The distances are very small and it's hard to be confident, but I'm an amateur machinist and happy to take some measurements.
The film holder I ran the newspaper test with is a Lisco Regal II (modern plastic, not wood). I do notice a bit of a rise in the edge just under where the logo is, but it's flexible there and it gets pushed flat by the rear standard when in the camera (otherwise it would leak light). I measured T for the film holder at the 4 corners where it's easy to measure using my old browne and sharpe depth gauge and gut checked the measurements with my modern mitutoyo digital vernier calipers. The corners measure 0.2670" and 0.2675" at the bottom. At the top where the rim bows up I get 0.2750", 0.2740". I believe these are all a little high but within the spec for an 8x10 film holder. I don't have an appropriate flat bar to check the center, but a 150mm machinist rule placed across the span of the septum does not show any light through peeking underneath. It is flat enough for my purposes and I think the film holder is not the issue. Anecdotally, I saw the same focus issue across all 8 of my film holders, which are various brands of modern plastic holders.
Attachment 222294
Bookmarks