Camera Modifications

The bottom gear of the Arri 35II models runs at 1800 rpm through a slight reduction for 24 FPS. The inching knob and shutter run at 1440 at 24 FPS. A 5 to 4 reduction. Driving the inching knob 24 fps and higher can be hard on the camera because the gears would be driven opposite their design and the claw mechanism is forced toward the film. Driven by the bottom gear the gear pressure pulls the claw cam away from the film. At low speeds and for stop motion we have had no trouble driving the inching knob. Using a 4 to 5 speed gear up to reestablish a 1 to 1 ratio from the bottom gear for stop motion might leave too much play.

The older flat bases can be cut in two to create a long and narrower camera. With a longer gear belt the motor can be put behind the camera and magazine. An 1800-rpm sync. motor would work for sound at 24 fps driving the bottom gear.

We drove Arri II Cs to 80 fps with governor controlled permanent magnet motors. We used $15. surplus motors to 50 fps. These motors required 30 volts. The controls had current limiting circuit beakers in case of a jam.

Many stock and homemade motors can drive Mitchells. These cameras need more power than Arri IIs.

Arris, Mitchells, Eymos with one to one shafts can be driven with elliptical gears to give a more than 180 degree effective shutter angle. This adds more exposure and eliminates strobbing in the film. Sterling Instruments have made these gears in the past.

Mitchell rack over cameras can accept almost any lens mount. There are many motors possible too. Simple gear reduction motors will run the camera at very low speeds. Having a built in separate shutter makes the Mitchell ideal for stop motion. Mitchells with the original turret can rising front adjustments by moving lens turret up or down. The viewfinder may not see all the light rays from the lens and you may get more vignetting than the film sees.

Some Eymos have been modified with one to one shafts out the back. They are OK for applications where registration can be poor.

Arri IIs can be driven at 1800 rpm with a modified fiber idler gear from the bottom for 24 fps.

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