True, the Epson 750 scan is not at the focus plane, where I describe that comment near the end of the PDF...
The Epson 750 example scan is approximately one, to one point five millimetres below the optimum plane for my Epson 750, but properly applied sharpening brings the image back into reasonable focus for the result comparison. If I scan the 8X10 negative at the proper focus plane within the Epson 750 and, or the Epson 4990, I would apply less sharpening to the image. My Epson 4990 had this issue too, but its focus plane was nearly three millimetres above the scanner glass.
Again, the discovery height above the scanner glass for each Epson instrument, was discovered while experimenting with frosted white optical plastic as a diffused source, in a completely different post about scanners within this group. The negative was wet mounted to the optical plastic in that discussion thread, and the negative was correctly positioned and supported with shims above the scanner glass, prior to a scanning event.
jim k
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