Jim> This is to report that the Umax Astra 1220S - the latest in
Jim> the 1200 series - appears to work fine with SANE, with no
Jim> nasty noises from the mechanism! The resolution on xscanimage
Jim> only goes up to 600dpi though (but see below)
...snip...
Jim> Why does my scanner provide resolutions up to 9600 dpi when
Jim> the default resolution for photos seems to be 100 and OCR
Jim> 300? What use is 9600 - scanning postage stamps?
FWIW, most consumer level scanners on the market scan with a maximum
optical resolution of 300-600dpi. Many scanners are marketed with
such claims as "scans at up to 9600dpi (interpolated)". This is a
joke, false advertising, and ought to be illegal.
Interpolation is a process of creating a new pixel based on the pixels
surrounding it. Interpolation can look ok at low levels, but it is
not adding any real, captured information to your scanned image. I'm
always surprised that the marketing folk stop at 9600dpi, they might
as well advertise "scans at 1billion dpi (interpolated)", since
besides disk and memory space, there's no limit to how many
interpolated pixels can be added to a scan.
As for the proper resolution for scanning, the probably reason for
those defaults is that when printing with a laser printer, one must
use halftones to produce levels of gray. A halftone consists of a
bunch of dots of varying size, placed along lines which are often at a
45 degree angle. The fineness of a halftone screen is measured in
lines-per-inch (lpi). A newspaper might print at 85lpi, a magazine at
above 105lpi. Printing in color gets a bit more complicated, let's
not go into it.
The lines-per-inch of a halftone and the dots-per-inch of the printer
that produce it are not directly related. A 1200dpi printer could
print at 15lpi, 50lpi, 80lpi, 105lpi, or any other number under
600lpi. As you increase the halftone screen, the resolution and
fineness of the printed photo increases, but at the same time the
number of levels of gray goes down, leading to photo reproduction that
looks rather xerox like.
Theoretically, a 300dpi laser printer could print at a resolution of
150lpi, as you could print that many distinct lines of single pixel
dots. However, the dots along these lines would not be able to vary in
size without running into one another and becoming black, or
disappearing altogether and becoming white. If you were to print at a
lower lpi, say 55, the halftone screen would be clearly visible, yet
many levels of gray could be reproduced. I used to have a formula for
calculating this lying around. Suffice it to say that 55lpi is a
pretty optimal halftone screen for a 300dpi laser printer, although
it's a bit low for many folks' tastes.
And if you know what halftone screen you're printing at, you can then
calculate the resolution at which you should scan your photos. I
don't remember the exact formula for this either, but if you print at
65lpi like my college paper used to, your photos should be scanned to
be at least 135dpi at the size at which they're finally printed
(i.e. if you scan at 135dpi and then enlarge the photo to fill a whole
page, it's no longer 135dpi). We used to scan at 150dpi so that we'd
have a little room to play with.
But the basic idea is that the halftone screen limits the amount of
detail that will be visible, so scanning at higher resolutions is
unnecessary.
I'd bet that the 100dpi default is with laser printed halftoning in
mind, probably at 40-55lpi. If you're scanning photos to print on one
of those super-duper epson stylus printers or some other high-res
inkjet, you might want to scan quite a bit higher. 300dpi ought to be
high enough for just about anything, unless you're printing a
magazine.
72dpi is the standard computer monitor resolution, should you be
scanning for the web.
Suffice it to say that you'll probably never need to scan at higher
than 300dpi, unless you're trying to magnify a detail. And scanning at
resolutions beyond 600dpi will probably not capture any more detail
than a 600dpi scan would, it'll probably just create a whole bunch
through interpolation and make your filesizes larger. Unless of
course the Astra 1220S has 1200dpi optical resolution.
-Zach Fine
czyz@u.washington.edu
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