Gah, why wasn't the forum notifying me of replies?
Okay, to clarify what I need this for, I want to do some wide field astrophotography, so the wider the better. I'm probably looking more for an equivalent
28mm rather than an actual 28mm, which means around 17mm or so actual focal length.
I think I can do that pretty nicely with my 15-85mm when pulled to its widest, but it's still fairly slow, and without an equatorial mount I'm risking star trails (which, admittedly, isn't much of a concern at this short focal length, but still a concern). The 50mm is fast enough, but is probably a little tight on the FOV for what I need since I only have a crop sensor camera.
On that note, any idea how focal length translates into FOV in the photo? The Andromeda galaxy, for example is about 2 degrees wide, and I think 85mm's FOV is about 15 degrees on a full frame, so I can probably capture Andromeda nicely on my lens and crop it, provided I have an equatorial mount.