Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Clayton 
Jones" <cj@c...> wrote:

.....
> 
> What he IS saying is that we cannot exactly duplicate what the 
BW film
> can do.  There is a difference between emulate and duplicate.  
Whether
> the difference is discernable to the photographer is another 
matter. 
> Both I and another photographer have reported that we have so 
far
> failed in our efforts to get a color->bw result that satisfies us, 
and
> I think that fact is an important part of the topic.  In fact, it's
> really what the topic is about.  Anthony's information is the first
> plausible explanation for a question I've asked on and off for 
months
> and gotten almost zero response to.
...
> I appreciate very much what Anthony has brought to this and 
I'm sorry
> he has become so frustrated at the obtuseness of some of the 
other
> participants.  His explanations have been crystal clear.
> 
> Regards,
> Clayton
> 

Clayton, 

I think you are right on in your assessment of the
situation.   Anthony's explanations seemed pretty clear as
well but the nagging thought is -- does it make any difference
in the real world photography?  Notch filters are pretty rare
in ordinary photography so I decided to look at the response
curves of real black&white filters.  B+W is kind enough to
show response curves for many of their filters.

The question is: if you take a B&W picture with a filter and
B&W film, can you get the same grayscale response by
taking the picture in color and then channel mixing the RGB.
My first reaction is that you can do pretty well -- like make a
blue sky darker by not using much of the Blue channel.

But on considering what filters I do use -- several yellows,
oranges, reds and greens -- I have a lot of options on
what spectral response I want for a particular image.  Then
looking at the response curves for the various filters it
evident that the frequencies selected have fairly sharp cut-offs.
Some of the filters that I choose between are closer than
50nm apart and quite steep (0-80% in less than 50nm).

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