Journal of the Society of Motion Picture Engineers (1930-1949)

Fig. 3. Schematic drawing of basic principle of planetary gear system. friction of the mounting plate (m). In shifting from one lens to another, the focus dial (o) will rotate one complete revolution thereby returning to the same distance calibration as the preceding lens. Example: In the case of a three-lens turret assembly, as illustrated, the gear ratio between the control knob (a) and the internal ring gear (c) is 3 to 1 with three indexing notches on the lens turret plate, one to index each of the three lenses. Focus dial (o) will rotate one complete revolution in shifting lens-mount assembly to next lens. This will allow the use of only one focus dial for all three lenses. The follow-focus operation incorporates, in addition to the foregoing system of lens focusing, a method of actuating the focusing viewfinder so that the optical elements of the finder sharply focuses the image on its viewing glass in synchronization with the lens focusing of the image on the film. A stop pin within each lens mount prevents the lenses from rotating beyond their infinity position. This in turn prevents the control knob from being further rotated due to the lens turret plate assembly being locked by the turret locking pin. When a release button is pressed and the control knob turned, the entire lens turret assembly rotates until the next lens is in place, indexed and locked by the turret locking pin. Synchronized with this function is the cam follower which rides out of its cam scroll into an inclined circular groove and drops down into the next cam scroll and is synchronized to the next lens that comes into place. The linkage from the cam follower to the focusing viewfinder is so constructed to permit racking over the camera for lining up a scene and also Richardson and Gaisford: Focus Device and Blimp 121