Magazine article American Cinematographer

Speed Aperture Computer Refined

Magazine article American Cinematographer

Speed Aperture Computer Refined

Article excerpt

As more and more television commercials as well as feature films call for visually creative effects there will be a demand for special equipment to satisfy those needs. One of those is the need to vary the speed of the action in one shot. When this is necessary, a smooth, gradual exposure compensation must be executed. (For example, the point of view of a race horse jockey or an automobile driver as they accelerate.)

In the past, this effect was achieved manually as the camera assistant monitored the frame rates of the camera while 'timing' the iris ring of the lens. With small frame rate changes of 24fps to 48fps the problems were not so apparent. It proved to be more difficult as well as expensive at more extreme ranges of speed. Production companies lost large quantities of film due to bad takes. What was needed was a device that would execute an iris change automatically as the speed of the camera changed.

When Howard Preston of Preston Cinema Systems developed his first speed aperture computer it was designed to work with the Arri 35111 camera in conjunction with the old-style Arri high speed unit. The outfit utilized a 'pulley system' where a tight rubber belt revolved from the lens iris ring to a Heden motor which was either mounted on the camera's carrying handle or the iris rods. The camera assistant would manually change the frame rate via the high speed unit while the speed aperture control box powered the iris ring of the lens. The system caught on fast in the industry and was recognized by the Academy of Motion Picture Arts and Sciences in March of 1985, presenting Preston with his first Technical Achievement Award. This version of the SAC has been used on several feature films, such as The Right Stuff, as well as commercial spots including H.I.S.K. Productions' now famous IBM personal computer series featuring the Charlie Chaplin campaign.

Since the development of his original model, Preston has improved upon his idea. His new speed aperture computer completely eliminates the Arri HSU. Instead, the camera speed control is integrated into the new control box, doing away with the extra cables. This allows for a built-in speed ramping circuitry that develops exponential speed ramps; the speed is slow at low frame rates and changes more quickly as the camera speed increases. The control box is now hooked up to a small 'switching box' which is in turn, hooked up to the camera. The switcher box has a dual function: it allows the unit to switch from 12 volts to 24 volts as the camera speed rises above 48fps (also the function of the Arri HSU) and acts as a buffer for the signals coming into and out of the camera. This is necessary because the Arri 35111 is not designed to drive accessories over long cables. An assistant can now remote the SAC at a longer distance and not have to worry about intermittent power.

The SAC control box now includes an LED readout for camera frames per second down to a tenth of a frame. The frame rates can virtually be adjusted from -zero up to 125fps. It is designed much like the Arri 35III in that if it were to exceed 125fps one of the battery cells would automatically cut off and drop down the speed. There are two dials to pre-set upper and lower frame rates and a toggle switch to activate the change. One could also switch from variable speed to 24fps crystal if necessary.

Other nice features on the new SAC are the separate "camera run" and "power" switches. …

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