SLRs and mirrorless cameras take interchangeable lenses, but the fitting, or ‘lens mount’, varies from one brand to another, so can’t usually use one brand of lenses on another brand of camera. Independent third-party lens makers offer their lenses in two, three or more different lens mounts to fit different camera brands, but lenses made by camera makers typically only fit their own cameras.
Modern lenses attach to the camera via a twist-lock ‘bayonet mount’. You line up two dots on the lens and the camera body, then twist and lock the lens into place. To remove a lens you press a button no the camera body to release a catch, then turn the lens the other way to remove it.
These bayonet mounts have different diameters, different physical connections on different brands of camera, together with a complex set of electrical connectors that are specific to that brand. Manufacturers may even use different lens mounts for different camera ranges.
For example, Canon uses the EF lens mount for its full frame DSLR cameras, but a modified EF-S mount for its smaller format APS-C DSLRs. You can fit EF lenses to EF-S mount cameras, but not the other way around. Canon introduced a new EF-M mount for its APS-C mirrorless cameras. These take new lenses designed for this mount, though it is also possible to fit Canon DSLR lenses via a lens adaptor.
Lens adaptors, or ‘mount adaptors’, are often used to fit lenses to camera bodies they were not originally designed for. They are available mainly for fitting DSLR lenses on to mirrorless camera bodies, and they work because the flange distance – the distance from the rear of the lens to the sensor – is smaller on a mirrorless camera, which leaves room to fit a lens adaptor between the lens and the body. Otherwise, lens adaptors can only be used if the rear lens mount is narrower than the lens mount on the body.
To return to Canon, the company introduced a fourth lens mount – the RF mount – with the full frame mirrorless camera range it launched in 2018. This has brought its own range of native RF lenses, though again it’s possible to use EF/EF-S DSLR lenses with an adaptor.
Canon’s great DSLR rival Nikon has been using the same Nikon F mount for its DSLR cameras since 1959, though this has been steadily modified over the years so that although early lenses will still fit, the latest autofocus and exposure options are available only with the latest camera bodies and lenses. Like Canon, Nikon makes both APS-C and full frame DSLRs. Nikon calls these ‘DX’ and ‘FX’ format cameras respectively, but they both use the same lens mount and you can use full frame FX lenses on DX cameras and vice versa – though attaching a DX lens will make an FX camera swap to a ‘crop mode’ which uses an APS-C sized area of its sensor.
Like Canon, Nikon has introduced a range of full frame mirrorless cameras with a new lens mount. The new Nikon Z-mount is wider than the Nikon F mount with a shorter flange distance, and while it comes with a new range of native Z-mount lenses, it is possible to fit Nikon DSLR lenses via an adaptor and retain all their autofocus and exposure capabilities.
Pentax has a similar arrangement. Like Nikon, Pentax has been using its own lens mount – the Pentax K/KA mount – since before the advent of digital cameras, but it has steadily evolved with new electronic connections and capabilities. Pentax also makes both APS-C and full frame cameras. You can use the same lenses on both, but a lens designed for an APS-C camera will be used in ‘crop’ mode on a full frame camera. Pentax also makes a medium format camera, the 645Z, which has its own, larger Pentax 645 A mount.
The situation is quite complicated in the Sony range. For a while, Sony made SLT (single lens translucent) cameras using a sort of hybrid mirrorless/DSLR designed with a fixed translucent mirror. These are still available, though in decline, and use the Sony A-mount. Again, these cameras came in APS-C and full frame formats, with lenses designed for each.
Sony is now committed to mirrorless cameras, and introduced its own E-mount lens mount for these. Later, it added full frame mirrorless cameras to the line-up and these use the Sony FE mount to distinguish them from the smaller format cameras. In fact, however, the mounts are identical and E-mount and FE mount lenses are interchangeable.
What does make things confusing with Sony is that it calls both its older SLR cameras and its new mirrorless models ‘Sony Alpha‘ cameras.
Fujifilm cameras are simpler. Its APS-C format X-series mirrorless cameras all use the same X-mount lens mount. Fujifilm has however introduced a range of medium-format GFX cameras which use a new, larger G mount.
All these different lens mounts are confusing and irritating for consumers, but makers have at one time or another joined forces develop lens mounts jointly.
The Micro Four Thirds format was developed by Olympus and Panasonic and is used to this day by both makers. It’s even possible to use Olympus lenses on Panasonic cameras and vice versa.
More recently, Panasonic, Leica and Sigma have joined forces to create the L-mount alliance, where Panasonic and Leica full frame mirrorless cameras use the same lens mount (developed by Leica) and Sigma’s optical expertise has been used to quickly build a wide range of compatible lenses. Sigma has also announced its own full frame mirrorless camera, also using the L-mount.