Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "John Vitollo"
<jvlist@...> wrote:

> Helen Bach posted on another list that some of the RGB values are
suspect with some films. I 
> just take all those values with a grain of salt and just pick the
best one for the image I'm 
> using...nothing more, nothing less.


If anyone is interested, here is what I wrote over on the Colorvision
group, with a small addition:

I'm not sure what those exact numbers mean, because they will depend
on what light the film has been shot in - and of course they would
also be changed by the use of filters. However, there is an
interesting feature: for Ilford and Agfa films, the B number is lower
than the G number. For Kodak films it is the other way round.

Why is this? Maybe it is because the mixer settings have been
estimated from the published spectral response curves. Agfa and Ilford
published wedge spectrograms made in tungsten light (2850 K for
Ilford), at one density only. Kodak publish equal energy response
curves at two densities. This makes Kodak films look as if the blue
response is higher than the green response, and vice-versa for Ilford
and Agfa. It's just the way the spectral sensitivity is shown. You
need to compare wedge spectrograms or equal energy curves. You can't
compare one with the other - that is misleading, and whoever wrote
those channel mixer numbers appears to have been misled. Not that it
matters one bit for imitating B&W film digitally because that will
only be a close approximation at best, but it does matter
if you wish to understand the differences between real B&W films.

Best,
Helen