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[OM] B&W thoughts

Subject: [OM] B&W thoughts
From: HI100@xxxxxxx
Date: Tue, 18 Sep 2001 01:11:10 EDT
Cc: image66@xxxxxxxxx
Ken N. wrote:
>>
Just a comment regarding scanning resolution vs grain:  I did a maximum 
resolution scan of a Tri-X neg and compared the "grain" with a chemical print 
I made of the same frame.  Even though the film grain is larger than the 
pixels, the pixels are not round, nor do they vary in size and position.  It 
really takes about 9 pixels (3x3) just to get a halfway realistic 
representation of a single grain.  The scan appears NOISY as compared to the 
photographic print as during the scan a pixel may pick up the average of 
several grains or it might pick up just the center of a single grain.  This 
is a subject of interest to me, because I KNOW that Provia-F is as close to 
grain-free as possible, but digital scans seem to pick of "grain" that isn't 
there in a normal photographic printing process.
<<

Ken,
       This is related to "Aliasing"  (in photo speak "moire"). We need to 
sample the picture with more than 2 pixels per line of resolution in any 
direction to avoid this. (this is the so called theoretical Nyquist sampling 
criterion). In practise you need a bit more than the theoretical 2 pixels , 
especially since the pixel spacing is not even the same on the diagonal as it 
is in vertical or horizontal scan directions. Aliasing, from pixel sampling 
adds low frequency noise that does no exsist in the original. The fact that 
the pixels have spaces between them also increases noise slightly because the 
sample does then not average over the theoretical whole pixel area.Your 9 
pixel estimate for a dot translates to about 2 pixels per "line" 
(line=1dark,1light transition) in any direction. Right on the limit. Even 
cheap scanners can improve aliasing a little by scanning and averaging 
overlapping pixel positions in the direction of the scan.


                  Regards,
                 Tim Hughes
                >>Hi100@xxxxxxx<<

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