Thanks for presenting this experiment. This is more or less a problem for all users of these cameras.
Do you think this error is caused by focusing-problems like lacking fine-adjustments on the camera,
or do you think it's the sliding-back thats causing it? (that the ground-glass is not in the same position
as it sensor is placed)
I have had the same problems with the sliding-backs. These system should have some kind of shimming-
system. The backs are not prodused to 0 tollerance. In fact the producers of DB normally have a 30-40 microns
tollerance in there systems. Without shimming there is no way to get this accurate with a ground-glass.
My evaluation of the result of my own camera system (Linhof Techno with standard ground glass and Linhof sliding back, plus Linhof-made H-mount adapter plate and H-mount Aptus 75 digital back) is that there is negligible focus shift problem, that is the precision in the system is adequate. I base this on that I cannot see that the ground glass image and resulting digital back image is shifted in any way. There still could be a shift -- since GG resolution is lower than the digital back resolution it is not possible to see where *exactly* the focal plane.
From what I have heard, Linhof is one of the manufacturers that have good reputation concerning good precision in sliding backs, and this experiment fits into that. I have heard much more criticism of Arca Swiss view cameras that the ground glass is not matching. I have no possibility to check whether this is true or not though.
The small focus error (which I think is acceptable) -- that I aimed for 130 cm mark on the GG but the resulting image shows ~138 cm as far as I can see -- is not due to focus shift or difficulty to turn the knob precise enough, but due to that GG shows detail corresponding to about 15-18 megapixels and the sensor is 33 megapixels i e it is not feasible to with the eye see better precision than that. I had no problem to use the knob and turn the lens into the position my eye said it should be in. It would be more elegant with a dual geared focus rail, one finer gear for fine-adjustment, but it is really not necessary, with tight gearing as Linhof has micro-adjustments can be made by hand.
I may not been that clear but my thinking around this is that focus precision of the ground glass is adequate, but the manufacturers (Linhof in this case) do not take the need for magnification seriously and that is the real problem.
I'm a bit of a view camera fanboy
so I'm also of the view that there's a little bit of FUD coming from the pancake camera manufacturers, exaggerating precision errors and also exaggerating the practical impact of such errors. You don't really need to nail focus within 0.0001 cm at 10 meters when you're doing f/11, and an important principle is that with reduced focal length the DOF increases correspondingly so the precision requirement on the linear focus rail does not increase in practice - it is indeed harder to hit a specific distance but DOF is wider so it does not matter.
The experienced increased difficulty to focus wide angles I think is mostly due to lack of magnification (and dimness of ground glass) rather than a precision problem in the equipment.
I'm a bit skeptical about the need for shimming, unless there really is poor manufacturing precision, which would be a scandal concerning the price of these things. It may be a greater need for shimming in pancake cameras since I guess there's more of a risk to focus past infinity there than with proper view camera focusing technique.