Flange focal distance

The range of this distance, which will render an image clearly in focus within all focal lengths, is usually measured to a precision of hundredths of millimetres, and is not to be confused with depth of field.

Optics to correct for distance introduce more cost and can lower image quality, so non-optical lens adapters are preferred.

If the difference is small, other factors such as the sizes and positions of the mounting flanges will influence whether a lens can be adapted without optics.

Professional movie cameras are rigorously tested by rental houses regularly to ensure the distance is properly calibrated.

Any discrepancies between eye focus and measured focus that manifest them­selves across a range of distances within a single lens may be collimation error with the lens, but if such discrepancies occur across several lenses, it is more likely to be the flange focal distance or the ground glass (or both) that are misset.