There are quite a few 3rd party options in the 10-20mm focal length range. 18mm is certainly wide... but 10mm is ultra-wide.
The f-stops aren't so important in these lenses for a couple of reasons. (1) you're likely to be outdoors taking photos in daylight and if it's night, you're likely on a tripod. (2) while it's interesting to have a sharp subject and deliberately blurred background... ultra-wide lenses don't do this well almost regardless of the f-stop (I have a 14mm f/2.8 and I can focus that lens to about 3' and the entire world is mostly in-focus... even at f/2.8). When shooting landscapes, you typically want the whole field to be focused.
If, in addition to your 100-400mm lens... you could only choose to bring ONE lens... then I'd probably favor the 18-135 over a 10-20mm range (Canon's is a 10-22).
In addition to an ultra-wide lens giving you a wider angle, it's going to stretch the depth and distance on the scene. Objects that are really just a little far away will seem a LOT farther away (and smaller). When using these lenses, it's often a good idea to find a subject which is VERY close to you and feature that foreground object against the background. Landscapes are often shot from a low height above ground to get some foreground interest (rocks, grasses, etc. especially if you can find an interesting object) .... otherwise the shot just looks like a panorama of very distant and very tiny objects -- none of which are large enough to inspect with very much detail.
If you read the reviews on the lenses in the 10-20mm range, Canon's tends to get the more favorable reviews.
Tim Campbell
5D III, 5D IV, 60Da
