ARRI 35 BL 2 – The Great Ground Glass Adventure

/ Cameras, Restoration / 35mm, Arri, Arri 35 BL 2, Arriflex

It has been a few weeks since my last update on the ARRI 35 BL 2. Since then, the Mitchell FC Fox Grandeur and VistaVision Butterfly cameras have both gone on to wonderful new homes. I was busy getting them both ready for the last little while, plus working on other projects, but now I’m back to finish up this repair. When last I updated you, I thought the flange focal depth issue had been resolved. Alas, after I put the camera back together and while I was doing a final check, I discovered that the ground glass was not correct. To fix this, the camera had to come apart again. Yay! (Sarcasm)

Five images of an ARRI 35 BL 2 camera in the process of being repaired

Once out of the housing, and with the camera midrib back on the fixture plate, I removed some nail polish and shims from a previous technician and began to decipher what was causing the ground glass problem. I don’t know if my method is the best one, but I was taught to check and adjust ground glasses using an autocollimator, a first surface mirror machined to fit at the film plane, and a carefully collimated lens as a master.

A grid of 4 images of the process of repairing an ARRI 35 BL 2 camera: (top left) the camera midrib on a fixture plate; (top right) collimating a lens on the autocollimator; (bottom left) a close-up of the lens mounted on the camera, both sitting on the autocollimator; (bottom right) machining a fixture on the surface grinder.

The only problem with this method is that I didn’t have a first surface mirror that fits the ARRI BL aperture. So, I had to make something. I modified a fixture that I had made years ago for a Mitchell NC to fit in the ARRI aperture, then surface ground and polished it so it had a reasonably reflective and perfectly flat surface. Then, I precision collimated a lens to create my master. All this allowed me to put the camera on the collimator and, looking through the master lens, see if the ground glass matched the aperture. If there is any error, the micrometer on the autocollimator will let you measure the error so it can be corrected. I was finally able to see the error and correct it.

A grid of 4 images of the process of repairing an ARRI 35 BL 2 camera: (top left) preparing to lap and polish to modified fixture; (top right) the modified fixture during the polishing process; (bottom left) a close-up of the finished fixture about to be fitted on the film plane; (bottom right) the fixture mounted on the film plane.

As a double check, while the camera midrib was still on the fixture plate, I put the camera on a tripod, set up a focus chart at several measured distances, and checked to see if the focus marks on the lens were accurate. All of this seemed to work to my eyes, so I think this was a success. Finally, I reassembled the camera, ran it, and double checked all my calibrations. As I was about to run film through the camera, I noticed a problem with the 400’ magazine. So that is next week’s problem.

Two images of the process of repairing an ARRI 35 BL 2 camera: (top) the camera on its fixture plate sitting on the autocollimator; (bottom) the 400’ magazine disassembled.