Digital BW, The Print

The key to this answer and most of this discussion is the filters. Any 
filter for electromagnetic energy - microwave, optical or Xray have 
characteristics. These filter characteristics come in two flavors, 
amplitude response and phase response. A black and white sensor captures 
only the amplitue, i.e., equivalent to the energy energy, of the wave 
that hits it at a specific frequency. The response is not flat - as we 
all know nor should it necessarly be or else we would not use filters 
when shooting B&W film. When you shoot in color you have three amplitude 
responses to contend with - one for each filter measures the amplitude 
at each frequency. It's the area in the fall off of these filters where 
the problem arises.

While you would have to know quite a few specifics about the color 
filters used to answer such a question, in general the calibration to 
ture a color negative into a "TriX" image would be quite an undertaking.

The issues of dynamic range also comes to play in the shadow and 
highlight areas. As long as you are in the linear range of the sensor ( 
or film ) you can do all the manipulation you want. When you get into 
the nonlinear range, e.g., the shoulder and toe, all bets are off and 
calibration of color to black and white that maintains the fidelidity 
you get from a black and white sensor is most likely not possible.

Truman

Peter Nelson wrote:

>All I asked for was a real-world empirical example, not a 
>gedankenexperiment.  Why can't you name a specific wavelength that 
>is picked up with a certain density on some black and white film, 
>but "falls in between" two colors on some color film in such a way 
>that if converted to b+w it would look different?
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