Digital BW, The Print

what is your high re scan? 400odpi?

> From: "pbard2002" <phil@...>
> Reply-To: DigitalBlackandWhiteThePrint@yahoogroups.com
> Date: Wed, 05 Dec 2001 17:19:22 -0000
> To: DigitalBlackandWhiteThePrint@yahoogroups.com
> Subject: Re: [Digital BW] 16-bit Scanning: Why?
> 
> Sorry for the long post, but I thought I might add what I've discovered
> with my images. After reading through this thread about high bit image
> editing, I thought it was worth noting that one important distinction
> needs to be made. I've spent some time experimenting with 8 and 16 bit
> BW scans and trying to determine if there is a perceptible difference
> in the output image.  Also have had numerous discussions with Antonis
> on this, who's knowledge of PhotoShop is truly phenomenal, and we've
> recently done some testing with hi-res piezo's output on the 7000. Here
> is what my current conclusions are, based on what I've been able to
> see:
> 
> There is no doubt that when you drop from 65,000 levels of gray (16
> bit) to 256 (8 bit), there are pixels that shift in tonality, since
> they are moved to the nearest 256 level gray value. But whether or not
> you will see this, either as a slight compromising of the tonal range
> or as flat-out posterization has to do with the resolution of your
> scan, if it is a B&W film neg we are talking about.  Computer generated
> images are a different matter.  The key seems to me to be the
> distribution of pixels and their relative values to each other.  If, in
> 16 bit, you have a large region of pixels that are the same value next
> to another large region of pixels that is one value lighter, you
> probably won't see a difference in a print, as they are only 1/65,000th
> of the tonal spectrum apart.  But if you convert to 8 bit, either PS
> will assign them to the same gray value OR to adjacent values depending
> on the choices it makes, and the two regions then become only 1/256th
> apart, and the border between these areas will be visible in print, ie.
> posterization.
> 
> However, and this is the important part, if your scan is high enough in
> resolution, at the pixel level you will see very few regions where
> there are many pixels very close together in value.  In the scan, each
> silver grain in the film is made up of numerous pixels, and these will
> vary substantially in value depending on whether they are located in
> the middle of the grain, the edge, the clear film base between, etc.
> You will see this if you look closely down at pixel level, the image
> actually looks "noisy."  But the perception of tone these pixels render
> is effectively an averaging effect, since we are seeing them from very
> "far away" in a sense, since our prints are not such severe
> enlargements.  So if you take a 16 bit scan and drop to 8 bit, those
> pixels will each shift marginally lighter or darker, but since the
> viewer's impression of tonality is made up of their "average," one
> usually cannot perceive this. And presumably the number of pixels that
> shift lighter and the number that shift darker will be relatively equal
> as this is a random process. Posterization will only occur if you have
> a predominance of one color pixel in a region that is visible at the
> viewer's level of resolution.  This can, BTW, develop in smooth tone
> areas of a hi-res image if you res down, or print _very_ small, as this
> may force pixels that were once only slightly different in value to
> become the same, again producing posterization.
> 
> I print a lot of skies, and in low-res scans I have trouble with
> posterizing if I use severe levels or curve tweaks in 8 bit. Or if I
> blur to smooth out grain. But I can get away with the contrast tweaks
> just fine in hi-res scans.  There is no visible difference in the
> output of an image that is 16 bit going to the printer and an 8 bit
> one.  Even though there is combing in the histo's of some of the layers
> in my PS files I can see no difference in print, which is what matters
> to me.
> 
> Hopefully this is coherent enough, it's a bit hard to explain using
> just words. Also hope it sheds some light...
> 
> Phil
> http://philbard.com
> 
> 
> 
> 
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