It’s very difficult to quantify the performance of Autofocus systems, but we can work with the lenses under similar, controlled circumstances and make some fairly subjective observations. The f/1.2 lens is pitched as having “… a Ring-type USM (ultrasonic motor), high-speed CPU and optimized algorithms” to deliver much improved autofocus. In practice, we observed much better performance than the f/1.8 in dealing with low contrast subjects. We saw slightly better performance in dealing with low light conditions, and a general increase in focus speed over the f/1.8 lens. Clearly, at low light and low contrast levels where this lens will be needed, the improved autofocus is a slightly better performer.