Canon 50mm F1.8
Sat down a few days ago with a friend to chat on our L lens and what it could do. Then cropped up a topic on prime lenses. He said a friend introduced him to the Canon 50mm F1.8 - cheap and best! Converted it is only USD80.
Has a plastic body but glass lens. Using it would make you a believer.
Tested it against my 28-105 zoom and 80-200 IS f2.8. Of course the L lens was top but against other zoom lens set at 50mm on a 5D with full frame, distortion was at minimal with a good tonal range (though a bit contrasting)
Have used it last week on a group photo session in an institution outdoors. It was really good. Prime lens stuff. Minimal distortion and sharpness was great.
Being light and inconspicuous, I am now carrying it around together with my L lens in case a 50mm is required.
For this price it is worth its weight in gold.

