The Dioptre adjustment is mechanical, sets the focus for the eyepiece of the viewfinder.
See page 44 of the PDF in this link:
http://www.bgfoto.net/upload/downloads/pdf/Nikon/nikon_f80.pdf
The page numbers for the original manual and page numbers of the PDF index are not the same, I had to read through the thing to get the correct section.
The correct section:
"
Dioptre Adjustment/LCD llluminatorThe finder dioptre enables near- or far-sighted photographers to adjustthe eyepiece dioptre to suit their vision. Remove the rubber eyecup and slide the dioptre adjustment lever whilelooking through the viewfinder until the focus brackets or other displays in theviewfinder appear sharp. Attach the rubber eyecup again after adjustment. The adjustable range of the finder dioptre is -1.8 DP to +0.8 DP. Nine optionaleyepiece correction lenses provide a viewfinder dioptre range of -5 to +3 DP(P. 96).
NOTE: Using the dioptre adjustment lever_________________
Since the dioptre adjustment lever is located next to the viewfinder, becareful not to poke yourself in the eye with your finger or fingernail whilesliding the lever."