Your pixels-per-mm calculation is a useful sampling metric, but it is not the whole story. If you zoom in or move closer so the subject covers more pixels, you usually increase image sampling on the subject, which can improve usable detail. However, true spatial resolution is limited by the entire imaging system, especially the lens and aperture, not just pixel count.

Lens resolving power is affected by diffraction and optics. As aperture gets smaller (higher f-number), theoretical resolving power decreases; off-axis areas are also typically worse than on-axis areas. Focus accuracy matters too: poor focus reduces resolution. Imaging at an angle can reduce effective detail on the subject and may make resolution less uniform across the field.

So yes, sensor, lens, aperture, focus, subject angle, and capture geometry all affect spatial resolution. Sensor pixels set a sampling limit, while the lens determines how much real detail reaches those pixels.

If this is for formal measurement or software that must produce official results, use recognized standards and documented test procedures rather than an ad hoc metric. For internal use, choose a consistent method and stick with it. Also, don’t judge suitability by spatial resolution alone—overall image quality and consistency for your analysis task matter too.