Here's a short report on how the focussing screen by Bill Maxwell works on an Arca F-line with 141 standards. Maxwell makes after market focussing screens that are supposed to be quite bright (see other threads on this forum), as replacements for OEM screens. A potential problem with the Arca is, that the Arca uses a fresnel/ground-glass sandwich, so the question of focus shift arises, when one replaces the Arca sandwich with Bill's single screen (plus protector glass).

I tested it with a desk-top set-up, using my new Rodenstock 180 mm Macro Sinaron wide open at about 1:1 and focussed on a slanted surface on the tip of a pencil with the OEM ground-glass/fesnel. After locking everything down, I unscrewed the OEM pieces, and put the Maxwell groundglass/protector glass in. The focus stayed exactely the same. In theory, the removal of ~ 2 mm fesnel should cause a focus shift of ~1/3 of its thickness = 0.6 mm, which is definitely noticeable. But I could not see a focus shift with a 6x loupe, and I could not improve that focus at all. I want to emphasize that this is not a test with lasers on an optical bench, but what you will actually see when you use the equipment for picture taking.

What I did notice is the brightness of the screen, looks about 1 f-stop brighter. What a difference! Took it out for a spin today to shoot some mushrooms and a Peonia. The screen performs just as well outdoors. In the past when I checked the corners of the screen, I had to tilt the loupe and look from the outside towards the center of the screen to get reasonably bright image. With the Maxwell screen, it's rather the reverse: to check the corners you look from the center of the screen out, which is much easier as I hardly have to move my head around.

I also use Ron Kulkas' ground glass protector (piece of moded plastic). The Maxwell pieces are a bit thicker than the OEM, so to make it fit again I had to file a groove in Ron's potector, which took the better part of 2 minutes.

So, if any of you Arca users wondered about it, here's the answer. Keep the Arca pieces for back-up.