Optical adapter that shortens a lens's focal length, expanding the image circle and increasing light transmission.
Technical Details
Focal reducers consist of a multi-element optical group with 4-6 elements that compresses the light beam before the sensor. For example, the Metabones Speed Booster reduces the focal length by 0.71x and improves the light intensity by 1.3 stops. With a 50mm f/2.8 lens, this results in an effective focal length of 35.5mm at f/2.0. The flange distance between the lens mount and the sensor determines compatibility – Canon EF to Sony E-mount requires 31mm of space for the reducer optics.
Electronic versions with aperture control and passive versions without electronic communication are available. Anamorphic focal reducers like the SLR Magic Anamorphot additionally compress the image horizontally by a factor of 1.33x.
History & Development
Metabones introduced the first cinematic focal reducer in 2013 with the Speed Booster, developed by Zeiss optics designer Caldwell Photographic. The technology adapted findings from astrophotography for film production. In 2015, Viltrox and other manufacturers followed with more affordable alternatives. Modern versions since 2019 support full electronic lens communication, including autofocus and image stabilization.
Practical Use in Film
Focal reducers allow the use of full-frame lenses on Super35 sensors without a crop factor. Roger Deakins used Speed Boosters for "Blade Runner 2049" to optimally utilize Zeiss Master Primes on an ALEXA Mini. In documentaries, they compensate for smaller sensor sizes of cameras like the BMPCC 4K. The light advantage reduces ISO values and thus image noise in low-light shoots.
Disadvantages include slight softness at the edges, vignetting with wide-angle lenses under 24mm, and additional reflections from extra glass surfaces. The mechanical length can become problematic in compact rig setups.
Comparison & Alternatives
Compared to normal mount adapters, focal reducers offer an optical advantage but incur higher costs (€300-800 vs. €50-150). Speed boosters differ from teleconverters, which extend rather than shorten the focal length.
Alternative approaches include native lenses for the respective camera system or cameras with larger sensors. Modern full-frame cameras like the Sony FX6 or Canon C500 Mark II make focal reducers partially obsolete, as they already utilize the full lens performance.
Current News
The Zhongyi Lens Turbo II is frequently discussed as a focal reducer for the Blackmagic Pocket Cinema Camera 4K. This adapter combination allows full-frame lenses on the Super35 camera—this reduces the crop factor and increases the light intensity. Users report practical experiences with this cost-effective alternative to more expensive speed booster solutions.