American cinematographer (Jan-Dec 1946)

The power unit which motivates the dolly consists of a 14 h.p., 110 v d-c motor which has a top speed of 1725 rpm. The shaft of the motor is con¬ nected to the speed reducer box by means of a rubber coupling that takes up all motor vibrations as well as start and stop jars. The speed of the d-c motor is reduced 50 times by the reducer box, and a sprocket gear pulley from it engages the sprocket chain which in turn rotates the dolly axle and bronze power wheel. A rheostat governs the speed control, while standard reversing switches determine direction. Essentially, the same type of power unit has also been installed on the dolly and is applied to the dolly tilt arm so that it may be raised or lowered with ease even when the dolly is in motion. To the rear of the unit a seat for the dolly operator is provided, together with a control panel which contains switches and controls that govern the speed and movement of both the dolly and the tilt arm. The control panel also con¬ tains dials which show any given posi¬ tion of the dolly and the tilt arm. Once the dolly operator knows the conditions of the shot, he can duplicate these con¬ ditions any number of times without fear of error, for any error that he might make would be plainly indicated on the control panel after the shot was completed. Also, additional switches make it possible for the operator to control the entire series of movements by the throwing of a master switch. The most important mechanism in¬ stalled on the dolly, however, is the automatic follow-focus device. As stated above, this device was developed and installed because of the gx-eat need for accurate and positive focus, particularly on close follow shots where the narrow depth of field characteristic of photo¬ graphic lenses as they closely approach a given target demands extremely ac¬ curate focusing. This need was of par¬ ticular importance in special effects work where follow shots are concerned mainly with extremely accurate framing and the extreme proximity of the lens to the object or target. Such examples can be cited as the need to move from a close-up of an individual to his mouth or eyes, or in some cases, to one eye. Another common case could be cited such as moving up to or away from small sections of maps or titles. Although this device could be used in many instances in standard set proce¬ dure, no intent was made to displace current production methods and it was conceived only for those highly difficult follow shots which are almost impossible to accomplish when the cameraman must depend upon the judgment of the operator or assistant to focus the lens by hand. The automatic focusing of the photographing lens is accomplished in the following manner. The focus unit receives its activation from the right front dolly wheel. (See Fig. 2). It transfers this energy to a cam, which has a contour pitch, com¬ plementary to the curvilinear action of a two inch lens or of the particular focal length lens desired. This action is applied to a small gear on the end of the shaft of a Selsyn generator mo¬ tor. The rotation of this motor is trans¬ mitted to and received by a Selsyn re¬ ceiving motor. A small gear, same size as on the generator, is mounted on the end of the receiving motor shaft. This activates a pinion gear, which turns the actual lens gear itself. (See Fig. 3). There is a distinct advantage in using electrically connected Selsyn or inter (Continued on Page 22) Fig. 3. Dolly lens gear assembly. Fig. 4. Title and insert stand. American Cinematographer • January, 1946 9