For this lens, the most useful range for sharpness is generally toward the long end: about 35–55mm, with 55mm being the best choice for maximum magnification when using extension tubes. Aperture-wise, f/5.6 to f/8 gives very good results, and f/8 is the safest recommendation if you want strong sharpness across the frame with lower distortion, vignetting, and chromatic aberration.

So a good starting point is: 55mm at f/8.

A practical limitation is the lens’s minimum focus distance, so extension tubes may be necessary to fill the frame with film. Even then, a true macro lens would generally be the better tool for this job.

For setup, control stray light. If you’re using a light table, mask off the unused illuminated area and surround the film with black material (like matte black card or foam board) to reduce glare and reflections. That can improve contrast and overall scan quality as much as lens settings do.