What is a Camera's Flange Focal Distance?
The Flange Focal Distance (FFD) of a camera is the distance from the film plane or sensor to the mounting flange. The mounting flange consists of the ring mechanism on the body of the camera and the back portion of a lens. The FFD is important in cameras that support interchangeability of lens and have a lens mounting system. The value of FFD will be different in cameras of different categories and manufacturers.
Understanding Flange Focal Distance
Flange focal distance plays a vital role in determining the clarity of focus at different focal lengths, and a minute difference of even a hundredth of a millimeter generated by a seating error of the lens can cause error in focusing according to the marks on the lens. The FFD has to be precisely correct so as to get a clear focus of an object at all focal lengths, or to get the correct depth of focus.
Among movie cameras, the Arri PL is the most common used mount and it has a flange focal distance of 52mm. According to recent research, to get the optimal FFD settings, it is considered better to measure the distance from slightly within the emulsion layer of the film rather than its surface.