Sprocketless/Clawless Projectors

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From Peter Swinson, December 2009:

Rob,

With all the talk about sprocketless transports, I attach some images and an explanation of the tracking multifacet prism system used many many years ago by Cintel. I believe this was a high quality version of the system used in many film flatbed editing tables such as Steenbecks, KEMs, etc.

The basic principle is that the faceted prism is rotated at the same rate as the film passes by. Yes a sprocket was attached to the prism circumference, but as the wrap could be very large, many sprocket teeth were engaged into many film perforations. Additionally as the prism rotated at the same speed as the film there was no friction on the film. This all led to very good film handling, even where there was a lot of film damage. Each frame is held in place against a single facet as it rotates past the fixed centre optics, the image obviously moves down the fixed optics face. However at the camera end, the facet facing the camera is moving up and its angle, relative to the camera is changing in the opposite direction to the facet at the film plane. Therefore the film vertical motion is cancelled at the camera end. As each frame transits the fixed optics the camera sees each frame dissolve into the next. Neat eh!

This is my simple explanation I am sure others can elaborate.

Of course as each facet must match the film image dimension, it is not suitable for multiple formats and I guess shrunken material caused some minor image jitter.

I rescued the 35mm optics in the photo from being dumped many years ago when Rank Cintel had a clear out. The prism is a about 140mm diameter. While the prism is in good condition, apart from a few scratches the fixed transfer optics cylindrical lenses at each end of the light guide are separating, loads of Newton rings!

Cheers

Peter

Images Courtesy Peter Swinson
Continuous Motion Prism System
Film tracking prism.jpg
Cont motion prism system1.jpg

Rob Lingelbach 16:02, 6 December 2009 (UTC)