I was taught, when setting the flange focal depth of a camera, the tolerances were +0” -.0002”. The logic behind these tolerances was that any error should be within the layers of the film and not in front of the film. So I have always followed that advice when I set up a camera. When we discussed this ARRI 35 BL2 last week, I said that the flange focal depth was out of tolerance. The deeper I looked into this, the worse things looked. I was getting inconsistent readings from the center of the film plane to the four corners of the aperture, all out of tolerance differently.
When I took everything apart, I noticed non-factory shims from previous work on the lens mount block. I took these old shims off the lens mount, cleaned all the surfaces, and reinstalled the mount to the camera. I then re-measured the flange focal depth and, while it still wasn’t correct, it was now much more consistent, being .002” off on one side of the aperture and .0035” off on the other side of the aperture. A known issue with the BL2 is that, sometimes, when used with the heavier zoom lenses of the day without proper lens support, it would distort the lens mount. I believe this is what I was seeing here, because the readings I was getting were telling me that the lens mount was too far away from the film plane.
As a sanity check, I re-indicated all of the fixtures and tools I was using to make sure that my gauges weren’t giving me false readings. Everything checked out, measured down to .0001”, so I think I’m still sane. Now that I knew what needed to be done to complete this repair, and while the lens block was off the camera, I decided to replace the broken lens lock arm. I found a new lens lock arm from stock that I’ve been hoarding and installed it so that both upper and lower lens locks were working. Then I began the process of lapping the lens mount block to the correct dimensions and checking it over and over again until I got it right. That took up most of my time this week. I still need to put the camera back in the housing and test everything, but that will have to wait until next week.
