Lens Mount

A lens mount is the connection between a camera body and an interchangeable lens. It sets the shape of the locking ring, the distance from the mount to the sensor (the flange focal distance), the width of the opening that light passes through, and the electrical contacts that carry power and data. A lens only fits a camera with the same mount, or with an adapter that bridges two mounts.

Every camera maker uses its own mount for its own cameras, and most makers have changed mounts at least once, usually when moving from film to digital or from mirror cameras to mirrorless cameras. This page explains the types of lens mount, what flange distance and throat diameter mean, why some lenses adapt easily to other cameras and others do not, and how to find out which mount you have.

Types of lens mount

There are three ways a lens can lock onto a camera.

• Bayonet mount. The lens has metal tabs that pass through matching gaps in the camera’s mount. You line up the index marks, push the lens in and turn it, usually less than a quarter turn, until a spring-loaded pin clicks into a notch. A release button pulls the pin back. Almost every interchangeable-lens camera made today uses a bayonet, because it is fast, precise and holds the lens at the same position every time.
• Screw mount (thread mount). The lens screws into a threaded ring on the body. Screw mounts were common on film cameras. They are simple and cheap but slow to change, and the lens does not always stop at the same rotation, which makes it hard to pass aperture or focus information to the camera.
• Breech-lock mount. A rotating collar on the lens or body clamps the lens in place while the lens itself stays still. It combines the precision of a bayonet with no rotation of the lens against the mount. Few cameras use it now.

Flange focal distance

The flange focal distance (also called flange distance or registration distance) is the distance from the mounting surface of the camera to the sensor or film. Each lens is designed to sit exactly that far from the sensor. If it sits even a fraction of a millimeter too far forward or back, it will not focus correctly at the ends of its range, most noticeably at infinity.

The flange distance depends mostly on what is inside the camera:

• SLR and DSLR cameras have a mirror that swings up in front of the sensor, so the lens has to sit far enough forward to clear it. Most of their mounts have flange distances of roughly 42 to 47mm.
• Mirrorless cameras have no mirror box, so the lens can sit much closer to the sensor. Most of their mounts have flange distances of roughly 16 to 20mm, with medium format mirrorless mounts a little longer.
• Rangefinder cameras also have no mirror, and their mounts sit in between, at around 28mm.

A short flange distance makes the body thinner and gives lens designers more freedom, particularly for wide-angle lenses, which no longer need extra elements to push their optics forward past a mirror.

Throat diameter

The throat is the opening inside the mount that light passes through on the way to the sensor. Its diameter, together with the flange distance, limits how large the rear element of a lens can be and how steeply light can reach the corners of the sensor. A wider throat makes very fast lenses (wide apertures such as f/1.2 or faster) easier to design and helps light reach the edges of the frame evenly. A throat that is small for the sensor behind it makes corner sharpness and even illumination harder, and contributes to vignetting with fast lenses.

Electronic contacts: what passes through a mount

The small gold pins on a modern mount carry power from the camera to the lens and data in both directions. Through them the camera can:

• drive autofocus and read the focus distance;
• set the aperture, which on most modern lenses has no mechanical ring;
• coordinate lens-based and body-based image stabilization;
• apply lens corrections for distortion, vignetting and color fringing;
• record the lens, focal length and aperture in the photo’s EXIF data.

Older mounts did some of this mechanically, with levers and pins that stopped the aperture down or coupled the light meter. A lens that relies on those mechanical links may lose them when adapted, and a lens that relies on electronic control may lose aperture and focus control entirely on an adapter that does not pass the signals through.

Same mount, different sensor sizes

Many mounts serve cameras with more than one sensor size. A lens made for a smaller APS-C sensor projects a smaller image circle. On most such mounts it will physically fit a full-frame body, but the image circle will not cover the whole sensor. Many cameras switch to a crop mode automatically, using only the middle of the sensor; otherwise you see dark corners. Lenses made for full frame work on the smaller sensor without any problem, with the narrower field of view described by the crop factor.

Wondering why the same lens looks longer on a smaller sensor? Crop factor explains the multiplier and what it does and does not change.

Adapting lenses between mounts

Whether a lens can be adapted to another camera depends almost entirely on flange distance.

• Lens with a longer flange distance onto a camera with a shorter one: usually easy. The adapter is a precisely machined tube that holds the lens exactly as far from the sensor as its own camera would. This is why mirrorless cameras, with their short flange distances, can take adapters for almost every SLR and rangefinder lens ever made. Adapting vintage lenses covers how to do it well.
• Lens with a shorter flange distance onto a camera with a longer one: usually not possible. The lens would need to sit closer to the sensor than the camera’s own mount allows. The only ways around it are an adapter with a glass element, which works like a weak teleconverter: it lengthens the focal length, costs a little light and some image quality, or giving up infinity focus and using the lens only for close-up work.
• Similar flange distances: rarely possible. When the two distances are within a millimeter or two, there is no room for an adapter, so it cannot be done without modifying the lens.

