With practically all camera manufacturers having launched several different camera systems, there seems to be quite a bit of confusion among photographers in regards to camera lens mounts. This is especially true for things like “throat size” and “inner diameter”, where different measurements are used to wrongly quantify a mount’s potential. Unfortunately, there is a lot of misinformation out there regarding mounts and their exact sizes, which is why I decided to write a detailed article talking about different mounts, their differences and take more precise measurements for different systems to present accurate information.
Table of Contents
What is a Lens Mount?
Simply put, a lens mount is an opening of a specific size on an interchangeable lens camera that allows attaching lenses designed for that mount. Although in the early days of photography lenses were only mechanically attached to cameras without any communication between the two, with the rise of automatic metering and autofocus systems, it was necessary to create electronic contacts on both cameras and lenses to allow them to communicate with each other. As a result, a lens mount became more than just a physical hole, but more like an interface between lenses and cameras.
Today, practically every lens mount for photography is a bayonet mount (the name “bayonet” comes from the type of fitting soldiers used on their rifles to quickly mount bayonets), where three to four tabs are used to lock a lens tightly in place, but other lens attachment systems have been used in the past, such as screw-threaded lock and breech-lock. With a bayonet mount, one aligns a marked section of a lens (typically a colored dot) with the marked section on a camera body, then after coupling the two, twists the lens either in clockwise or counter-clockwise direction (depending on the brand/mount) until it locks into place.
The locking mechanism is mechanical, with a spring-loaded pin locking the lens in place in its proper position, requiring the pin to be retracted when a lens needs to be detached (which is done using a button next to the camera mount). The bayonet mount has a number of advantages compared to other mounts, which is why they are the most popular today. First, it makes it quick and easy to attach and detach lenses. Second, it allows for a tight and precise fit, which is especially important when shooting with modern high-resolution cameras, as any sort of wobble or play could end up negatively affecting parts of the image. And lastly, a bayonet mount also allows for easy incorporation of electronic contacts between lenses and cameras to enable two-way communication.
Due to the proprietary nature of all camera mounts, each one of them can differ in mounting direction, mount size, flange distance, number of electronic contacts and even where the electronic contacts are physically placed. Let’s take a look at each in more detail.
While most lens mounts require attaching lenses by twisting them clockwise, some brands like Nikon have a reversed way of doing it. On one hand, it does not matter which direction a lens is mounted or dismounted, but on the other hand, it might cause some confusion and might take time to get used to it, especially for those who decide to switch brands. Below is a table that summarizes the mounting direction of lenses among the popular camera brands.
Please note that the mounting direction is relative to the front view of the camera.
|Brand||Attaching Direction||Detaching Direction|
Mount Size (Throat Size, Inner and Outer Diameters)
When it comes to mount size, there are a few important measurements that need to be done correctly, especially when a lens mount is being compared to another one (as you want to compare apples to apples). There is throat size, inner diameter and outer diameter, and all of them mean different things. Let’s take a look at different measurement criteria and their differences.
The throat size of a lens mount is the inner diameter of a mount, minus the tabs that are used to mount lenses. Throat size gives us a more accurate representation of the potential of the mount and is important for calculating the angle of incidence, which we will discuss below.
Take a look at the below image to understand how the throat size is measured on a system:
As you can see, the distance measured is between the two tabs on the inner side of the mount.
The inner diameter of a lens mount represents the size of the lens opening ignoring the tabs on the mount. This measurement is often provided by camera manufacturers to give us an idea of the overall size of the lens mount.
Below is the image of how the inner diameter of a lens mount is measured:
Please note that the measurement is taken of the outermost inner part of the mount here. Due to the small additional recess within the inner mount in the case of Nikon F, there is an additional loss of 0.5mm to clear it. Hence, while the inner diameter is 47mm as measured above, it is technically 46.5mm between the inner parts of the mount.
The outer diameter of a lens mount is the full diameter of the bayonet mount, which in most cameras represents the end of the metal mount. The outer diameter of the mount plays an important role in determining the approximate outer diameter of the rear part of a lens, as it must be able to wrap itself over the outer diameter.
Here is the image of how the outer diameter of a lens is measured:
Again, there is another small recess that is right below the outer part of the mount, but we do not measure the outer diameter from it.
Flange distance, also known as “flange focal distance”, “flange back distance” or simply “register”, is the distance between the mounting flange (which is the outer part of the lens mount when viewed from the side) and the film/sensor plane. Just like different mounts have differences in throat size, inner and outer diameters, flange distances also often vary greatly between different camera systems.
Mount Size, Flange Distance and the Angle of Incidence
Now that we have defined the mount size and the flange distance, let’s talk about the pros and cons of large vs small mount sizes, as well as the impact of the flange distance on a system.
The size of the mount is an important factor in a camera system. Generally, a larger mount size allows for larger lenses that can provide more light to the sensor. Hence, faster lenses can be designed by optical engineers. At the same time, mount diameter is not the only variable that impacts lens design – flange distance is also equally important. Shorter flange distance allows lenses to be placed closer to the sensor, which in itself allows lens manufacturers to start building simpler, smaller, lighter and less expensive short-focus lenses instead of the retro focus types.
