A lens mount is the handshake between camera and glass: a bayonet, a set of electrical contacts, and a very precise distance between the mount face and the sensor. Nothing in photography is more irritating than discovering the lens you have just bought will not sit on your body — and few things feel better than realising that an old prime in a drawer will.

Most of the confusion comes down to three things: the physical mount, the flange distance behind it, and whether the electronics can talk to each other. Get those straight and buying lenses stops being a gamble.

What a lens mount actually is

Physically, most mounts are bayonets. Three or four metal lugs on the lens drop into matching slots on the body, and a short twist locks them in place. Older systems used screw threads or breech-lock rings, which is why a 1970s M42 lens needs nothing more exotic than a threaded adapter.

The interesting part is what travels through the mount. On a modern mirrorless body, the contacts carry autofocus commands, aperture control, stabilisation data and EXIF information such as focal length. On a lens with a manual aperture ring, you are the electronics. That distinction shapes everything about adapters later on.

The two families: DSLR and mirrorless

DSLR mounts

Digital SLRs inherited mounts designed around a mirror box, and the names are familiar: Canon EF and EF-S, Nikon F in its various forms, Pentax K, Sony A, Four Thirds. These mounts sit a long way from the sensor because the mirror needs room to swing up and down. That extra space is the reason you can adapt these lenses so easily onto newer bodies.

Mirrorless mounts

Mirrorless cameras removed the mirror, so the mount could sit much closer to the sensor. That gave designers more freedom with wide-angle and fast lenses, and it created the modern line-up:

• Canon: EF, EF-S (DSLR) and RF, RF-S (mirrorless)
• Nikon: F (DSLR) and Z (mirrorless)
• Sony: A (DSLR era) and E (mirrorless, shared with many third-party makers)
• Fujifilm: X, an APS-C mirrorless mount
• Micro Four Thirds: used by Panasonic, OM System and others, with a 2x crop
• Leica L: a shared full-frame mirrorless mount used by several brands

Flange distance: the number that decides what fits

Flange distance — sometimes called register distance — is measured from the face of the mount to the sensor. Nikon F is roughly 46.5mm, Canon EF about 44mm, Sony E around 18mm, Nikon Z around 16mm. Those few millimetres explain almost all adapter compatibility.

A longer flange distance can be adapted to a shorter one. The reverse only works if the adapter contains glass, and that glass costs you image quality.

So a Nikon F lens on a Nikon Z body needs an adapter around 30mm thick to make up the difference in spacing. A Sony E lens on a Canon RF body would need a negative-thickness adapter, which is why no such thing exists. Canon EF glass on Sony E, or Nikon F glass on Fujifilm X, works fine with the right tube in between.

Adapters, and when they work

Adapters fall into three broad types, and it pays to know which one you need before you order.

• Dumb adapters are a metal tube with no contacts. You focus by hand and set aperture on the lens, if it has a ring. Perfect for legacy glass, useless for lenses with electronically controlled apertures.
• Electronic adapters pass signals through, so autofocus, aperture control, stabilisation and EXIF data keep working. Quality varies enormously, and firmware updates sometimes matter for newer bodies.
• Focal reducers, often sold as speed boosters, contain glass to concentrate a full-frame image onto a smaller sensor. You gain roughly a stop of light and your wide angles stay wide, but corners can soften and edge performance often drops.

What adapters cannot fix

Screw-drive autofocus lenses rely on a motor inside the camera body. Nikon AF and AF-D lenses, and several older Minolta and Pentax designs, work this way. Very few adapters include such a motor, so those lenses become manual focus on mirrorless bodies. The lens will still take photographs; it just will not do the focusing for you.

Also worth knowing: some adapters with glass elements, sold cheaply for mounting one brand's DSLR lenses on another brand's DSLR, tend to soften images and reduce light. Treat them as a compromise, not a solution.

Crop factor and the labels on used lenses

Crop-sensor lenses project a smaller image circle. Canon calls them EF-S or RF-S, Nikon DX, Sigma DC, Tamron Di II. They mount and work on crop bodies, but on a full-frame body you will get dark corners, heavy vignetting, or the camera will drop into crop mode and quietly throw away pixels.

Full-frame lenses on crop bodies are harmless. You simply use the middle of the image circle and lose some wide-angle coverage — a 24mm lens looks like roughly 36mm on APS-C.

Before you buy: a five-point check

1. Read the mount name stamped on the lens barrel or rear cap, not the wording in the listing title.
2. Check the flange distance works without glass if you intend to adapt.
3. Confirm whether autofocus and aperture control are electronic or mechanical.
4. Establish whether the lens is full frame or crop only, and match it to your sensor.
5. Work out where stabilisation lives — in the lens or the body — and whether your camera can use it.

Matching a lens to your camera in practice

Start with your own mount. Write it on a note in your phone, because it saves you from half-hearted guessing in a shop or an online listing. Canon RF, Nikon Z, Sony E, Fujifilm X, Micro Four Thirds — whatever it is, that is the target.

If you shoot one system and value convenience, buy native lenses. Autofocus is faster, weather sealing lines up properly, and you avoid the small wobbles and firmware quirks that come with third-party adapters.

If you are adapting, buy a good adapter once rather than three cheap ones. Test it at infinity focus on the day it arrives, and test the widest aperture you own. Return anything that wobbles or will not reach infinity.

And if you are weighing up a used bargain, ask two questions before handing over money: which mount is it, and does it have an aperture ring? Those answers tell you whether you are buying a lens or a project.

Photo: cuncon / Pixabay