Lens Mounts Explained

With Nikon and Canon having recently launched brand new mirrorless system mounts and talking about their benefits, 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.

Nikon Z7 vs Sony A7R III vs Nikon D850

1. 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.

Bayonet Mount Terminology

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 the 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.

2. Mounting Direction

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
Canon Clockwise Counter-Clockwise
Fujifilm Clockwise Counter-Clockwise
Leica Clockwise Counter-Clockwise
Nikon Counter-Clockwise Clockwise
Olympus Clockwise Counter-Clockwise
Pentax Clockwise Counter-Clockwise
Sony Clockwise Counter-Clockwise

3. 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.

3.1. Throat Size

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:

Nikon F Throat Size

As you can see, the distance measured is between the two tabs on the inner side of the mount.

3.2. Inner Diameter

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:

Nikon F Inner Diameter

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.

3.3. Outer Diameter

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:

Nikon F Outer Diameter

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.

4. Flange Distance

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.

Camera Flange Distance

5. 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 retrofocus types. A shorter flange distance also allows to design 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.

6. 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
Fujifilm X 40.7mm 43.5mm 17.7mm 35.34° APS-C
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
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