Camera Lens Mounts Explained

Rajib Mukherjee Avatar
Rajib Mukherjee
18 January, 2023 • Updated 1 year ago
Camera Lens Mounts
Camera lens mounts are mechanical and electrical interfaces that allow a connection between the camera body and the lens. Camera lens mounts are primarily a feature of interchangeable lens cameras – mirrorless, DSLRs, and rangefinders.Lens mounts also allow the attachment of other optical tools like mount adapters, enabling the attachment of a lens or other optical device.

Back in the old days, when film cameras ruled, the roost camera lens mounts were merely holes with mechanical connections that ensured that a lens was firmly attached to the camera body. The lens mount had little other function without autofocusing and metering, aperture control, and such advanced technology.

However, now with autofocusing and metering and other functions, it has become necessary for the lens mount to have electronic connections that establish a link between the lens and the camera body.

Also Read: Camera Lens Filters Explained


There are a few specifications that are important in order to properly grasp camera lens mounts.

Bayonet Mount

DSLR camera with lens showing bayonet mount

Nowadays, almost all of the lenses we use are bayonet mounts. You will hear this term concerning lens types and other discussions. But what is the bayonet mount? The term comes from the mechanism allowing soldiers to fix their rifles’ bayonets.

The bayonet mount’s beauty comes from the fact that the system is very robust. This system uses three or four tabs that secure a lens to the lens mount and establish the connection between the lens and the camera body. The camera body’s mounting plate has corresponding recesses for those tabs (in the lens). The system is designed so that the lens can only be inserted in a particular way. Once the lens is inserted in a specific orientation, it travels by turning a bit and gets locked into position using a spring-loaded pin. When you need to take a lens off the mount, you must operate the spring using a lens disengagement button near the lens mount on the camera body. The spring disengages, and you can rotate the lens in the opposite direction to dismount it.

Before bayonet mounts, there were other lens mounts, such as the threaded screw type and the breech-lock (also known as friction lock) types. Without a doubt, the most popular lens mount at the current time is the bayonet mount. The screw-threaded type looks pretty logical, but it had a significant issue – they were very fragile, which kind of undermines the whole purpose of having a lens mount in the first place.

Throat Size

camera lens closeup showing throat

Throat size is a term that’s used in relation to lens mounts. It refers to the inner diameter of a lens mount sans the tabs that help mount the lens. Throat size is a good indicator of how big the lens mount will be.

Inner Diameter

camera lenses closeup showing inner dia

The definitions of inner diameter and throat size are similar, except in the case of the inner diameter, the tabs are considered when the opening size is measured. That means the inner diameter is the size of the opening, including the tabs that help mount a lens.

Flange Distance

DSLR camera with lens in the hands of the photographer

To explain in simple words, flange distance refers to the distance between a camera sensor and the lens mount. This distance is essential when considering buying a lens for your camera. You will often hear this term in association with a lens mount.

The difference in flange distance also determines whether some lenses will mount or not. The key reason for flange distances differing in each camera system is the reflex mirror’s size. SLR cameras have a flapping mirror, which requires the flange distance to be more prominent on SLR (and DSLR) cameras. Otherwise, if the lens were too deep into the camera or the flange distance was too small, the mirror would hit the rear lens element every time it flapped.

Canon’s EF mount system has a flange distance of 44mm. On the other hand, Nikon’s F-mount camera system has a flange distance of 46.5mm. Both are DSLR camera systems and have a flapping mirror inside the body.

The Sony E mount comes with a flange distance of 18mm, and the micro four-thirds systems have a flange distance of 19.25mm. Both of these are mirrorless camera systems.

A Word оn The Electronic Connections Between Lens аnd Camera

camera lens contact pins

If you remove a lens from a lens mount, you will notice that the lens mount has contact pins. The job of these contact pins is to ensure the exchange of information back and forth between the camera and the lens. This controls many functions, including autofocusing, aperture, image stabilization, and metering.

One of the main differences between lens mounts is that these pins are not similar in design, number, and arrangement. Even lens mounts designed by the same brand vary. Canon’s mounts are an example. The EF mount has seven pins, while the L series lenses under that segment have 10.

One may argue that more pins are certainly better but this isn’t the case. It all depends on how the pins are used and for what purpose.

There can also be compatibility issues within a series. The EF and the EF-S lens mounts, for example. The EF-S mount is derived from the EF mount. Physically they’re identical because all lenses designed for the EF lens mount work on the EF-S mount. The problem is the converse isn’t true. No EF-S lens will work on the EF mount. EF-S lenses sit a little deeper into the body of Canon cameras, and the bigger mirror of the EF camera gets in the way, making it impossible to mount EF-S lenses.

