[Guest]

You're right, "rotate" is ambiguous. It's option A.

 

But I disagree with your answer. Neither the subject nor the camera moved, thus the distance is exactly the same.

 

*** edited to add: I understand there's another ambiguity in my post. The subject is relatively small, a bell tower at 500m, so there's not a "center" or a "border" of the subject. The "center" and "border" are those of the framing.

 

PS Apart from the fact that with a subject at 500m I believe there's plenty of DOF to play with: in the worst case, 70mm, at ƒ/8 focused at 500m, the near DOF limit is 29m... but as I said, this shouldn't be ever needed in this case.

 

The distance is the same on a spherical curve if you rotate the camera in a circle. However, the lens can have some field curvature, but most likely, it's usually not spherical. Ideally, the focus plane of the lens would be flat.  So, now the question is, how much did you turn the camera. That depends of course on the focal length you used, because that gives you the field of view. A wide angle might give you a 90 degree horizontal angle of view, so you can turn your camera about 45 degrees to get your bell tower from the center to the left or right border of the image.   So, the distance is 500meters, let's assume the lens has a planar plane of focus, no field curvature. Simple trigonometry now  That means, at the border of the lens, which is set to 500 meters, the optimal sharp focus plane is at a distance of square root ( 500 x 500 + 500 x 500 ) = 707 meters.

If you had a tele, say a 500mm lens, then there would be very little difference, because you can't rotate it much to get to edge.

 

Given you don't know the field curvature of the lens, you probably need to make some tests. I.e. using life view set the correct distance at the edge, note the distance setting (say 600meter). Then using life view, set the correct distance for the center (say 500meter).  To have a compromise now, set the distance to 550meter, and choose an aperture that gives enough DOF to have 500 to 600 meters sharp.   Ok, this was just an example, it's pretty impossible to set such distances on a lens, but it illustrates the concept.