Digital BW, The Print

cirkutguy writes:

> I'm curious as to what information is missing from the color image
> that would be in the black and white.

Well, it's a long story ...

The light in a real-world image consists of a blend of an (essentially)
infinite number of frequencies at an infinite number of amplitudes, for
every point (pixel) in the image.  No capture device is capable of
recording an infinite number of amplitudes for an infinite number of
frequencies for each pixel, so a great deal of information is discarded
when image capture occurs.  In a color image, all of the original
information is reduced to three numbers, based on a fixed, one-way
conversion function.  In a black and white image,
all of the original information is reduced to one number, using a
different conversion function.

It's mathematically impossible to reconstruct the original scene
information from either B&W or color image captures.  Most of the
original information is gone, and cannot be recovered.  Since both types
of capture are functions of the full range of original image
information, there is no transformation that can be performed on a given
set of image data that will convert it into the image data that would
result from a different type of capture.

For example, it's intuitively obvious that a color image cannot be
reconstructed from a black and white image file. However, it is also
true that a black and white image cannot be reconstructed from a color
image file. It's possible to produce black and white images the capture
of which would be a subset of the function that produced the color
images (i.e., in which all information necessary has been preserved),
but not anything else.

In consequence, there exists an infinite number of black and white
images that can only be produced by capturing an original scene directly
in black and white with an appropriate monochrome capture method. No
manipulation of a color scene can produce these black and white images.

In summary: if you want to shoot black and white, you must _capture_ the
original image in black and white.  Converting color always gives an
inferior result.

> I too find that I often can't match the look of black and white
> film with a color conversion, but have always assumed that some
> amount of messing around would do it.

You can come close to the look of some black and white films, but you
cannot duplicate the look.  If you really must have exactly the look of
a particular B&W film, you must shoot that film directly.

This is even more true if you are using any kind of filters when taking
the shots.