It seems to depend on the lens more than any other setting for me. With my 50mm f/1.8d, it hunts a bit in low light. With my 85mm f/1.4, focus lock is extremely quick compared to my D300 in very low indoor light. I played around with high ISO shots in the kitchen last night and could help but grin after every shot! It really is a huge step up from my D300, which I loved using.