The other question is what still works. A fully manual lens with its own aperture ring works on almost any adapter, with manual focus and the camera metering at the working aperture. A lens without an aperture ring needs either an adapter with its own aperture control or an electronic adapter that passes signals through. Electronic adapters between systems can give autofocus and stabilization, but focusing is often slower and some features may not work. Adapters made by the camera’s maker for its own older lenses usually give the most complete support.

Open and shared mount standards

Most mounts belong to one camera maker, but a few are shared standards with published dimensions that any manufacturer can build to:

• M42: Type: Screw, 42mm diameter, 1mm thread pitch. Flange distance: 45.46mm. Where you find it: Many film-era SLRs and their lenses, now widely adapted.
• T-mount (T2): Type: Screw, 42mm diameter, 0.75mm thread pitch. Flange distance: 55mm. Where you find it: Telescopes, mirror lenses and other optics, with an adapter ring per camera.
• C-mount: Type: Screw, 1 inch diameter, 32 threads per inch. Flange distance: 17.526mm. Where you find it: Small cine cameras, security and machine-vision cameras.

Some camera makers also share a bayonet mount with other camera and lens makers by agreement, so lenses from several brands fit the same bodies.

How to find out which lens mount you have

1. Check the lens and camera markings. The mount name is usually printed on the lens barrel, the lens box or near the mount on the camera body.
2. Look up the camera’s specifications. The manual or specification sheet always lists the lens mount.
3. Look at the mount itself. The number and shape of the bayonet tabs, the color of the index dot, and the number of electrical contacts differ from mount to mount.
4. Check the lens description before buying. Many lenses are sold in versions for several mounts. Make sure the version matches your camera, and whether it is designed for full frame or a smaller sensor.

Why the mount matters when choosing a camera

Lenses usually last longer than camera bodies, and a collection of lenses ties you to its mount. Moving to a different system later means selling or adapting those lenses. Before choosing a camera, it is worth checking which lenses are available for its mount (from the maker and from independent lens makers), whether the focal lengths you want exist, and whether lenses you already own can be adapted. What lens to buy first covers how to plan a lens collection, and mirrorless vs DSLR compares the two kinds of body.

Need more reach from the lenses you already have? Teleconverters explains what a 1.4x or 2x converter adds between lens and mount, and what it costs in light.

Common mistakes

• Buying a lens in the wrong mount version. Fix: many lenses come in several mounts. Check the exact mount name against your camera before you buy.
• Assuming an adapter keeps every feature. Fix: check what the adapter supports (autofocus, aperture control, stabilization, EXIF data) and expect slower focus with cross-system electronic adapters.
• Buying a cheap adapter of the wrong thickness. Fix: an adapter that is slightly too thick stops the lens from reaching infinity focus, and one that is too thin can make the focus scale wrong. Test it on a distant subject.
• Forcing a lens. Fix: line up the index marks first. A bayonet lens should slide in and turn smoothly; resistance means something is misaligned.
• Leaving the mount open. Fix: keep a body cap on the camera and a rear cap on the lens, and change lenses with the camera pointing down to keep dust off the sensor.

Frequently asked questions

What is a lens mount on a camera?

It is the ring on the front of the camera, and the matching ring on the back of the lens, that locks the two together. It holds the lens at the right distance from the sensor and carries power and data between them.

Are lens mounts universal?

No. Each camera maker uses its own mount, and many makers have more than one. Lenses are made in different versions for different mounts, and adapters can join some combinations but not others.

What is the most common camera lens mount?

There is no single standard. The most widely used mounts are the current bayonet mounts of the largest camera makers. Among open standards, the M42 screw mount was used by many film cameras, which is why so many old M42 lenses are still adapted today.

What is flange distance?

It is the distance from the mount’s surface to the sensor. Most mirrorless mounts are around 16 to 20mm, while most SLR mounts are around 42 to 47mm because the lens has to clear the mirror. A lens can be adapted to a camera with a shorter flange distance, but not usually to one with a longer distance.

Can I use a lens from one brand on another brand’s camera?

Often, with an adapter, as long as the lens was designed for a longer flange distance than the camera has. Manual lenses adapt most easily. Autofocus lenses need an electronic adapter, and some features may be slower or missing.