Thanks for the succinct summary of grain aliasing, Winsor. I knew that
was a possible cause of Matt's problem, but didn't have the energy yet
for writing the long, round-about way I would have tried to explain it.
I think there is some good stuff, with examples, in on the net, one is
at <http://www.photoscientia.co.uk/Grain.htm> .
I've never had trouble with this efffect on my Can*n FS2710 with either
slide or negative film (from 100-800 iso). The only difference I find
between reversal and neg film is that slides with significant dark areas
require multiple passes to control noise.
Your post is also timely because a friend just e-mailed that he was
seeing a lot of grain on negs, but not slides as he tried out his new
Epson 2450. Now I can pass on your summary and the web site I
rediscovered to him
Moose
Winsor Crosby wrote:
There is a common phenomenon called grain aliasing that makes a scan
of a fine grain film look grainy when scanned. It seems to have to do
with the relationship between the size of the film grain and the size
of the pixels so that the way the scan sums things up the image is
pixellated in a way that looks like exaggerated film grain. It is not
really. It is a digital artifact. A rough way to think of it would be
if you had a CCD that corresponded 1:1 exactly to film grain size. A
smooth gray field in grain would yield a smooth gray field in pixels.
If you decrease the grain size slightly, the first pixel will catch
the smaller grain plus part of the space between the grains. Going
down the line, eventually the center of a grain space will line up
with the center of a pixel and you will get a light pixel Some where
else you will get a darker pixel. Instead of a smooth gray field you
will get a pattern of light and dark that will look like grain.
People will frequently experiment with different fine grain films to
see which works best with their particular scanner.
At least that is my poor understanding of it.
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