapter's precise functionality will depend on the lens and body used. The version to use Canon EF lenses on Sony NEX bodies will support electronic aperture setting and image stabilisation, along with (slow) autofocus when used with newer Canon lenses. However the Nikon F mount adapter offers only mechanical aperture setting with G-type lenses; there's no promise of either AF or VR at the moment. 

Press release:

Metabones and Caldwell Photographic introduce Speed Booster

Petersburg, VA, USA, January 14, 2013 - Metabones® and Caldwell Photographic jointly announce a revolutionary accessory called Speed Booster™, which mounts between a mirrorless camera and a SLR lens. It increases maximum aperture by 1 stop (hence its name), increases MTF and has a focal length multiplier of 0.71x. For example, the Canon EF 85mm f/1.2L II lens becomes a 59mm f/0.9 lens on a Sony NEX camera, with increased sharpness. The faster F-stop allows for shallow depth-of-field and a lower ISO setting for decreased noise.

Speed Booster is also particularly pertinent to ultra-wide-angle SLR lenses. The combined focal length multiplier of Speed Booster and an APS-C mirrorless camera is approximately 1.09x, making the combination almost "full-frame". Full-frame ultra-wide-angle SLR lenses largely retain their angle-of-view on an APS-C mirrorless camera when Speed Booster is used.

The optics of Speed Booster is designed by Brian Caldwell, PhD, a veteran of highly-corrected lens designs such as the Coastal Optics 60mm f/4 UV-VIS-IR APO Macro lens with exemplary MTF performance (focusing done with visible light requires no correction whatsoever for the full spectrum from UV to IR).

Speed Booster serves double-duty as a lens mount adapter, from Canon EF lens (but not EF-S) to Sony NEX, with auto-aperture, image stablization, EXIF and (slow) autofocus support for late-model (post-2006) Canon-brand lenses. It will be available in January 2013 from Metabones' web site and its worldwide dealer network for US$599 plus shipping and applicable taxes and duties.

Other mount combinations will follow shortly afterwards. Leica R, ALPA, Contarex, Contax C/Y and Nikon F (with aperture control for G lenses) will be supported, as will Micro 4/3 and Fuji X-mount cameras. Support for other mounts will be added in the future.

To learn more details about this revolutionary technology, read the Speed Booster White Paper on Metabones' web site.

Specifications

 Magnification  0.71x 
 Maximum input aperture  f/1.26
 Maximum output aperture  f/0.90
 Lens elements/groups  4/4
 Objective lens mounts  Canon-EF, Nikon-F
 (Leica-R, Contax C/Y, Contarex, Alpa planned)
 Capabilities, Canon-EF version  Electronic iris control, AF and IS
 Capabilities, Nikon-F version  Manual iris control of G-type lenses
 Camera mounts  Sony NEX, Micro Four Thirds (Fujifilm X planned)
 Length reduction (Sony NEX version)  4.16mm
 Length reduction (Micro Four Thirds)  6.17mm
 Dimensions Sony NEX version
 (diameter x length)
 69mm x 27mm (with tripod mount removed)
 Weight Sony NEX version  194 grams
 Tripod mount  Removable type with Arca Swiss compatibility