A shorter flange distance also allows designing thinner cameras, thus making them smaller and lighter compared to cameras with longer flange distances. In addition, it allows lens designers to place a more powerful actuator on lenses for faster autofocusing and makes it possible to adapt lenses from other camera systems with longer flange distances via adapters. Lastly, the throat diameter combined with flange distance determines the maximum possible angle of incidence of the marginal rays from the lens, which is important in designing lenses – generally, the larger the angle of incidence, the easier it is to make high-performance lenses.
One downside of a larger lens mount is lens size and weight issues. The larger the throat diameter, the larger the lens has to be at its mount point, which obviously does impact its overall thickness and weight as well. Also, the shorter flange distance can lead to increased vignetting or discoloration in the corners of an image (although this can be mitigated by making the lens design longer to simulate a longer flange distance), due to sensor microlenses not being able to pass enough light at extreme ray angles.
To understand the importance of a larger mount and a shorter flange distance, please check out the below video from a Nikon engineer that explains the benefits of the Nikon Z system when compared to Nikon F:
When adapting lenses from other mounts, it is important to point out that due to the proprietary nature of autofocus systems, differences in exchange of information between camera body and lens via electronic contacts (the number of which also vary from system to system) and other issues, most adapters end up being “dumb” adapters with manual controls, manufactured by third party companies. Camera manufacturers themselves often provide adapters with the release of shorter flange distance systems to be able to mount lenses from other camera mounts that they developed in the past, but they almost never provide adapters for competing systems.
In addition, some adapters are capable of changing the physical properties of lenses by using glass elements in them, while others make it possible to insert a lens filter (such as neutral density or polarizing filter), making it possible to use filters on wide-angle lenses with oversized front elements, without involving bulky filter mounting rigs and large filters. When adapting lenses from other systems, it is important to make sure that the target lens mount has a longer flange distance in order to be able to achieve infinity focus. Furthermore, the difference in flange distance between the source and the target system has to be big enough to have enough room for an adapter to sit between the lens and the camera for a dumb adapter. Smart adapters that can establish communication between the camera and the adapted lens must have even more legroom for contacts and electronics to fit between the two.
Camera Mount Comparison
Now that we have gone through all the terms and definitions, let’s go ahead and compare different camera mounts based on their throat and inner diameters, as well as flange distance and angle of incidence:
|Description||Throat Diameter||Inner Diameter||Flange Distance||Angle of Incidence||Format|
|Leica M||40.0mm||44.0mm||27.8mm||16.05°||Full Frame|
|Minolta SR||42.0mm||45.0mm||43.5mm||11.69°||Full Frame|
|Sony E||43.6mm||46.1mm||18.0mm||28.58°||Full Frame|
|Nikon F||44.0mm||47.0mm||46.5mm||12.14°||Full Frame|
|Pentax K||44.0mm||48.0mm||45.5mm||12.40°||Full Frame|
|Leica L||48.8mm||51.0mm||19.0mm||33.13°||Full Frame|
|Canon EF||50.6mm||54.0mm||44.0mm||16.82°||Full Frame|
|Canon RF||50.6mm||54.0mm||20.0mm||33.62°||Full Frame|
|Nikon Z||52.0mm||55.0mm||16.0mm||41.19°||Full Frame|
|Fujifilm G||62.1mm||65.0mm||26.7mm||28.67°||Medium Format|
NoteTo find the angle of incidence, I calculated the angle between the throat diameter and the top center of each system’s camera sensor. I assumed all full-frame sensors to be 24mm in height, the Fuji X camera sensor to be 15.6mm in height, and the Fuji G camera sensor to be 32.9mm in height (note that other websites sometimes do this calculation with inner mount diameter rather than the throat diameter, and they may calculate from a corner or the center of the camera sensor rather than the top center).
Frequently Asked Questions
Below is a list of FAQs related to lens mounts that I put together for our readers:
While many camera mounts have existed in the past, the most popular camera mounts today are: Canon EF, Canon RF, Fujifilm X, Fujifilm G, Leica L, Leica M, Nikon F, Nikon Z, Pentax K and Sony E. Each one differs from each other by a combination of throat diameter and flange distance.
Find your camera’s brand and model, then search for its manual online. The manual should contain information relative to the mount, as well as a list of compatible lenses. Another option is to look up the lens that is mounted on your camera – its description should contain the name of the mount.
While all lens mounts have their strengths and weaknesses, lens mounts with the largest throat size and shortest flange distance typically provide the most flexibility for lens design. At the moment, Nikon Z, Canon RF and Leica L mounts have the largest throat diameter and shortest flange distance among full-frame cameras.
No, they are not. Nikon has two different lens mounts: Nikon F and Nikon Z. The former is for Nikon’s DSLR cameras, while the latter is for Nikon’s newer mirrorless cameras.
