Remember that only a loupe on the ground glass tells the truth. Levels on camera bodies do not--they provide a reasonable starting point.
1. The oblong nature of some of the round shapes is caused by rectilinear distortion, which is visible with this close camera position. More distance would reduce this effect, and that's why a longer lens might help.
2. I saw many photos that were not level with the edges of the frame. The closer you are to the subject, the more critical such things become. So, we may tilt the back so that it is vertical using the level, but then we have to check the subject carefully, with a loupe and with measurements (or an accurate grid) on the ground glass, to be sure there is no convergence or slanting horizontals.
3. When you tilt the lens with respect to the film to a given angle, the focus plane tilts by twice that angle. Depth of field surrounds that focus plane. But depth of field is always less when closer to the camera and greater when more distant, so the depth of field space around a tilted focus plane is generally wedge-shaped, with the narrow part of the wedge being closest to the camera. If you chose a focus plane to make it possible to bring in your three-dimensional subjects and keep the table-top in focus, you would have needed a compromise focus-plane position that would allow depth of field to accommodate the subject material not in that plane, and more tightly at the bottom of the photo where that material is closer. With three-dimensional subjects, this is tricky and you have to check your result using a loupe, with the lens stopped down. That's not always easy but that's the gig.
4. Squaring a camera to the subject may not be as easy as it seems. When at the limits of the depth of field, even small errors will yield visible results. Again, the proof of the pudding is in the tasting, not the recipe: Evaluate your positioning and movements on the ground glass.
Rick "noting that you also have to view the whole ground glass for subtle cockeyedness that might not be visible with a loupe" Denney
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