Epson Expression 1680 Reference Manual page 64

If TPU for Neg. Film or TPU for Pos. Film is selected as the
Document Source, the value of the focus position is automatically
set to 2.5. If you are using film holders, you do not need to change
the focus setting. If you set the film directly on the scanner's
document table, EPSON recommends changing the focus
position to 0.0.
If Flatbed, or Auto Document Feeder is selected, the value is
automatically set to 0.0. EPSON does not recommend changing
the setting unless you want to have the scanned images out of
focus or you are scanning an object that is not flat.
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Understanding EPSON TWAIN Pro Features