Fast Prime

A fast prime is a fixed focal length lens with a maximum aperture of f/1.4 or wider. The term fast refers to the ability to use a faster shutter speed at the same ISO as a slower lens in identical lighting. Fast primes are used for photography and video in low light without flash, for portraits where shallow depth of field and subject separation from the background are desired, and for video work where raising ISO is undesirable.

On M43, fast prime options span a wide range of focal lengths. OM System produces f/1.2 autofocus primes at 17mm, 25mm, and 45mm. Panasonic and the Leica-branded M43 line include f/1.2 and f/1.4 primes. Sigma has produced fast primes for M43 in the Art series. Voigtlander's Nokton series offers manual focus M43 native primes at f/0.95, the widest apertures available in the M43 system. Third-party brands including 7Artisans and TTArtisan offer fast prime options at f/1.4 and f/1.8 at lower prices.

Depth of field on M43 fast primes is affected by the 2x crop factor. A 25mm f/1.4 M43 prime gives a 50mm full-frame equivalent field of view but produces depth of field equivalent to approximately f/2.8 on full-frame at the same framing. To approach the shallow depth of field of a full-frame f/1.4 or f/1.8 prime, M43 users need f/1.2 or f/0.95 lenses. Background separation is clearly visible with M43 fast primes at wide apertures but does not match the extreme shallow focus of full-frame f/1.2 lenses.

Fast primes typically deliver higher optical quality at a given aperture compared to zoom lenses, because the optical design can be optimised entirely for one focal length. Many premium M43 primes, including OM System's f/1.2 Pro series, achieve sharp results at or near their maximum aperture with well-controlled aberrations. The trade-off compared to a zoom is the lack of focal length flexibility, requiring the photographer to move physically to adjust framing.