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James Malamatinas

35mm Film Cameras - Registration pin for top shots

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Someone was recently describing a pin/claw mechanism that comes into play with some film cameras when riggng them in a top shot mode. Apparently the pin ejects itself when in a specific vertical orientation to maintain correct pressue on the plat and should then collapse back when the camera is bought level again.

Does anyone know which cameras this applies to and have any more information?

The topic arose due to an assistant's experience where the pin failed to be retract, affecting the depth of the film plane and thus focus. I'm curious to learn more for any future, simlar situations.

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maybe he/she meant the detachable pressure plate's cam mechanism which is rotated in place to secure the pressure plate? this can be found on certain cameras with for example Mitchell style movement (that cam-like thingy holding the pressure plate in place. just above the reg pins)

movement2.gif

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Thanks Aapo, Ill follow up on this and see if this is what they were referring to. I thought they said it was present with more modern Arri film cams too but Ill take a deeper at this and see where it takes me!

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I think maybe you're talking about the Arricam FFD displacement mechanism, which uses a counterweight to shift the inner membrane in the opposite direction of gravity when the camera gets tilted. The inner part of these cameras is suspended by rubber shock mounts to isolate vibration and so tilting the camera would otherwise cause the inner membrane (along with movement and gate) to shift away from or towards the lens mount (which is fixed to the outer shell) and so change the crucial flange depth. So this mechanism counteracts that.

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Thanks Dom, that sounds precisely the mechanism that was described to me. Is it possible that on occassion when returning the camera back level from being tilted that this can get stuck and that a manual check should be done to make sure it has been reset? If so how would that be done. I'm sure any issues are extremely rare but now I'ved learned of it I'm curious how it works! I've tried searching for further details on the mecahnism itself but documentation appears scarce.

Are you also aware of similar mechanisms on other camera e.g. PV, Movicam?

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I can't recall if it's also in a Moviecam.. could be. Maybe just the Compact MkII.

 

You probably won't find it described anywhere outside of parts diagrams. It's basically a sort of weighted pendulum fixed to a shaft with a cam that pushes the membrane in the opposite direction of the pendulum. The more the pendulum is offset, the more the membrane is pushed back into position. You can see it when you take off the motor side cover.

 

I had one camera where the rubber shock mounts had been damaged by a fall, and it affected that mechanism, since the elasticity of the mounts was compromised. But it affected other things too, so the fault was identified and fixed pretty quickly.

 

To check it you would need to measure the flange depth with a depth gauge to ensure that the FFD was still in tolerance. If your lenses suddenly seemed to not quite line up with their focus marks that would be an indication the FFD had shifted (the ground glass and film plane are still locked together, so the ground glass focus would match the altered flange depth but the lens scale would be a bit out).

 

From memory the shift in the membrane caused by tilting the camera without being compensated by that mechanism is only a few hundredths of a mm, so if the mechanism happened to fail, it wouldn't be a huge discrepancy.

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Thanks Dom, your level of expertise and willingness to share is exactly why I visit this forum. I've got a good understanding of the mechanism now I think so much appreciated. I imagine I'll never run into the issue but if I do at least I'm well armed for it!

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