Digital BW, The Print

Peter writes:

> There's no reason why that should be true.

There's an excellent reason why it is true, which I've just explained in a
separate answer to Seth's post.

The only accurate B&W you can get from an RGB image is "perceptual" B&W that
precisely matches the spectral sensitive of the RGB capture device in the
first place.  All other B&W renderings are impossible.  You cannot get the
look of Tri-X from an RGB image, although you can try to simulate it by
cranking down the red channel a bit.  For real Tri-X, though, you need to
shoot Tri-X.

As a more extreme example, there is no way to get infrared from an RGB
image.  You can pretend by dropping the blue and green channels and
desaturating, but that's just pretend ... it's nothing like real infrared.
To get real infrared images, you must capture the image originally with a
device that can record infrared, such as IR film or a CCD with a filter that
transmits IR and only IR.

> Scanning black and white film should produce
> flawless grayscale images.

And it does.  But in that case you are not capturing the original image;
that has already been done for you.

> Why wouldn't it?   What is lost?

Nothing, in a scan.  Information is lost only when originally capturing the
image from real life.

No form of image capture today can capture information for the entire
visible spectrum for any image.  Color imaging reduces that entire spectrum
to three values; black and white imaging reduces the entire spectrum to one
value.  It's a one-way process.  You can do anything you want when making
that conversion, but once it's made, it's impossible to transform it into
the result of any other conversion, because the required original spectral
information is gone.

> If you scan T-Max or Tri-X the shape of the
> curve should be the same for all three channels,
> with just a translation applied due to the film
> base color.

It is.  And no adjustment for base color is required.

> I agree that digital cameras are another story ...

In this particular domain, it doesn't matter whether you use digital or
film.  The constraints of the conversion to RGB or grayscale are _identical_
for both types of image capture.  It doesn't have anything to do with
digital at all.

The only disadvantage digital photographers have is that they cannot shoot
straight black and white with their color digicams--whereas a film
photographer can replace color film with B&W film.