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Flange Focal Distance

From here.

For an interchangeable lens camera, the flange focal distance (FFD) (also known as the flange-to-film distance, flange focal depth, flange back distance (FBD), flange focal length (FFL), or register, depending on the usage and source) of a lens mount system is the distance from the mounting flange (the metal ring on the camera and the rear of the lens) to the film plane.

FFD

It’s easier to design a smaller lens for a short FFD camara. That’s why the lenses for SLR cameras are normally much bigger than the lenses for rangefinder cameras.

The reason why an SLR camera has a longer FFD is that, it must contain the “reflex mirror”. So it has to have a longer FFD than a rangefinder camera which doesn’t have a reflex mirror. That’s the price of having a “what you see is what you get” view finder.

That’s also one important reason why SLR cameras and rangefinder cameras coexist: SLR has better view finder but bigger body and lenses, and rangefinder has compact body and smaller lenses without a “through-lens” view finder. Either one cannot replace the other.

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