The electro-optical mechanism of the scanner can only scan at one dpi,
determined by the actual sensors in the mechanism. Any scanning at less
than that dpi means the driver is downsampling and any higher dpi means
it is interpolating up. To avoid artifacts and generally goofy results
(some a bit and some a lot goofy, depending on specific dpi chosen), you
should always scan at a dpi which a whole number and which is the result
of division of the native hardware dpi by a whole number. Thus, with a
600 dpi native resolution, acceptable dpis would be 300, 200, 150, 120,
100, 75, 60, etc.
If you ask it to downsample to 66 dpi, you are pretty close to 600/9 =
66.666..., whereas as 168 dpi, you aren't anywhere close to a number
that allows simple downsampling. That may be a the reason the supposedly
higher scan resolution gives poorer sharpness. The same rule applies
when doing one's own downsampling in an image editor.
Moose
Brian Swale wrote:
Hi folks,
This might possibly open a can of worms, but anyway, here goes.
I've been playing with a print, about 8 x 12 inches, and scanned it at a variety
of resolutions. Mostly this was due to my scanner misbehaving horribly, and
swapping parts of the image around, and I tried several scanning resolutions
to find if that would get around the misbehaving.
In the process, I found this.
When I scanned at 66 pixels/inch (25.98 px/cm) I got an image that was
(height = 20.01 cm / 520px / 7.87 in.)
(width = 29.56 cm / 768 px / 11.6 in.) as a pdd file
I presume the jpeg I converted it to is the same.
I adjusted colour and brightness, and did one simple sharpen. I did not re-
size. The image has come out quite sharp with none of the over-sharp
artifacts one sometimes sees, including none of the white halo effect.
When I scanned at 168 px / inch then after adjusting colour, brightness and
contrast, resized and sharpened in that order (I think) I just could not get it
as sharp no matter what method I used, and sharpening artifacts appeared
quite quickly.
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