Digital BW, The Print

> Is it better to take colour slides and then
> convert them to black and white before printing?

As compared to ... what?

As compared to shooting black and white film to begin with ... no, I don't
think shooting color slides is a better option.  There are zillions of
black-and-white films out there, each with its own characteristics, and this
gives you tremendous flexibility in getting the exact kind of B&W "look"
that you want.

Color slides, on the other hand, are designed to give lifelike colors, and
lifelike luminance.  This is fine, as long as your final image is required
to be lifelike, too.  Problems arise, though, when you remove the color,
because areas that might be very distinct in color often turn out to be
equivalent shades of gray in luminance.  The B&W conversion is accurate, but
not necessarily pleasing to the eye.  My experience is that most B&W images
converted from color look mysteriously "flat," even though they are
technically accurate as far as relative luminance goes.

The situation is different with B&W film.  For example, Tri-X is relatively
insensitive to red light.  If you shoot a subject with isoluminant areas of
red and blue with Tri-X, the red areas will be a darker shade of gray than
the blue areas in the final image, even though, in theory, they should both
be the same shade of gray.  If you shoot the same subject in color and
convert to grayscale, the two areas _will_ be the same shade of gray.  The
problem here is that you might want to be able to distinguish them in black
and white, and Tri-X will allow that, whereas a simple grayscale conversion
will not.

Tri-X thus has a distinctive "look" to it.  It's not an accurate reflection
of real-world luminance--but then again, black and white isn't an accurate
reflection of the real world in the first place (since the real world is in
color).  The response of Tri-X lets you see details that might not be
obvious if you just grayscaled a color image.

Other black-and-white films offer similar advantages.  Tech Pan is unusually
sensitive to red (almost the opposite of Tri-X), so red objects tend to
"glow" a lot more in Tech Pan, giving the film its own distinctive look.
The sensitivity to red is often flattering for portraits.

Then there are issues of grain.  You can simulate grain in a converted color
photo, but you can't exactly recreate it.  Conversely, any grain that is
present cannot be removed.  So you won't be able to simulate Tri-X with
converted Velvia because Velvia doesn't have enough grain, and you won't be
able to simulate Tech Pan with converted Velvia because Velvia has _too
much_ grain.

Even more extreme examples can be given.  Infrared photography, for example,
requires infrared film; no amount of manipulation of any color image can
produce anything like it, because color images contain no information on
infrared.

All of the above applies to color digital images just as much as it applies
to color film images.  Digital images are crippled in an additional way,
though, in that they have lower resolution than equivalent film images, even
when the same number of pixels is used.  That is, since each original pixel
in a digital camera only receives one color, a significant amount of
luminance information and detail is lost in the digital image.  This is made
more obvious by a conversion to B&W.  Even with the same number of pixels,
the converted digital image may not look as sharp as an image scanned from
B&W film.

So, in my own estimation, if you want the best black and white, you must
shoot black and white film exclusively. If you are investing in a dedicated
solution for printing the best possible B&W photos, it only makes sense that
you'd want to pursue a similarly dedicated solution for taking the B&W
photos in the first place.  Black and white is more than just images without
color; it's a whole different domain.