Flange Focal Distance

The Flange Focal Distance or Flange Distance (or flange focal depth, flange back distance, flange focal length, back focus, or register) of a camera refers to the distance from the mounting flange (the metal ring on the camera body where the lens is attached) to the film plane or image sensor. This distance is usually specified in millimeters.

In other words, the flange focal distance is the distance between the rear surface of a camera lens (where it mounts to the camera) and the camera’s image sensor when the lens is focused at infinity.

Film Plane Indicator / Focal Plane Mark

Depending on the manufacturer a camera body may have a mark that indicates where the film or sensor plane is. It is signified by a circle with a line through it.

For Canon 35mm film cameras, such as the Canon AE-1, the mark is to the left of the prism. The distance from that mark to the front of the lens mark represents the flange focal distance.

Example shown on a Nikon D750 DSLR. The digital camera obviously doesn’t use film, so Nikon referrers to it as the focal plane mark.

How To Know If A Lens Can Be Adapted

The flange distance is important because it determines which lenses can be adapted for use with other cameras. For example, if the flange focal distance of a lens is longer than that of a camera, it can be possible to use an adapter to fit the lens to the camera. The extra distance can provide enough room for the physical coupling of the second lens mount.

However, if the flange focal distance of a lens is shorter than that of the camera, the lens cannot be adapted to the camera without either modifications to the lens or camera, or an additional lens in the adapter to correct the optical path. This is because the lens would need to be placed inside the camera body to achieve focus at infinity.

Adapters that use a correcting lens should generally be avoided. The optics on these adapters are of poor optical quality and will severly degrade the image quality.

Different camera systems have different standard flange focal distances. For example, as of my knowledge cutoff in September 2021, the Canon EOS EF mount has an flange focal distance of 44mm, Nikon F mount has an flange focal distance of 46.5mm, and the Sony E mount has an FFD of 18mm.

Vintage Lenses

Vintage 35mm film lenses are easily adapted onto modern mirrorless cameras. Simple adapters are enough to allow vintage lenses to work because they have physical aperture rings.

Smart Adapters

Lenses without physical aperture rings require “smart adapters” to provide electrical contacts so that the aperture of a lens can be controlled. For instance adapting a Canon EOS lens onto a Micro Four Thirds camera body.

Adapters Without Infinity Focus

There are some lens adapters that do not include optics to correct for infinity focus that are for mounts with a longer flange distance. One that I found was a Minolta MD-mount to Nikon F-mount.

These can be useful for macro photography for attaching a lens onto a bellows. In my instance I wanted to attach a Minolta bellows lens onto a Nikon PB-4 bellows.

Common Camera Lens Mount Flange Focal Distances