If you have the time and patience, you can try stitching together photos if the landscape is static. I use a 70-200mm f2.8 lens and it works great, you can literally go as wide as you want. There are a lot of tutorials on the web. Just a thought.
Quoted Message: What Ultra Wide Angle lens do you use for your Canon 5D Mk2? Comments on why you like it or don`t like it. I want to purchase one down in the 10-20mm or 10-22mm range but it must be SUPER SHARP. What make do you recommend? Why?Thanks!
If you want SUPER sharp you will have to go prime lenses. What exactly do you want to photograph with a super wide?
Also any distortion these days can usually be fixed on photoshop to a certain degree
A 14mm Samyang is a cheap alternative for a FF... its ultra wide and has some distortion but that can easily be fixed with a lens profile in lightroom for example... Ive got a Canon 5D Mark III and heres one shot I took over the weekend with it...