Well, I thought I was at double or more since since the image resolution
was probably 500 ppi or less. But I didn't know about the film grain
impact. Anyhow, since the image is already scanned at 4000 ppi it
should only require downsizing and not rescanning. But my little
experiment did show that grain was almost invisible when downsized to
31%... getting close to my 1000-1200 ppi... at least as final resolution.
Chuck Norcutt
ws wrote:
> 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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