决定自动聚焦速度的因素

I understand large aperture lenses allow higher shutter speed - people call them fast lenses. On the other hand, I am not sure I understand when a lens is said to AF slow, independent of camera body.


Under the low light situation, some slow lens does not have enough light to the sensor so the system "hunts", taking time to lock on the object. That is a combination of camera body and the lens aperture.

With bright sun shine and a good contrast object, will one lens AF slower than others on teh same camera body?

I have a Sigma 18-50mm F2.8. When I first used it on a D300, I was amazed by the AF speed comparing the same lens on a D80. I had to let it focus on my hand to see if AF is working. So, I may be wrong, but I think the aperture and the body are two biggest factors for AF speed.