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Ethan Brake

Does the pressure plate actually touch the image area on the film?

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Maybe this is obvious, but I can't find any information on this. On the magazine to an Arri 16sr1, there appears to be a raised part of the pressure plate that would touch the image area. It is also has it's own subtle springiness to it, separate from the rest of the pressure plate. When reading about super 16 conversion of these cameras on cinematechnic.com, they recommend making the image area contact-free. Also, when watching a video on loading the magazine of an Arri 416, the pressure plate has a raised area as well, but it appears to be machined as a solid part of the pressure plate, not a separate springy part as on the sr1 magazine.

My questions being,

Does it touch the image area, should it touch the image area, and if not what is its purpose, and why does this part on the sr1 magazine have its own springiness to it separate from the rest of the pressure plate?

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The pressure plate should push the film against the edges of the gate to get the frame edges flat against the gate. It also needs to support the center of the frame area to keep it at the correct distance so that the center can be focused at the same plane as the frame edges. so the center support is very crucial for image sharpness.

But there is a catch. The film does not stay flat on the gate and it may even be beneficial to let it develop a small controlled curve so that it  is easier to make the lens's focus curvature flat (it is easier to let the film plane to be a little bit concave than to make it perfectly flat and correct the focus curve in the optics). So the center area of the pressure plate may be machined to slightly lower plane than the areas which push against the frame edges. This is to let the film to develop the tiny amount of concave curve instead of staying perfectly flat. Don't know which cameras do and don't do this correction on the pressure plate areas but it has some benefits so I assume the newer Arri cameras would use it.

If the plate surfaces have the correct smooth finish without defects there should be no problem them touching the "picture area" which is the backing of the film and not the actual image

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Thank you very much aapo lettinen, that was a very informative response that cleared up my confusion on that matter.

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SR mags have 2 seperate spring pressure areas that mate to the camera gate The larger main one uses the 4 corner feet to press against the indents in the camera gate side walls to create a narrow film channel, with just a little more space than the thickness of the film. The second smaller sprung pad in the middle is the actual pressure plate that exerts slight pressure on the film just over the gate aperture to keep it flat. There is no film curvature allowed.

On high speed mags the small middle pad is fixed not sprung, creating a tightly spaced channel for the film to pass through rather than applying pressure to it. For this reason the flange depth of high speed SRs is set 0.03mm shorter than normal SRs.

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