Unfortunately, they are not. Aside from a couple of alliances, most camera manufacturers typically create their own proprietary mounts, resulting in many different lenses for different lens mounts. However, some third-party lens manufacturers make the same lens for different camera mounts.
It depends on the lens mount. You cannot mount a Canon EF lens on a Nikon F camera, because the Canon EF mount has a shorter flange distance. However, it is technically possible to mount a Canon RF lens on a Nikon Z camera.
A lens mount adapter is a device that allows a lens from one camera mount to be used on a camera with a different mount. While some manufacturers provide lens mount adapters with full compatibility and autofocus features, most adapters are “dumb”, and cannot transmit electronic data.
Please note that the above information is based on my research and submitted data from our readers. If you use a system that is not listed in the table and would like to help expand it with more data, please use the images in this article to properly measure the lens mount, then provide the information in the comments section below. We will do our best to update the table as soon as possible!
The FFD of the L mount should be 20, not 19
A good article provokes comments…
I don’t understand why a lens “must be able to wrap itself over the outer diameter”. If the manufacture is good enough to prevent light leaks and the inner is covered, why does it need to overlap the outer?
Have you heard of people physically changing mounts on lenses? (eg screw mounts to bayonet, to avoid lenses unscrewing). It should be possible, right? The mount is a plate, fixed to the lens barrel….
I would like to know whether I can adapt a 135mm bayonet mount lens to my pentax KX. Thanx
That’s a good read! But i got a question here. So do the throat diameter affect image quality from corners? I’ve been adapting f mount sigma lenses to my fujifilm gfx 50r, does the canon ef mount version has better image quality in the corners?
Is it possible and feasible to change the lens mount from Sony to nikon f , looking at a sigma lens that has a sony mount but I have nikon f camera
Thanks for this very useful article for comparing the basics of different systems that I’ve just found.
It’s coming up for 10 years since I switched from a Pentax manual focus SLR system to the Nikon F mount DSLR system, but I still find the left-hand thread mounting action awkward, despite Nasim’s comment that it doesn’t matter. I don’t think it’s because I haven’t got used to the change of brands yet. I think it’s:
a) Because in everyday life I’m regularly taking off and replacing jar lids, bottle caps, nuts and screws, etc, that all have a right-hand thread action.
b) Because Nikon lens hoods use a right-hand thread action, and I probably reverse my lens hoods more frequently than I change lenses.
I had hoped that Nikon would switch to a right-hand thread action for the Z mount, which would have been an additional incentive to switch to that system, on top of IBIS, the lighter weight, and a live-view type experience through the viewfinder, with dioptre adjustment and protection from the sun, when using a tripod. But I’d need to continue with some legacy F mount lenses, so even if they had switched, I’d sometimes have needed to mount a left-hand thread lens between the right-hand thread adapter and right-hand thread lens hood!
I have a number of old Nikon lenses from my film photo days (bayonet mounts) – and an old nikon body which I want to replace with digital. My lenses are lovely, and I would like to use them on a digital camera body but I want to make sure that is possible. I have read all these posts with some excitement and was hoping you might have some guidance for me, advice as to which digital camera would be appropriate, or perhaps what to look for in a camera body?
What about the number of electronic contacts on the mount? This is not mentioned in the article above. Canon RF mounts introduces 12 pins versus the EF with 8 pins. Does this have an impact on the speed of data being transferred from lens to the camera body? Is it possible that future Canon RF Lenses will take full advantage of the 50% increase of the data flow ? From a layman perspective, it sounds promising. Ben Yee
Although they are meant to be extremely sharp, the new F1.8 lenses for Nikon Zs (35mm & 50mm) are longer than their F-mount equivalent lenses by 12mm and 30mm respectively, and around 100g heavier. Does the short flange distance mean all lenses have to be longer, or will it be possible for a pancake style lens to be made for the Zs?
Speaking of legacy mounts, I think the original Canon FD mount was the greatest fully mechanical lens mount of all time. The breech lock automatically compensated for wear, so no matter how many times a lens was mounted and removed, its distance to the film plane remained exactly the same. The breech lock mounting ring was spring loaded, so all you had to do to mount the lens was press it against the mounting flange on the camera body. The ring would turn automatically and the lens could be used. (Yes, you had to turn the ring further to tighten it, but it was technically useable. This capability was no longer available when Canon updated the FD mount to the “New FD” mount, which made the system behavce more like the bayonet mounts found on other camera brands.) Its design allowed very accurate stopping down of the lens aperture, so even early auto exposure Canon SLRs could use shutter-priority automation. And, in one of the most forward-looking designs of all time, the mount included something called a “reserve” pin, which was a pin which had no current function, but was literally reserved for the development of future accessories and mount features! It is one of the great mysteries of photography what the reserve pin would have been used for, but it’s almost certain that that functionality was included in the electronic EOS mount with which Canon replaced the FD mount in 1987 — and so we’ll never know. : (