Back focus is a common term used to describe the Flange Focal Distance (FFD). Or the distance between the rear surface of a lens mount and the film plane or sensor. The most common FFD in my line of work is relative to the Positive Lock (PL) Mount system used on many motion picture cameras and lenses. The FFD for a PL camera is 52mm (2.0472441″) This means that when a lens is mounted to a camera correctly, the image produced by the lens should come to focus at exactly 52mm from the rear of the lens mount. Continue reading “Focus back on back focus”