Digital BW, The Print

Alex writes:

> The spectral response of BW film is a fairly
> simplistic curve that 'could' be somewhat
> represented by three points.

Just because the curve seems simple doesn't mean that the conversion is
possible.  Even a flat response cannot be simulated when filters are used.
For example, using a yellow filter on film with a flat response will produce
results that cannot be duplicated by any manipulation of an RGB image.

> OR, in retrospect, perhaps the film curves
> subtile sensitivity curves Illustrate Your
> Point of not being able to reconstruct
> them perfectly with 'three points' .... ???

The fundamental problem is that the B&W film response (or indeed the
response of any image-capture device, be it film or digital, color or B&W)
is a function of the spectral distribution of light in the original scene,
which includes an infinite number of different frequencies of light with an
intensity for each frequency, and this for each point in the image.  No form
of image capture records all this information, so once an image is capture,
there is no way to recreate the distribution in the original image, and
since converting an image from one form to another in order to simulate the
response of a different image-capture device requires this information from
the original image, there is no way to convert a RGB image to accurately
represent a B&W capture, or anything else.