Camera lens-mount × adapter terminology
Lens-mount glossary
Plain-language definitions of the terms used across this site — flange focal distance, image circle, crop factor, focal reducers (a.k.a. Speed Boosters), infinity focus, autofocus levels, image stabilisation, and aperture control. Every entry cross-links to the matrix cells and per-mount pages where the term applies in practice.
Mount geometry
- Flange focal distance
- The distance from the lens-mount flange to the sensor (or film) plane. Often called register distance or flange-back distance. A lens is designed to focus at this distance — adapting it onto a body with a longer flange pushes it beyond infinity focus (or past macro range), while a shorter flange leaves clearance that a mechanical adapter can occupy without optics. Every mount's flange distance is listed in the compatibility matrix and the per-mount mount index.
- Image circle
- The circle of light a lens projects onto the sensor. A full-frame lens projects a ~43.3 mm circle (the diagonal of a 36 × 24 mm sensor); an APS-C lens projects a smaller ~28 mm circle. When the image circle is smaller than the body sensor the corners vignette to black; when it's larger, the body crops to the centre and the effective focal length is multiplied by the crop factor. The matrix flags both cases via the
vignettesandN× cropbadges. - Crop factor
- The ratio between a sensor's diagonal and the 35 mm full-frame diagonal (43.3 mm). APS-C is ~1.5× (Nikon, Sony, Fuji) or ~1.6× (Canon); Micro Four Thirds is 2×; full-frame is 1×. Multiplying a lens's focal length by the crop factor gives the equivalent angle of view on a 35 mm body — a 50 mm lens on a 1.5× APS-C body frames like a 75 mm on full-frame. The matrix shows the crop factor on every cell where the lens and body formats differ.
- Vignetting
- Darkening — often to full black — at the corners of the frame because the lens's image circle is smaller than the body sensor. Common when adapting APS-C / DX / EF-S lenses onto a full-frame body without auto-crop. Some bodies auto-switch into a smaller crop region to suppress it; others leave the corners black. Flagged as
vignetteson the matrix.
Adapter types
- Mechanical adapter
- A passive ring that converts one mount's bayonet geometry to another — no electronic contacts, no optics. It spans the flange gap so the lens reaches infinity focus. AF, IS and aperture become manual. Fine for fully manual lenses (Olympus OM, M42, Leica M) and a frequent compromise for photographers willing to give up autofocus.
- Smart adapter
- An adapter with electronic contacts on both faces, passing AF drive, IS data, and aperture commands between body and lens. Examples: Canon EF-EOS R, Nikon FTZ II, Sony LA-EA5, Sigma MC-11, Viltrox EF-NEX III. An electronic-mount lens on a smart adapter behaves like a native mount: full AF, full IS, and electronic aperture.
- Focal reducer (Speed Booster)
- An adapter with optical elements that compress the lens's image circle to better fit a smaller sensor. Widens the effective angle of view (typically ×0.71 for APS-C, ×0.64 for MFT) and gathers an extra stop of light. Required when a long-flange lens is adapted onto a shorter-flange mirrorless body but the geometry leaves no clearance for a plain mechanical adapter. Sold as Metabones Speed Booster, Viltrox, Kipon Baveyes, Mitakon Lens Turbo. Flagged as
Speed boosteron the matrix. - Infinity focus
- The ability to focus on subjects at infinity (the horizon, the moon). Possible only when the total optical path from rear element to sensor equals the lens's designed flange focal distance. A mechanical adapter that's too thick keeps the lens always focused closer than infinity; too thin and the lens can't focus past macro range. A focal reducer can absorb up to ~12 mm of flange deficit while preserving infinity — see the M42 → mirrorless case study.
Autofocus
- Phase-detection AF (PDAF)
- Autofocus that compares the phase of light from two viewpoints on the lens's aperture to compute focus direction and magnitude in one read — fast and predictive, the basis of continuous AF. Modern mirrorless bodies host PDAF pixels on the imaging sensor itself, so adapting a DSLR lens via a smart adapter usually retains on-sensor PDAF with slight performance penalties (a few % slower acquisition, occasional hunting on long telephotos).
- Contrast-detection AF
- Autofocus that ramps the lens focus while sampling image-contrast peaks at the sensor. Slower than PDAF (the lens must overshoot then confirm the peak) but accurate on stationary subjects. Older mirrorless and DSLR Live View modes are contrast-only; modern mirrorless combines PDAF + contrast as “hybrid AF”.
- Manual focus (MF)
- Focusing by turning the lens's focus ring with no body-driven motor. The only option for mechanical-only lenses on any body, and for screw-drive AF-D lenses on mirrorless bodies that lack the screw-drive coupling (Nikon Z, Sony E adapters). Modern bodies provide focus peaking and magnified view to assist MF.
Image stabilisation
- IBIS (in-body image stabilisation)
- A stabilisation system inside the camera body that shifts the imaging sensor on 3 to 5 axes to compensate for hand-shake. Works with any lens — including manual vintage glass on a mechanical adapter — but is most effective when the body knows the lens's focal length and aperture (a smart adapter passes those through; otherwise enter focal length manually on the menu).
- Lens IS / OIS (optical image stabilisation)
- A stabilisation system inside the lens that shifts a floating element to compensate for hand-shake. Canon calls it IS, Nikon VR, Sony OSS, Sigma OS, Tamron VC, Panasonic Power OIS. Survives a smart adapter; a mechanical adapter usually leaves it dead because the lens cannot tell the body when to engage it.
Aperture control
- Electronic aperture
- An aperture controlled entirely by the camera body over the electronic mount protocol — no aperture ring on the lens. Every Canon EF, RF, and EF-M lens uses electronic aperture, as do most modern Nikon F, Z, Sony E, and Fujifilm X / GFX lenses. A smart adapter carries the aperture command through; a plain mechanical adapter leaves the lens stuck wide-open (or requires an adapter-side aperture wheel as a workaround).
- Mechanical aperture (aperture ring)
- A physical ring on the lens that opens and closes the aperture diaphragm by mechanical linkage. Standard on every pre-1990s SLR lens (Olympus OM, M42, Pentax K, Nikon AI/AI-S) and many modern manual primes (Voigtländer, Zeiss ZF.2, Samyang, TTArtisan). Survives every adapter — the ring opens or closes the diaphragm independent of any electronics.