A Word оn Lens Mount Adapters

Lens Mount Adapter

Lens mount adapters are designed to enable specific lenses to be mounted on lens mounts that they weren’t originally designed for. The job of lens mount adapters is to fill the gap between the flange distance of the original mount and the flange distance of the mount to which the lens is adapted.

Let’s look at an example to explain this further.

Let’s say you want to mount a Nikon F mount lens on a micro four-thirds camera. We have just read that the flange distance of Nikon’s F-mount camera system is 46.5mm. And the flange distance of the micro four-thirds system is 19.25mm. Therefore, to mount a Nikon F mount lens on the micro four-thirds camera, the adapter must be 27.25mm.

The reverse isn’t going to be possible. Why? Because the micro four-thirds lens sits more profound into the camera body. If you were to make the micro four-thirds lens to mount on an F-mount camera, you would have to make an adapter of -27.25mm, which is impossible.

Another thing that you need to keep in mind is that the intended flange distance must be long enough to manufacture a lens adapter. Let’s say you decide to mount a micro four-thirds lens on a Sony E-mount camera. The difference in flange distance is only 1.25mm, and it’s impossible to make an adapter that thin.

This is a significant reason why most DSLR lenses can be adapted to their respective mirrorless mounts, but the converse is never possible.

Lens adapters look a lot like hollow tubes. Something of similar nature to extension tubes. However, there is one significant difference between extension tubes and lens adapters. Lens extension tubes result in light loss, while with lens adapters, there is no light loss.

Lens Mounts and The Importance of Compatibility

DSLR cameras with lenses

Every camera manufacturing company has launched its own mounts, adding to buyers’ confusion.

When choosing a lens, selecting one that matches the lens mount on your camera is paramount. You cannot use a lens that’s not designed for the lens mount of your camera without using an adapter or other accessory. Any forceful attempt can damage the fragile electronic couplings and other mechanical bits and pieces on the lens mount, and you will not be able to use the camera or the lens.

What Determines the Size of Lens Mounts?

DSLR cameras with lenses on the table

The first influential factor is the camera that these lens mounts attach to. Mirrorless camera systems have a smaller flange distance but a much narrower diameter than DSLRs. Thus, the lens mount of DSLR cameras is much bigger than mirrorless cameras’ lens mounts.

Additionally, the sensor at the back of the camera also determines what the lens diameter would be. Nikkor makes a 1-inch sensor-powered camera system, and these camera systems have a much smaller lens mount diameter than an APS-C or a full-frame camera system.

That said, some camera lens mounts are also compatible with smaller sensors. For instance, Nikon’s F-mount camera system is designed for the full-frame camera system. However, the same mount is also compatible with the smaller sensor size – APS-C. That means the F mount accepts both lenses for the larger sensor of full-frame camera systems and the smaller APS-C sensors.

Frequently Asked Questions

Over the years, there have been many lens mounts. In modern times, however, the following lens mounts are popularly used: Canon EF, Nikon F, Sony E, Pentax K, Nikon Z, Leica M, Fujifilm X, Canon RF, Fujifilm G, and the micro four-thirds. There are a bunch of other cine mounts and full-frame mounts as well.

It’s difficult to point out any mount and say this is the best. However, in terms of flexibility, the mount with the shortest flange distance and the largest throat size wins. The Nikon Z mount has the shortest flange distance and largest throat diameter.

Yes, you can, but with an FTZ adapter. This will allow you to mount your F-mount lens on a Z-mount camera system.

It is very simple. The camera box will have some indication as to the lens mount that your camera uses. Otherwise, look for the details in the manual. If all else fails, check online to find out what camera lens mount your camera uses.

A few things need to be done to make this possible. The main factors are: the target camera must be an interchangeable lens camera, the lens must have a lens mount with a longer flange distance than the target lens mount, and finally, there must be a suitable lens mount adapter.

Yes, it is. With a shorter flange distance, a camera can theoretically use a range of lenses designed for camera systems that use a larger flange distance. Albeit, you will require a lens mount adapter for the lens to work.

Rajib Mukherjee Avatar
Rajib is an avid travel photographer and an overall shutterbug. The first time he ever clicked an image was with an Agfa Click IV back in 1984. A medium format film camera. From that auspicious introduction to photography, he has remained hooked to this art form. He loves to test and review new photography gear. Rajib travels quite a lot, loves driving on Indian roads, playing fetch with his Labrador retriever, and loves photography. And yes, he still proudly owns that Agfa Click IV!
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