Flange-back (back focus) Flange-back (commonly called back focus) is the distance from the flange surface of the lens mount to the image plane of the pick-up sensor. Each camera type has a specific flange-back distance (for example, 48 mm in air) and any lens fitted to that camera must be designed with the equivalent flange-back. There is usually a flange-back adjustment mechanism of the lens with which the flange-back can be adjusted by about 0.5 mm. It is important when changing lenses on a camera to check the flange-back position is correctly adjusted (see opposite) and to white balance the new lens.