UWA shots of large groups does not look good. The folks on the outter edge usually get distorted and look bigger than those in the center. Or you get loss of detail in the edges. That UWA look that is so cool for landscapes and scenics, just looks bad for shots of groups.
You still need to be at least 10-15 ft away for proper perspective. Lets look at the closest focus, worst case scenario.... At 10ft, 24mm gives you a 10ft x 6ft 8in dimension. That is big enough for three rows of about four people each if you only get upper body in the shot (12 people). That is good enough for most small to medium weddings. (angain, only upper body shots - not full length shots).
At a more realistic 15ft, the same 24mm gives you 15ft x 10 ft, which is enough for 3 rows of 5 people (maybe 6 in the center as you stagger and you don't have any large girthed subjects). That is 16 peole which is adequate for most mid-sized weddings.
For very large wedding parties, where you have 25 people in the wedding party or family shots.... you are going to need a lot of room. For those situations.... Just rent the lens you need. (20mm at 20 ft would give you 24ft x 16 ft, which is plenty for three rows of 8, with 9 in the middle row to stagger = 25 people).
For anything over about 10 peole in the shot, you are probably going to need to bring lights (speedlights in umbrellas will do for the small stuff). And for the 25 subject shots, you are probably going to need to rent monolights, unless you shoot outdoors.
If you get caught on-site and need more width... then in a pinch, you can use the old stand-by of the 18-55 VR. You are going to be stopping in down to f/11 anyway to get the DOF you need, and it is plenty sharp at those apertures.
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Catallaxy
