Arie Vandervelden , Oct 29, 2008; 05:04 p.m.
20/2.8 is wide only on a full-frame body. On a 40D the field-of-view is similar to a 35 mm lens, just short of normal.
For landscape, what you want is a lens that does 10 mm or 11 mm. Have a look at Canon 10-22, Sigma 10-20, Tokina 11-16. Tamron 10-24 should be out soon. All of these ultrawides are sharp and will mate nicely with your 24- 105. Can't go wrong.