[pure-silver] Re: Why does B&W Film Scan so poorly
- From: Sauerwald Mark <mark_sauerwald@xxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Sun, 27 Feb 2005 16:59:41 -0800 (PST)
--- "Justin F. Knotzke" <jknotzke@xxxxxxxxxx> wrote:
>
> Hi,
>
> I love B&W. Really, I do. But I am shooting with
> it less and less
> because it scans horribly. It never looks nearly as
> good as when I scan
> colour and then desaturate.
>
> Why is that? Any websites with tips on how to
> improve this? I did
> some searching and only came up with a few replies.
> At worst, maybe
> someone would be as so kind as to explain to me what
> B&W scans worse
> then colour.
>
> Thanks
>
> J
>
Justin
A few thoughts on this.... But first a bit of
background. I am an amateur photographer, and do the
bulk of my photography in B&W. For a profession, I am
an electronic engineer, and have spent much of my
career working on Analog To Digital (A/D) converters,
many of which are used for image digitization.
When scanning an image, there are two, completely
separate things going on. You are sampling spatially,
and you are measuring the opacity of the film. In
sampling spatially, the determining factors are the
size of the spot which is sampled - which you rarely
if ever see specified on scanners, and the number of
samples per unit width. There is a theory (Nyquist
Theorem) which dictates how small this spot must be,
and how many samples you need to be able to sample the
image without loss of information, and the basic
answer is that the spot needs to be about half the
size of the smallest grain particles on the film.
This is where there is a significant difference
between colour film and B&W - in the colour film
process, the grains of silver are converted to dye
clouds, and they are on different layers of the
emulsion, with the end result being that there is not
as much high spatial frequency information on a colour
negative or transparancy as there is on a B&W
negative. Most scanners are designed to scan colour
images, and as a result, have their sample sizes
optimized for the colour die clouds. If you were to
design a scanner purely for B&W you would want to have
a much smaller sample size, and a greater number of
samples per inch than what you find for colour
scanners.
The second dimension to the scanning process is the
dynamic range of the image. This is another place
where there is a real problem. On a microscopic
level, a B&W negative is just that - BLACK and WHITE -
it is only when we step back, and average the
transmission through the negative over an area that is
large compared to the grain size that greys appear.
The ideal scanner for B&W would only need a 1 bit A/D
converter, which would look over a very small area
(smaller than the grain size) - to generate a digital
black and white file, which could then be manipulated
digitally to give us what our eye sees when we look at
a good print. Alas, this is not what we have, what we
have is an A/D converter which averages the
transmission of light over an area large - but not
very large compared to the grain. The result is that
there is a relatively large amount of noise that ends
up in the image, which is created by exactly how the
grains within one of the samples from the scanner.
There are other things that can complicate matters -
there is something called ICE which is used to reduce
such things as scratches and bits of dust on colour
negatives or slides. It works on the theory that
negatives and transparancies are transparant to IR,
regardless of the density in the visable part of the
spectrum. This assumption does not hold true for B&W
or for Kodachrome - so if using a scanner with ICE -
turn that feature off for scanning B&W.
Where I have had the best results, is not to try
scanning the negative, but to generate a print in the
wet darkroom, then scan the print.
Mark
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