On Checking Lens-Sensor Parallelism

As for camera body film-plane alignment, some people can see it, some not, but here goes on a way to check for good alignment...: (The F-series pro-Nikons have the best viewfinders for checking alignment, since the screen focus is correct and sharp all over the screen [it isn't in most newer cameras]. Any common screen is fine.)

1	2	3
0	0	0
0	0	0
0	0	0
4	5	6
0	0	0
0	0	0
0	0	0
7	8	9

This is easiest on a tripod, using a fine-detailed, contrasty, VERY distant subject (like a very distant bare tree with sky behind, or a detailed very distant building - the distance is needed to eliminate rotational errors, though this is less of a problem with teles...). First focus carefully at location 5 on the viewing screen, then check to see that all the other points are at exactly the same focus. Next focus at 4 (or 6), and see if 6 (or 4) is exactly at the same focus. Then do the same with 2 and 8. Then with 1, 3, 7, and 9. If all agree exactly, all is probably fine. If some don't agree slightly, it could be lens or mirror misalignment. If there is gross focus disagreement, it could still be lens or mirror misalignment, but it is easily checked by trying another prime lens on the body (zoom misalignment is common...). If opposite sides agree with each other, and all four corners agree with each other, but not with the center (and maybe the edges not with the corners), the lens has field curvature (few Nikkors do - the 135 f2 AI/AIS and the 55 f3.5 often show this, though). Field tilt is not common in primes, but I have seen it often enough (including in one 28mm f2, one 300mm f2.8, and one 400mm f3.5) that I want to check for it. Field tilt is a common fault in zooms, and is often easily spotted in the viewfinder. A further check (on film) can show that even if there is good visual alignment, there can be unequal sharpness, usually across the frame. This can be spotted by taking two identical vertical photos (with the diaphragm nearly wide open) of a line of distant detail (like a silhouetted row of trees, distant city skyline, etc.), the first with 3-6-9 at the top of the frame, the second (with the same framing, and without refocusing) with 7-4-1 at the top. On the film, the 3-6-9 and 7-4-1 edges (with the same subject matter) will appear across the frame line between the two frames, and it will then
be easy to compare opposite edge and corner sharpness agreement (1-2-3 vs. 7-8-9).

For more, see my article on lens checking.

"Hope This Helps"
David Ruether (d_ruether@hotmail.com)

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