Back in the days of shooting with actual B/W film, there were a number
of filters that could be used for varying contrast.  But once you'd
shot with the filter, you were stuck with what you got on film.  Same
with using a "filter effect" in the camera.

I'd *much* rather do my BW effects in Photoshop.  PS rules!  You
don't need the channel mixer, either; there are always many ways
to do things in PS.  The hue/saturation layer effects gives you
control over color filtering before going BW, and a second
hue/saturation layer effect lets you desaturate.  The neat thing
about layer effects is that they don't alter the base image;
you can change anything you want in them, even in later editing
sessions.

If you want to play with hand-tinting effects, you can edit the
mask layer on the hue/saturation mask layers to block selectively
the color filtering or the desaturation.  Or if you want single-
color effects like old hand-tinted photos, you can apply colors
in new layers and set the layer attribute and transparency to
get the effect you want.  The possibilities are endless.

Diane