On Jul 9, 2006, at 1:09 PM, Vic Mortelmans wrote:
> I'm looking into low-level postprocessing techniques and I'm a bit
> puzzled how film grain influences postprocessing.
>
> More precise: will a film scan that has no grain (e.g. scanned at low
> resolution) look the same as a film scan that has visible grain (e.g.
> high-resolution scan), when you apply exactly the same postprocessing?
Of course not.
> Because postprocessing is typically non-linear, the change applied to
> dark pixels may be totally unrelated to the change applied to bright
> pixels. But what happens to an image area that---to the eye---has a
> medium-gray brightness, but---at the pixel level---is a mixture of
> bright and dark pixels due to grain.
>
> I doubt that the effect is as desired, i.e. as you would expect on a
> smooth image that has no grain, even if the processing is as simple
> as a
> gamma-correction.
I'm not sure what you mean by "low level post processing techniques".
It is nearly impossible to make a good answer without knowing what
sort of processing you intend to do.
Overall, film grain is *like* digital noise when processing but the
scanning process itself introduces a lot of variables. The
granularity of a particular piece of film can interact with the
frequency of the scanning and cause an effect called "grain aliasing"
which can cause grain to look big and ugly.
In general, I find that you have to treat grainy film very
carefully ... it scans to a high frequency image with lots of micro-
edges which limits how much sharpening and other processing you can
do to it without building up a ton of artifacts that look ugly.
Defocusing the scanner by a small percentage and than applying
convolution sharpening ("Smart Sharpen" in Photoshop CS2) can often
produce superior results. Most tonal shaping, done in small steps,
doesn't affect the appearance anywhere near as much as sharpening and
resampling operations.
That's about as much as I can say unless I know more about what kinds
of operations you're planning to perform on the scan files.
Godfrey
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