Just to amplify upon Jim's comment, I have a 4990, but I’ve rented a v750 for a side-by-side, non-scientific comparison. [Executive summary: little difference]
I used a sheet of 4x5 Velvia-50 film (nicely exposed w/ minute details in the subject) and snugly dry-mounted w/ a film holder, similar to Doug Fisher's "Betterscanning" design.
I used Epson Scan for the comparison, selecting the "film w/ film holder" option in each case (i.e., not the "film w/ area guide" option). So I was comparing the 4990’s single fixed lens w/ the v750's “SHR” fixed lens.
I found my 4990’s best plane of focus, which is unique from model to model, to be about 2mm above the glass.
At this optimal plane of focus, my 4990's maximum resolution is about 2000 dpi, perhaps a touch less. Most 4990 users report about 1500 or 1600 dpi, so I think I was blessed with a very good machine.
I really have to squint to see improvement above 1500 dpi, but it’s there if I look carefully enough, even if it’s not a practical matter.
When I rented the v750, I likewise determined its best plane of focus; it occurred at a slightly higher elevation than my 4990.
I then compared the two scanners at 2000 dpi.
My personal result: I couldn’t tell the difference between these scans w/ several carefully inspected crops. However, there was a different "look" to the images that would be difficult for me to describe.
When I went to higher dpi, up to 2400, I still couldn’t tell the difference, but I was making comparisons w/ E-6 4x5 film, not a fancy line resolution chart. If I had used a chart, I suspect the v750 may have appeared a tiny bit better; and, it might have appeared better at 1800-2000, too. However, since I rarely print larger than 11x14, the slightly better performance, if I had seen that it existed, would probably be negligible to me.
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