Originally Posted by
RustBeltRaw
. . .It's worth noting that I use the 50mm f1.4 USM heavily for indoor action with rapidly-varying ranges, so I am stressing the crap out of the AF system. The USM versions tracks more finely, and I assure you that the adjustments are quicker. In a static test, one may not notice a few tenths of a second's difference in focusing time, but it becomes vastly more apparent when your subject is juking, diving, and twisting like mad. Sometimes I think roller derby was designed from the ground up to frustrate photographers.