Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Austin 
Franklin" <darkroom@i...> wrote:
> > There's no way that channel mixing from RGB can come
> > close to that shape of response curve and selectivity.  So
> > at this point I'm inclined to think that Anthony not only has a
> > theoretical point but an issue that can in some cases has
> > a very real effect on an image.
> >
> > Roy
> 
> Roy,
> 
> YOU may be very well right about what you postulate, but that 
isn't what
> Anthony said.  He said that you can't "duplicate" Tri-X by using 
color
> information and mapping the tonal curves.  He didn't say you 
can't
> "duplicate" Tri-X when used with colored filters...etc.  That's 
possibly a
> different problem.  That changes the response curve for Tri-X, 
which he
> never said was what he was doing.
> 
> Now, on to your theory.  Why do you believe that adjusting the 
tonal
> response with a colored filter can't be duplicated?  What is the 
frequency
> range represented by RGB, and what frequency do you believe 
RGB doesn't
> cover?
> 
> Austin

Austin,

There isn't any frequency that's not covered -- the issue is the
SHAPE of the frequency response curve.  Using a colored
filter at picture taking time, you can very selectively choose a
response curve based on the film and the colored filter.
However if you take the picture with color film you've reduced
the spectrum information to just 3 values RGB and the
response curves for those 3 values are fixed.  With channel
mixing you only get to pick percentages of those fixed
response curves -- NOT simulate or duplicate the SHAPE
of an arbitrary colored filter response.

The whole concept is very simple.  In the real world visible
light is a continuous spectrum of light frequencies with
arbitrary amplitudes for each frequency.  Conversion to RGB
is reducing an essentially infinite number of possibilities to
just 3 numbers.  The 3 numbers work as well as they do
because they were specifically designed with what the human
eyeball can distinguish.

Roy