A theoretical and maybe silly question. Suppose I have a camera with a well corrected lens at point A and a subject at the point S far from image centre. Let us say that I don't have a ground glass and have to deal with a foccusing scale in getting S in focus. Which distance should I set in camera focus rail? L1 or L2?
I "tested" that with an Apo-Symmar 150mm f/5.6. I focused a spot on a wall 6 meters away from lens, right in front, on the lens axis. Then I checked another point at the corner in a 4x5" ground glass. My assessment was that focus could be improved there. I fine tuned it and found indeed that in several attempts I had to bring the lens closer to the film plane in order to improve sharpness at the corner. That tells me, without any math, that at least for this lens, the actual oblique distance is the one that has a better chance of being correct if I had to use the distances scale.
The reason why I am asking that is that I am planning to print scale focusing and use a wire frame finder plus the aid of a laser measure. By doing so I could, in some cases, dispense the ground glass.
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