I'll almost certainly be using it outside with lots of sunlight. The Sigma has actually taken some very pleasing shots with great bokah but it never gives crisp sharp subjects, they are always just under sharp but still very nice. Thats a 5-6.3 variable aperture, does the IS f/4 Canon with a 2x teleconverteron that lens sound like a good idea or would I possibly step forward to lean backwards adding the teleconverter?