Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Paul D. DeRocco" 
<pderocco@i...> wrote:
> > From: Roy Harrington
> >
> > Here's another way to look at it.  At the low end say the A/D is
> > distinguishing between
> > a value of 1 and 2.  No matter how many samples you average
> > together you'll
> > get either 1 or 2 since it's integer math.  This is quite
> > different than in the upper
> > range.  E.g. say you are distinguishing between 100100 and 100200
> > where the
> > bottom 2 digits are in the noise.  If you average 10 samples
> > you'll be close to being
> > able to distinguish 100140 and 100150.  The point being that if
> > its in the bottom
> > bit the multiple-sample noise reduction doesn't get represented.
> 
> The averaging I'm talking about takes place in your eye or brain. Across a
> large number of pixels, even if the pixels are all 1's and 2's, you can
> still represent in-between values like 1.5 or 1.123, by varying the
> proportion of 1's and 2's. You just can't do it for an individual pixel. For
> that to work, however, there has to be enough noise to cause dithering,
> i.e., to break up any posterization.
> 
> Look at blue sky in any digital image, and it will look like a smooth blue
> gradient. Zoom way in on it and it will look like blue confetti. As long as
> you're not examining individual pixels, the dithering caused by the noise
> effectively increases the bit depth. (That's why GIF images work at all.)
> Unfortunately, for fine detail, you need those individual pixels, and that's
> where the noise limits your dynamic range.
> 
> My point is that the dynamic range we're talking about is the dynamic range
> for the highest spatial frequencies, where individual pixels matter. But for
> low spatial frequencies, you automatically get more dynamic range, because
> the noise is mostly high frequency.
> 
> --
> 
> Ciao,               Paul D. DeRocco
> Paul                mailto:pderocco@i...

Feels like we've drifted a little from the subject.  My comment only had to do with
how the dynamic range of a sensor directly influenced how many bits you needed 
in the A/D and why -- that it is an integer arithmetic reason not a physics reason.

I think you are confusing dynamic range with smoothness or resolution of gray
values thoughout the scale.  Dynamic range is only a measure of how far apart
the darks shadows can be from the brightest highlights.  It's a matter of seeing
detail in shadows without blowing out the highlights.  Dynamic range is just
a ratio -- no more.

What you say about resolving smooth grays and averaging many pixels I totally
agree happens.  It just doesn't play a part in dynamic range.

What I think is a common fallacy is the logic:
1) More dynamic range  implies  more bit depth needed
2) More bit depth  implies  more gray values
Therefore:
More dynamic range is one and the same as more gray values.

This almost seems obviously true but in fact it isn't true.
(1) is only true because you need more bits to get the max/min larger
NOT so there are necessarily more distinguishable values in between.

Roy