Re: Scan vs print resolution on Epson 1200

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Morrie Gasser wrote:
[re the difference between scanning at 300 dpi on a 600 dpi scanner,
and scanning at 600 dpi and then downsampling to 300dpi in Photoshop]

> Now, this implies that if Photoshop were simply to throw out every other
> pixel of the 600 dpi image, rather than averaging, this should be identical
> to scanning at lower resolution.  Is this true, and if not, why?

The unaccounted-for variable is the scanner software. If the scanner
software does what you describe when scanning at an integral fraction of
maximum resolution, then the result should be equivalent to resizing in
Photoshop with the "nearest neighbor" preference set. In other words, you
will drop out very small details entirely, and diagonal lines will look
stairstepped. The alternative (using Photoshop's default of bicubic
interpolation) is, as you suggest, some unsharpness. Resizing should
generally be followed by a bit of unsharp masking or increasing the amount
of unsharp masking you were already planning to do.

However, there's no guarantee that the scanner software does what you
suggest, and it certainly can't do just this if it supports scanning at
non-integral fractions of its optical resolution. Asking for 400 dpi from a
600 dpi scanner means that the scanner is running at 600 dpi and the
scanner driver is downsampling to 400 dpi (because the stepper motors in
the machine can't move 3/2 of a step). It may be using a nearest neighbor
algorithm, or it might be using bilinear, or bicubic, or some other
downsampling algorithm, and it might be applying some sharpening as it
goes. Only the driver writer knows for sure; if you're lucky and picky, the
info might be in the manual.

Botttom line: there's nothing that the scanner can do that will give you
better results than scanning at max optical resolution and then
downsampling and sharpening in Photoshop (or downsampling with "nearest
neighbor" if you prefer sharp, jagged edges). The scanner software *may* do
as well (especially for noncritical applications), or it may do much worse.
Check out the difference and then do whatever gives the best quality /
convenience tradeoff with your equipment.

Russell Williams
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