for starting off? im shooting on a 5d and currently only have a 50mm 1.8. i'm looking for a wider lens, and am planning to get a 70-200 (either f/4 or 2.8, havent decided yet) in the long run. i am starting to do portraits, high school senior pictures, and aspire to be an engagement and wedding photographer. does anyone think the 20mm is a good buy? does anyone happen to have any samples on a FF camera? thanks in advance!