This is the distance between the mounting plate on a camera that takes interchangeable lenses and the sensor itself. Mirrorless cameras have a shorter flange distance because there’s no mirror inside the body, and this makes the cameras slimmer. DSLRs have a longer flange distance because there needs to be space inside the body for the mirror that this design gets its name from. This makes DSLR bodies thicker. This difference in flange distances means that it’s sometimes possible to use lens adaptors to fit lenses of a different type, brand or lens mount to a camera. This generally works one way only – you can mount a lens with a longer flange distance (e.g. a DSLR or old film SLR lens) on a camera with a shorter flange distance (e.g. a mirrorless camera) but not the other way round.
In principle, you can’t mix and match different types and brands of lenses with different camera bodies. Each camera maker uses its own bespoke lens mount and different mechanical and electronic connections between the camera body and lens. However, it’s often possible to make lenses fit different brands and types of bodies with lens adaptors. These are usually from third-party makers and designed for users who don’t mind a few compromises in camera functions. For example, you may lose autofocus functions and have to use manual focus only, and it’s likely that you’ll have to use manual exposure and lens aperture control rather than the camera’s full range of exposure controls.
This is the lens mount for Fujifilm’s X-series mirrorless cameras. These include the Fujifilm X-T2, X-H1, X-E3 and others. Any X-mount lenses can be used on any X-mount camera, though note that Fujifilm’s medium format GFX 50S uses a different mount and different lenses.
The lens mount used by Sony’s Alpha SLT cameras. Because these cameras have a mirror in the body, even though it’s a non-moving one, the rear of the lens is further from the sensor, so Alpha mount lenses are physically different to Sony’s E-mount lens range. You can use Alpha lenses on an E-mount camera with an lens adaptor, but not the other way round.
This is a sensor and lens format used by Olympus and Panasonic for their mirrorless camera ranges. The MFT sensor measures 17.3 x 13.0mm, so it’s smaller than the APS-C sensors used in rival mirrorless cameras. This does have a modest effect on overall image quality, but the payback is the both MFT cameras and lenses are substantially smaller and lighter than rival APS-C models. The MFT format also has a slightly squarer 4:3 (four-thirds) aspect ratio, which some photographers might prefer.