Digital BW, The Print

Seth writes:

> The advantage of using the RGB before converting
> to B&W is in the channel mixer.  That really amounts
> to the same as using yellow, orange, green, etc.
> filters on a B&W film.

Unfortunately, no, it's not the same thing.

Imagine a scene that is lit in some areas with monochromatic yellow light
(as from low-pressure sodium vapor or something), and in other areas with a
blend of red and green light that looks--at least to human eyes--to be the
same color of yellow as the monochromatic light.

If you look at this scene with your eyes, everything appears to be yellow.

If you photograph this scene with a color camera (digital or film, it
doesn't matter), the resulting images also render the entire scene as
yellow.

If you photograph this scene with black and white film and a narrow-band
monochromatic yellow filter, however, you get a different result.  The final
image will be bright in the areas lit by true, monochromatic yellow light,
and dark in the areas lit by a blend of red and green light.  Thus, you'll
see sharp contrasts in the result that were not present in real life (to
your eyes, that is) or to color film/digital cameras.

Given this, it should be obvious that there is no way to convert the RGB
image to any kind of grayscale image that will emulate the black and white
scene photographed through a yellow filter.  Why?  Because both the
red/green areas and the truly yellow areas came out to the same shade of
yellow in the color images--nothing you can do in post-production will allow
you to separate them into light and dark.  So no matter what manipulations
you attempt, you'll never get the same result that you would have gotten
with black and white and a yellow filter.

This holds not just for the use of filters with B&W film, but for the use of
B&W film in general.  Black and white film responds differently to light of
different colors than does color film or a color CCD.  Once an image is
converted to RGB or grayscale, virtually all the information on the original
spectral distribution of light in the scene is lost.  As a result, nothing
can convert one RGB or B&W rendering into another one that emulates capture
of the original image with a different spectral sensitivity curve, because
the spectral information required for this is gone.

This rule is independent of the capture method you use.  Digital or film,
the rule is the same.  The only way to get true black and white with any
other rendering of light than standard, human-eye rendering is to shoot B&W
from the beginning.  It can be black and white film, or a monochrome CCD,
but it has to be B&W to start with.