I just got this lens a few months age and finally had a chance to shoot with it. After reviewing the images I noticed there is a dramatic drop off in focus just on the right side of the images. Never experienced this and am trying to figure out if it's the lens or camera 5D mk iii. The set up was single row of people (12) centered in the frame shot fairly loose so they are within the confines of all af points. Shot at 28mm F5 iso 100 and 1/200 sec. Any help would be appreciated.