At 08:06 PM 3/14/2009, Chuck wrote:
><snip> I'm wondering what happens to this image if you scan it at,
>say, 1000-1200 ppi or just enough to avoid aliasing. Does it look just
>as good?
Please ignore, technical engineer brain talking... Just a note on sampling
and aliasing. To avoid aliasing when sampling the rule of thumb is to
sample higher than twice the frequency of anything in the image. Which
means noise, grain, etc. and is not related to the resolution. Aliasing
takes high frequencies and moves them down into low frequencies.
This is why there is an anti-alias filter in front of the digital camera
sensor. For film scanning, there is no such filter, so to avoid aliasing
may mean sampling a lot higher than the final resolution required.
Then with proper post processing the image can be down-sampled
closer to the resolution needed.
So the sample rate from the scanner will depend on the scanner lens
resolution and the film grain frequencies. You can search the web for
"grain aliasing". At the top of the list is:
http://www.normankoren.com/Tutorials/MTF8.html
Take note of how to identify grain aliasing in the image, scroll down the page.
It just may be that with finer grain film and under-sampling that the grain
will actually look more pronounced.
But the important thing to remember is that the scanner sampling required
is more related to the grain structure of the film than the image resolution.
So I always sample as high as possible and use post processing
to down sample to avoid enhancing the grain, especially if the grain
is very fine. It may be that some scanners have less of a problem, and that
is probably due to the optics in the scanner providing some anti-aliasing.
Chris, you commented:
"I scan these 6x6 negs at 4000 dpi all the time, and yeah they look grainy at
100% even with films like Acros that have fine grain. In prints, however,
the grain isn't visible."
When printing a negative there is no such thing as aliasing. I'm wondering if
this is an example of grain aliasing? Probably not, quoting the site above
Norman says:
"If you haven't worked with fine darkroom equipment or haven't purchased
well-made custom prints, you may not realize how grainy film can be
(negative film is grainier than slide film). Grain aliasing and scanner
noise are often blamed for grainy appearance, when, in fact, straight
unaliased film grain is the cause."
Wayne
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