The smaller the individual photo receptor sites, the higher resolution
you need from a lens.  That's one of the reasons for going full-frame - as
an example, the 5D, even with 12 megapixels, still has larger receptor sites
than the 20D at 8 mp.  Because of that, you don't need *quite* as much
resolution from your lens.

 This really shouldn't surprise anyone with more than a passing familiarity
with photography, since going to a larger negative has, since the inception
of photography, been the method for obtaining greater detail despite
limitations in both optics and film.  To put some numbers to it, some of the
dSLR sensors today are theoretically capable of resolving 100 lp/mm.  If you
look at the gigapixel project, their design goals were, I believe, just a
modest 30 lp/mm or 40 lp/mm over the film plane - but because there's such a
large film plane, that works out to nearly unfathomable amounts of recorded
detail.

 If you want to compare the resolution requirements of film and digital, it
depends on what kind of film you're talking about.  If you're shooting some
400-ISO film that you bought at the supermarket, then the digital sensor
will provide far more resolution, and hence take more advantage of a
high-quality lens.  If you're talking about an ultra-fine grain, low-speed
slide film, then it's going to have the capability to resolve more than all
(or nearly all) dSLR cameras today.

 It should also be noted that in the situations under which most
photographs are taken, an increase in resolution of either the lens or the
film/sensor won't do any good, because the camera movement and/or other
factors (such as diffraction) will limit the overall resolution anyway.

 As for your DR XT, you're not wasing good glass on it, sticking a good
lens on it isn't going to disappoint.  Besides, there are lots of
differences between a mediocre lens and a good lens other than just
resolution.  Even getting aside from the non-visual aspects such as build
quality and autofocus speed, you still have contrast, color, resistance to
flare, the quality of bokeh, the shape of the aperture (visible in
out-of-focus highlights), barrel/pincushion distortion, presence (or lack)
of coma, astigmatism, spherical aberration, and various other deficiencies.

steve

Once more we see the urban legend of different lenses for digital than
film.

Just get the best lenses out there and don't worry about it. Your
lenses are going to be better than you are for a long time.

**********************************************************

"A combat photographer should be able to make you see the
color of blood in black and white"

                    David Douglas Duncan
                Speaking on why in Vietnam
             he worked only in black and white
     http://www.hrc.utexas.edu/exhibitions/online/ddd/

of picture quality for a 1.6 dslr sensor vs. a full frame dslr sensor
or
35mm film body. It seems to me that the lens quality would be more
critical for smaller
sized dlsr sensors. <<

Lens quality should have a correlation simply to FRAME SIZE as has been
proven in 100 years of film photography...a large format lens does not
need nearly as high spatial resolution performance as a microfilm lens!

In the context of crop vs. FF digital, the factor is that crop uses
LESS of the image circle of an lens than FF format, so the lens need
not perform as highly out at the edges of the image circle.  So smaller
is less demanding in this regard!

Beyond resolution (Wolfe and wilt replies), there are other qualities in
higher end lenses which include build, faster apertures, constant
aperture with zoom, internal zoom and focus, color rendition, proneness
to vignetting (no issue with 1.6x crop), proneness to flare and some
others I've likely missed.  Avoid high ratio zooms.

Since cameras continue to rise in resolution, and at least with Canon
there are FF sensors, it makes sense to err on the side of resolution
and all qualities that improve the image in your lens selections.

With the D200 touting 10 Mpix, and rumors of a new "prosumer" Canon
ahead of PMA 2006, moreso.  Even if you're happy with 8 Mpix, you _know_
you'll be lusting for 10 or 12, or whatever Canon do.

Cheers,
Alan.