75 dpi - approx 'full size'
depending on your screen. Not much detail - the cancel looks like a
solid impression on the outer circle
150 dpi - satisfactory for
most casual purposes. Can now see the cancel breaking up, but note
the background in the stamp image looks like a solid light brown.
300 dpi - now you can start
to see finer detail. Note how the background in the top left of the
picture can now be seen to be a series of individual lines.
600 dpi. Now you can
see almost all the printing detail in the image. Care to guess what
printing process was used? You probably couldn't tell with the 75
and 150 dpi scans, and could make a guess at 300 dpi, but it is only with
this detailed 600 dpi scan that you know, for sure, that it is an engraved
print, and therefore, Scott 2287, rather than a lithographed Scott 2291.
Lastly, a scan of part of
the stamp (the image is getting too big) at 1200 dpi. My HP ScanJet
6200C only goes up to 600 dpi of optical resolution, and at 1200 dpi it is
starting to 'invent' extra information, with the net result being that
although the image is bigger, it is starting to get fuzzier again because
the extra size isn't being matched with extra resolution.