That little tidbit about the 20D is not trivial.

I don't think the Canon wide angle lenses are a very good choice for
the small sensor SLRs. These lenses have too many design compromises
to give them good coverage of the full 35mm frame, and that makes them
heavy, slow, expensive, and not very wide for the cropped frame.

Instead, consider the 10-22mm zoom. It is excellent and gives much
wider angles than you could achieve with the normal Canon WA prime
lineup.

28mm is not wide angle on the 20D. It is a normal focal length.
Anyway, the Canon 17-55mm f2.8 zoom outperforms the 28mm f2.8 and
probably the f1.8 too.

KS

The Sigma 15-30 is a good performer on the cropped frame, yielding
approx 24mm on the wide end. It is slow and big, though not as heavy as
you would think looking at it. I found it to be a very sharp lens on my
10D though it is prone to flare. Nevertheless, it is a good value at
about 200 less than the 10-22 Canon new. You can find a used one for
around 300 or so.

Eric Miller

Although it isn't a Canon lens, I use the Sigma 20mm f/1.8 prime on a
300D.  It is a very good performer for the money, IMO.  It also has
great bokeh having a 9 blade construction and is from Sigmas EX line of
lenses.  It has great build quality.  For me, another plus was it can
work on full frame DSLRs should I decide to go that route in the future.
 Here's a link:

http://www.sigmaphoto.com/lenses/lenses_all_details.asp?id=3280&navigator=4

http://photography-on-the.net/forum/forumdisplay.php?f=33

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