currently using 17-50. but performance under low light or indoor still limited, so looking for upgrades. shortlisted some lenses:
sigma 30 f1.4 vs canon 28 f1.8 vs canon 35 f2 (1.4L is too ex :X)
sigma 50 f1.4 (heard of focusing problem)
canon 50 f1.4 (review show poor lens construction)
currently using 50D. intended use will be indoor... budget less than $800.