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Arri gate issues?


Webster Colcord

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From a recent test of my very old Arri 2B. This is at 2fps, and at the start of each take the image "breaths". I had swapped out the Academy gate with a high speed gate, in order to get closer to a full aperture frame. The Arri has quite a tight curve in the upper loop before the gate, and I think that when that goes thru the gate it still has a bit of the bend in it, hence the breathing? That problem is not unique to my camera, and it was also happening with the old gate.  

But I think the fact that it goes into focus during the breathing, but settles out-of-focus means that the new gate is not seated properly - probably half the width of the film too far from the back of the lens?

Caveats: this was a test with very old, badly stored stock from a friend. And the wide angle attachment seems to be soft in the middle. But the slate at the head of the test was shot with my trusty 35 Schneider, which has always been very sharp. 

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Well, I think the pressure plate doesn't quite have as much spring to it as the old Academy pressure plate. I'm going to put the old gate back in until I can get a pro to look at it. The breathing is normal, but the rest of the footage is actually soft so something's not right.

Edited by Webster C
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You generally can't assume a new gate will sit at exactly the same depth, it needs to be checked with a depth gauge and possibly re-shimmed. It only takes a deviation of two hundredths of a mm (under a thousandth of an inch) to throw the focus out on a wide lens.

Check also if there were shims under the original gate, they might be just little cut squares of shim, easy to fall out or get pushed out of position.

Your diagnosis of the top bend deforming the film and causing the footage to breathe at the start of each take sounds pretty good, especially if the film is old.

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Thanks Dom,

Yes this was my first time swapping out the gate in the Arri and I was shocked how it actually "pops" into place like a snap-together model kit. I'm used to the Mitchell and Wall gates, which as you know slide in on a very tight tolerance - there's no question as to the fit. 

I didn't see any shims in there and I think that the old academy gate is the factory original - have you seen shims used in cameras straight from the factory? I would imagine that adjustments like that would have been made by a camera tech later. 

Also, do you know what the difference is between the high speed gate and the regular gate? The only major thing seems to be that the aperture is larger (almost full).

Edited by Webster C
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Given that the factory stopped making 35-II variants back in the 70s, I've never seen one straight from the factory! But yes, shims are probably something techs add later.

We talked about high speed gates in this thread:

My understanding is that the only difference is adjustment screws for the pressure plate tension. I don't know about the gate aperture size being related to high speed, I thought the larger aperture was for Scope. 

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