Hello Ian,
Here you have a table with an extensive test showing effective dpi results scanning at different nominal dpi. V750 and V850 are mostly the same, beyond illumination.
As you can see is not necessary to scan 6400 to get most of the V850...
You may see that, usally as with any scanner, USAF 1951 based measurement depends on the pass, each time can be a bit different measurement because the smallest pattern you see may be more or less aligned with the pixels, having more or less aliasing.
Here it's reported that even 2900 dpi have been measured for vertical bars, for horizontal bars measurements are a bit lower.
When evaluating scanners should be considered the best value you obtain from several samples, because the lower measurements can be provocated by aliasing between the position bars position in respect to the pixels.
But well, this table reports from a trusted source that scanning 6400 dpi with vertical bars with have average 2643 dpi resolving power, and with horizontal bars we have average 2148 dpi.
If you scan 6400 and you want to downsample with Photoshop then go to "image size" dialog, at the bottom you can select the interpolation/binning algorithm, use "Bicubic ideal for reductions".
We have "3 kinds" of sharpening: Input, Creative and Output.
Normally we may sharpen the raw file from the scanner, while editting we can perform creative local sharpening like in the eyes in a portrait, but perhaps not in the cheek, and after resizing to match the output device (printer) we can perform another sharpening for the pixel level.
Those average 2400 effective dpi makes the V850 a very powerful scanner for 8x10, and a very good choice for 4x5. If extreme/important densities are there, then a drum is what works.
For rolls a Plustek 120 (or 8xxxi) is better if the shot is very sharp or if we want the grain structure.
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