 The internal lens does not use a pop out design and a fixed 28mm threaded lens mounting ring was included at the front.The diameter of the mounting ring happens to coincide with that of most clinical microscopes, resulting in the 9xx series becoming popular for hand-held eyepiece-projection photomicrography.This mounting ring is present on the entire series, making them system cameras; a set of intercompatible telephoto, wide-angle, and fish-eye converters were available, and worked on any of the 9xx cameras.