Hi i have just bought a canon 40D body only and am about to invest in a lens. I am looking at a lens for landscape photography and was recommended the ef17-40mm f/4L USM, can someone please explain what all that means?? I am new and I am sorry but it does not mean anythhing to me. ef ? 17-40mm ? f/4 ? L ? (why is it red?) USM ? is it a "fast" lens? and what is the zoom? any help would be gratefully appreciated and I would like to understand it before I buy it and will it be fine for landscape as recommended? thank you.