[pure-silver] Re: Why does B&W Film Scan so poorly
- From: Jim Brick <jim@xxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Sun, 27 Feb 2005 17:33:12 -0800
Great post Mark, many thanks.
I don't scan anything, I do wet stuff, so I'm not effected by scanning. I
have a close friend who switched from B&W film to color neg for his B&W
work. He converts the image to B&W in Photoshop. I have to admit that the
results look great.
:-)
Jim
>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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