Flange focal distance
Flange focal distance is exactly what the name implies: the distance (usually expressed in millimeters) from the front of a particular lens mount to the plane of focus, i.e. the film or sensor surface. Lens register, flangeback, or flange-to-film distance are all synonyms for this.
The flange focal distance is one of the key measurements defining a particular lens mount standard. Rangefinder cameras with interchangeable lenses typically have shorter lens registers than SLRs, because the latter must leave extra body depth for the reflex mirror.
Even between different brands of 35mm SLR, there is variation in their register distances. For example, Canon's FL/FD mount and Minolta's MC/MD mounts had comparatively short register distances. Nikon F mount and Leica R mount had comparatively long register distances. Register distance has become a more significant lately when the owner of a lens of one mount standard wishes to adapt it to a different camera type—these days, typically a digital one.
Lenses designed to work with short register distances become problematic, because on long-register bodies, they cannot sit close enough to the focal plane to reach infinity focus. For an adapter to work in this situation, it must contain negative lenses. In effect, it acts as a (very) weak teleconverter; and the optical quality at the widest apertures may be poor.
Lenses designed for longer register distances are more widely adaptable. Bodies with exceptionally short register distances (for example Micro Four Thirds] or Sony NEX) can accept practically any lens mount known.
