Flange distance? Mount diameter? Here’s why they matter.

Something that stood out in the announcements by Nikon and Canon of their full-frame mirrorless systems was their presentation of flange distance and lens mount diameter as key specifications. As completely new mounts aren’t introduced very often, one could be forgiven for not understanding exactly why these measurements should matter or how they impact the abilities of the camera system as a whole. Read on and be enlightened!

The basics

Flange distance: Also referred to as flange focal distance or flange focal length, the flange distance refers to the exact distance between the imaging sensor (or film) and the metal ring where the lens is mounted (the flange). This is the distance noted on the left side of the helpful image from our friends at Canon, above.

Inner mount diameter: The inner mount diameter is a little more straightforward, referring to the total diameter from one inner edge of the lens mount to the other, not including any outcroppings for gripping the lens. This is the dimension shown on the right side of the image above.

Why flange distance matters

An unchanging flange distance is of critical importance for lens designers, as a very shallow depth of focus is usually required to make lenses with the best balance of size and sharpness. Depth of focus is essentially depth of field (the “slice” of the image in view of the camera that is in focus), but on the other side of the lens: it refers to the range in which the image being reproduced inside the camera will match that from the outside. Film had a lot more potential to move around (due to heat warping, mechanical camera damage, etc.) and thus required slightly more depth of focus. Lenses for digital cameras, in contrast, can take advantage of a very narrow depth of focus indeed, thanks to the reliable positioning of an imaging sensor. With a precise target, autofocus systems become that much more effective, too.

The shorter the flange distance required, of course, the thinner the camera can be.

Why mount diameter matters

Mount diameter is a balancing act between compactness and ambition. Making the mount wider allows more light into the camera*, allows larger sensors to be used to match that larger image circle, and allows for more elaborate lens design in general (for example, bringing a larger rear element closer to the sensor itself or installing a more powerful AF actuator), but the larger the mount is, the larger the camera and lenses must be. You can bet that manufacturers spend a long, long time deciding on the balance they think is right before releasing a system to the public.

*This is why Nikon claims that the forthcoming Noct f/0.95 (!) lens is only possible thanks to the wide diameter of its new RF mount. The “f number” you set on the camera is a relative measurement calculated by dividing an instantaneous focal length by the physical size of the aperture (hence why the same aperture hole can be 1.8 at one distance and 3.5 at another). The bigger the back of the lens is, the bigger the aperture that can be accommodated.

Adaptation game

These are the considerations necessary in deciding native measurements. There remains, however, the coolest part of all this math: in most cases, if the native flange distance of your camera is shorter than the native flange distance of another mount, you can use those lenses on your camera! (Note that if the image circle of the adapted lens is smaller than your sensor requires, vignetting may occur.) Like a magic door opening in your very own garden, lens mount adapters (check out a selection at this link) unlock other worlds. There are three basic categories of adapters.

  1. The most simple adapters are metal rings like this which simply provide additional space between the lens and the focal plane. Remember, all that is required for a sharp image is for the flange distance to be correct. Because they are nothing more than metal spacers with specialized mounting hardware, these adapters will require you to manually control aperture and focusing.
  2. The next step up are adapters which offer communication between the lens and the camera. These include first-party adapters like the Nikon FTZ which are designed to maintain compatibility between their older and newer mount systems as well as third party adapters which enable communication cross-brand.
  3. At the most elaborate end of the spectrum are adapters which actually add features to the lens being adapted, like the Metabones adapters which include additional lens elements to boost the light being passed to the sensor, or the Canon adapters announced with the EOS R which add an extra control ring or space for a drop-in filter.

Why adapt lenses? Many people do because they already have a significant collection of high-quality lenses and would like to be able to use them on a newer, smaller, and lighter mirrorless body. Others, myself included, enjoy the ability to experiment with a host of lenses released over the past few decades, often at a great price!

Like this wide-mouthed beauty I got for literally nothing. Thanks, dad!

Don’t forget: Mike’s Camera is now your trusted source for quality used equipment, too. 😉

Hopefully that helped demystify some of the science that goes into your great pictures. Let me know in the comments if you have any more questions, and be sure to bring the best shots you get with your adapted lenses in to one of our stores so we can help you bring your masterpieces to life!

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