By Bjorn Petersen |

You may be thinking, "What a boring subject for an article," and you would be right. However, as boring as the interface between your camera and lens might be, the significance of your camera's specific mount, along with which lenses can be used on your camera, which lenses can be adapted to your camera, and why some lenses work and others do not, can prove to be a more interesting subject that may even inform which camera or lens system you eventually buy.

What Does a Lens Mount Do?

A lens mount is a standard or proprietary interface used by camera and lens manufacturers to ensure a secure and accurate means for attaching a lens to a camera body. Each camera system nowadays uses its own unique mount that limits compatibility between lenses and other manufacturers’ cameras, and also enables electronic communication between the lens and camera to achieve accurate focus and exposure.

Most lens mounts in current use are bayonet style, in which a lens is attached to the camera body by registering the lens in proper orientation with the camera body, then giving a slight twist of about 45-90° to lock the lens in place. Prior to the bayonet mount, a couple of other mounting styles were commonly used, namely thread and breech-lock mounts. Thread mounts, or screw mounts, are self-referential and describe the action of threading your lens onto the camera body, much in the same way you thread a nut onto a bolt. Breech-lock mounts are more closely related to bayonet mounts; however, they use a self-contained rotating ring on the lens itself to tighten the lens onto the camera body with friction.

Bayonet mounts are the most favored of these three mounting types, due to the ease and speed of installing and removing lenses from camera bodies, the ability to incorporate electronic contacts using this attachment method, and the repeatable precision afforded by a simpler design.

What’s in a Lens Mount?

Besides connecting a lens to a camera and sporting a certain design style, lens mounts also have a number of distinctions from manufacturer to manufacturer. Size is the main differentiator between the various mounts, and is a slightly more complex measurement than you may imagine. The number of tabs in a specific bayonet can vary from one manufacturer to another (although most use three tabs), the direction in which you rotate the lens to connect with the camera body varies among brands, and the incorporation of electronic contacts will also be unique to the camera and lens manufacturer. Furthermore, each mount corresponds, arguably, to the most important element of this article, a specific flange focal distance (FFD). This measurement, which describes the length from the mounting flange (the edge of the lens mount on the camera body) to the image sensor or film plane, varies from manufacturer to manufacturer, and is one of the true limiters of the interchangeability of lenses with specific cameras.

The flange focal distance of each camera system is factored into subsequent lens designs, and is a constant length used by each manufacturer to ensure accurate focus from a specific lens’s minimum focusing distance to infinity. Using a lens with a specific FFD on a camera system with a shorter flange distance, you will not be able to achieve infinity focus. This distinction is the key element in which lenses can be used on camera systems other than the original one the lens was designed for, via a lens adapter.

Lens Adapters

Spurred by the somewhat recent advent of mirrorless camera systems, a renewed interest in the ability to use a wide array of third-party lenses has also occurred. Due to the self-referential design of these cameras, the lack of having a mirror in a camera body design affords, besides a more compact overall design, a shorter FFD. By having this shorter registration distance, you can theoretically mount any lens with a longer FFD on a camera with shorter FFD through the use of a lens adapter. 

The lens adapter effectively serves to make up the difference in focal flange distance between the camera and lens—for example, a Nikon F lens to Sony E adapter makes up the difference of 28.5mm to provide the proper total 46.5mm of focal flange distance for a Nikon F-mount lens to achieve infinity focus. While this is the ideal situation, to mount lenses with a longer FFD on cameras with a shorter FFD, adapters do exist that allow you to physically attach lenses with shorter FFD measurements to camera bodies with a longer FFD. The caveat with these adapters is that you will not be able to achieve infinity focus without the inclusion of a corrective element in the adapter itself, and it is unlikely the quality of this corrective element will match the quality of the lens being mounted. However, without the corrective lens in place, this combination will afford the ability to work at focusing distances less than infinity, since the lens adapter is now functioning as an extension tube.

With this basic concept in mind, lens adapters can also be substantially more sophisticated and maintain electronic communication between the adapted lens and body through the use of dandelion chips, with some adapters even capable of retaining a lens’s autofocus and image-stabilization capabilities. On the other hand, completely manual adapters will not convey any information between the camera and adapted lens, forcing you to manually focus and adjust the aperture settings on a lens, and work in manual or aperture-priority mode on the camera.

One additional type of adapter that has gained tremendous attention over the past few years is a style designed exclusively for APS-C and smaller format mirrorless cameras, most commonly Sony E, Fujifilm X, and Micro Four Thirds systems, that has been popularized by Metabones and Mitakon Zhongyi. With the exception of Sony E-mount now being featured on full-frame cameras, these mounts typically correspond to crop-sensor sizes and, as such, are associated with terms like “crop factor” and “equivalent focal length.” This batch of lens adapters strives to make these terms somewhat moot by incorporating a condensing lens into their design to minimize, or in some cases eliminate, the crop factor and increase the amount of light reaching the sensor. This is accomplished by projecting all of the light gathered by the lens onto the image sensor, rather than just simply losing the light that would typically be cropped out by the smaller sensor dimensions.

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