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[filmscanners] Re: 8 bit versus 16

On 22/9/03 16:44, "Roy Harrington" <roy@harrington.com> wrote:

> On Monday, September 22, 2003, at 02:04  PM, Brad Davis wrote:
>> On 22/9/03 12:09, "Preston Earle" <PEarle@triad.rr.com> wrote:
>> ...
>>> 2. All visible files are the product of a final
>>> resize/pixel-combination
>>> of some sort, at least until we get 2800x4200 or larger video screens.
>> I don't think this is relevant, there are places in this note where I
>> suspect that you may be confusing DPI and bit depth at a pixel.  This
>> conversation is only about bit depth.
>>> ...
>>> For 16-bit processes to be relevant, wouldn't adjacent pixels have to
>>> be
>>> identical to more than 8-bit precision?
>> Yes, exactly, a 16 bit representation is more nearly continuous than 8
>> bit.
>> The fact that 8 bits look continuous is an artifact of our perceptual
>> system
>> which can't discriminate that many levels ( and some of the work of
>> printer
>> drivers to smooth things out for the resolution of the printer).  Most
>> people can't resolve much more than 2^5th (32) levels, some 2^6th
>> (64), very
>> few much more than that.  ( Of course I presume everyone here can
>> resolve
>> 2^7th or 128 levels.)  Much of the variability in this measure is a
>> function
>> of differing assumptions and different methodology in measuring it, the
>> generally accepted number is 6 +/-1 bits with few people getting much
>> over 6
>> bits.
>>> Preston Earle
>>> PEarle@triad.rr.com
> Brad,
> You are probably right that  --   "This conversation is only about bit
> depth."
> But I think that is one of the main shortcomings of the discussion.
> Resolution
> (DPI or more accurately PPI) and bit depth are certainly very different
> and
> clear properties of an image file.  However the criteria for comparison
> is always
> a print that the human eye looks at to evaluate.  At this level PPI and
> bit depth
> are no longer independent and well defined properties of the image.
> Fewer levels of gray (i.e. less bit depth) is easily compensated for by
> a higher
> resolution --- that's the basic principle of halftoning or of
> stochastic dithering in
> printing.
> Roy

First, I agree, I didn't think that the post I was responding to was about
PPI (Thanks, that clearly is more apt, although Polaroid rated my scanner in
DPI - but what do they know, they went bankrupt!).  Adding the issues you
introduce might make the case for 16 bit a little more certain - I know that
I started out with a strong bias toward 8 bits for a number of reasons, not
the least being the human perceptual system, but there are subtle things
that keep coming up which make me think I may have been wrong - the
interaction between PPI and bit depth being one - I was also able to think
of an example of how a color image might show banding (Austin's arguments
not withstanding), even though Henk refused to support his opinions with an

I still can't figure how a scanner can manage to be noise free
(electronically) at 1/65,000th of its voltage range (that would be necessary
for a full 16 bits - but I can imagine a somewhat smaller bit size as
working (12- 14?).  I also wonder how much dithering (effectively increasing
the apparent bit depth by increasing the number of levels between adjacent
pixels is done by say Epson or HP - or whoever.

Also, in all of this, I haven't seen any reference to the limitation of
paper to present more levels on the one hand, and the subtle effects of
increased levels on perception.  Simple psychophysics isn't sufficient to
explain all of our perceptual capacity - photography paper (silver) could be
much cheaper is we psychologists had it all locked up.


> -
> Roy Harrington
> roy@harrington.com
> Black & White Photo Gallery
> http://www.harrington.com
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