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

The requirements of the blimp were to contain the camera and follow-focus mechanism and be constructed of a sound deadening material, be lightweight, and reduce camera noise interference to a level permitting the use of a microphone within 3 ft of the camera. The Maurer Camera was ideally suited to this project as it comes equipped with a focusing viewfinder with a parallax compensating mechanism. Design and Construction A planetary system of gearing was selected as it made possible the functions of keeping the entire gear driving mechanism and viewfinder linkage permanently engaged. The fact that all three lenses are caused to rotate simultaneously in their mounts is not objectionable (Fig. 1). Each lens was set up in a dividing head and the amount of rotation from the 3 -ft mark to the infinity mark were obtained in order to determine the correct gear ratios for synchronizing the lens calibration with the focusing dial and control knob. The main drive gear consists of a ring gear having both internal and external gear teeth of 48 pitch with annular ball races ground on both faces of the gear and loaded with 480 -jVin. steel balls. Two retaining gear rings are grooved to match the ball races which hold it in place on the front of the camera case. The balls are slightly preloaded to allow the ring gear to rotate with minimum friction and no end play. Compound gears of proper ratio are driven by the internally cut teeth of the ring gear and are ball-bearing mounted on fixed stud shafts attached to the lens turret plate. These gears in turn mesh with the ring gears attached to the lens focusing barrel. A gear transfer case transmits motion from the control knob and dial assembly to the externally cut teeth on the ring gear and also to a master cam plate gear housed in the subbase of the camera. These two gears are synchronized with a ratio of one to one. The master cam plate has three scrolls (Fig. 2) each generated and cut to act upon a cam follower and linkage to the Maurer focusing viewfinder. The cam is spring loaded against the scroll to eliminate backlash. The viewfinder was modified to function with a minimum of friction and backlash by removing the rack and pinion and replacing the dovetailed slides with ball rollers. As the Maurer viewfinder has two parallax compensating cams, one for the wide-angle lens, 15-mm, and the other for the 25-mm and 40-mm lenses, an adjustable linkage was provided for manually shifting the finder only for the wide-angle lens. The control knob and dial assembly is one detachable unit and can be mounted on either the left or right side of the camera. A splined coupling permits engaging the control unit to several reading angles. Operation A simplified sketch is shown (Fig. 3) which illustrates the basic principle involved in the operation. Control knob (a) drives gear (b) which in turn rotates internal ring gear (c) causing gears (d, e, f) to rotate, in turn causing gears (g, h, i) to revolve, By proper gear ratios, (g, h, i) rotate lens focusing barrels (j, k, 1); thereby, keeping all lenses synchronized in respect to their focusing range from infinity to their nearest focal point. When lock (n) is released from notch in turret plate, and control knob (a) is rotated in either direction, the entire lens-mount assembly attached to the mounting plate (m) will rotate until by-pin (n) drops into next indexing notch in plate (m). All lenses will remain in the same synchronized focus position as the lens mount assembly is shifted from one lens to another because the resistance of the lens-focusing barrels and the gearing will overcome the lighter Richardson and Gaisford: Focus Device and Blimp 119