Depth of Field for scanning 35mm slides on flatbed scanners (Epson)

I have both an Epson V750 and V850 scanners and am trying to digitize a large amount of slides.

The software for scanning does great for color restoration (slides dated from 50s on) but the issue is focus. Slides are from film that ranges from PolaChrome, KodaChrome through Agfa and others. Mounts range from "factory cardboard" through "factory plastic" through after-market snap slide mounts.

I mention all this because all these factors seem to impact slide position in a scanner as far as focus and DOF (differing thickness, clamping, centering) as well as how "warped" the image is in the mount.

I used a set of 12 varied images for some testing in both and found that the DOF in both scanners appears to be less than the thickness of two layers of film. I can find no way to enlarge that and that indicates to me that this is way more of a issue than I had hoped. If that is true, then I think my only option is to create a test matrix for each scanner that cross references – film type – film age – mount type – film warp amount – scanner holder setting – and then use that to scan based on those parameters.

Am I missing something?

Just to scale the "job," this is for 22,000+ images accumulated over the last 60+ years

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I had used the 750 several years back for a 200+ slide effort for a friend and even with the "Unsharp Mask" the results were not up to the originals but worked for the low resolution video we produced, so the base question still remains, what can be done to improve the DOF for the scanners? If nothing, then I will again abandon the scanners. As far as dedicated film scanners (Nikon-Coolpix/etc.), I have tried at least five over the years and all with limited success (jams every night, no batch film warp compensation, batching scans by mount, film type, age, etc. creating a post scanning organizational issue) for any volume.

Since these are great slides with considerable detail, I have recently invested in a 50+mp Canon with a top of the line Macro Lens and tried to duplicate that way, only to have to send the body in for maintenance as the focus prism and the film plane did not match. So am awaiting return of that for a second round of tests. Did not want to bring that into the scanner topic but thought I would respond to BobT.

The Epson scanner does such a great job on initial scan correction range, but if the focus is still going to be a hit and miss based on the limitations of the technology, then I really would like to "put that to bed" since it appears to be unsolvable at volume.

I appreciate the input of others, but if the scanners are "doing their best" and that is not acceptable, then I will put my efforts on the the camera (to achieve the best focus) and eat the time for "all the corrections" needed for the "age range and variations" of the originals. I will post again when the body comes back and I can run yet another test.