Archived posting to the Leica Users Group, 2002/03/27

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Subject: Re: [Leica] Re: dMax and Dyanmic range - scanners
From: "Michael Abbott" <lists@mabot.com>
Date: Wed, 27 Mar 2002 10:32:12 +0200
References: <B8C62659.19DB%lists@johnbrownlow.com>

> >  I'm confused about what your formula means. It says that th. dyn. range
in
> > *stops* = number of bits.
>
> No, that isn't true. You can't talk about 'dynamic range in stops'.
Dynamic
> range factors in the tonal resolution you require.

OK, I think I see now. I didn't realise we use the same sort of numbers for
both densities (light absorbtion in film or filters, which we can talk about
in stops) and 'digital detail' meaning (log) how many steps betweem our
system's black and white.


> 256 shades of gray (8 bits) is usually considered sufficient to be
perceived
> as continuous tone, so theoretically the maximum Dmax any scanner requires
> is just under 3. This would however require you to get the scan absolutely
> perfectly right on the first pass.

Doesn't this build in assumptions about the maximum brightness range of the
final product? White compared to black on paper is only ever so-much
different, but if we had a device that could display scenes like snow and
wet black rocks in sunshine, then surely we would need more than 256 shades
for a visually continuous tone?


Perhaps rather than me asking a million basic technical questions, do you
know of a decent basic reference on all this? Most of the stuff I've seen on
the web is recipies for how to scan; I'd like to first understand what's
really going on. Maths is no problem, as long as they define the terms.

Michael


- ----------------------------------
  Michael Abbott lists@mabot.com

(Theoretical physics, clueless about signal processing.)


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In reply to: Message from John Brownlow <lists@johnbrownlow.com> (Re: [Leica] Re: dMax and Dyanmic range - scanners)