A key issue trying to digitize film with a camera has always been critical focus across a full film plain. No top ILC camera lenses are equally optimally sharp from frame center to frame edges and corners. The greater magnified detail desired, the more critical are the micro-dimensions of focus required regardless of added lens element optics tricks. Not only is film on any base not absolutely flat but also critical focus difference varies between say a center focus and frame points further away towards edges. Optimal optically would be a slightly concave film plain. There is tremendous potential resolution in slides but getting a critical focus from a single shot has been inferior to typical flatbed XY scanning processes, much less laser drum scanning. But laser drum scan cost about $80 each and I have hundreds of strong images.
But with focus stacking using a top medium telephoto prime digital lens, I can use sharpest apertures as F5.6 and take a number of shots at different film frame locations while setting Auto Spot Focus for each of those spots. At each point am taking an F5.6 and F8.0 shot. With 6 spots that is 2*6=12 shots per frame that later is optimized with Zerene Stacker that chooses the sharpest point for each XY spot to make the composite image. So 2*6*4=48 total shots. I do that for each of the 4x5 quadrants and then stitch blend combine that with Kolor Autopano for the full composite that I then perform Photoshop processes.
