even slow autofocus is faster then most people can focus manually
While most bench tests will confirm this statement, I find that in real situations this is not always the case (and I'm using, for the most part Canon L USM
Consider - aircraft sitting on the tarmac - pick camera up, point at it, and yes, the AF
will probably lock more quickly than you can manually focus. But then think about tracking an approaching aircraft - once the general focus is established, the differences required are very minor as the aircraft gets nearer, requiring only small adjustents. I find the AF
can sometimes result in losing focus completely for a brief period (perhaps as result of the key contrats area moving off the sensor) and taking another moment to re-aquire. Yes its pretty fast, but I find tracking using manual focus is more likely to result in the subject staying in focus all of the time = less missed shots.
Colin K. Work, Pixstel