Focussing - body or lens?

[Durbs]

Was reading some reviews of the Sigma 120-300 and a couple of people say the AF can hunt badly or fail in low light etc, however none of them say what camera they were attached to.

It was my understanding that with focussing, the lens dictates the speed (and volume) of AF, but it's the body that actually does the focus tracking?

Is this rougly correct?

 

[gcogger]

With a phase detect system, focusing is the responsibility of both the lens and the body. What happens is:
1) The body uses phase measurements to calculate how far out of focus, and in which direction, the system is currently set.
2) The body tells the lens "change focus position by X amount".
3) The lens changes the focus position, reports to the camera that it has done so, and the camera takes the picture.

Note that, in order to keep focusing speed high, the camera does not verify that the lens has focused to the correct point - it's an open loop system. Therefore the lens can affect focus accuracy at step 3 (because it moves the wrong amount). It can also affect accuracy because some types of lens aberration (specifically spherical aberrations) can cause the phase measurements in step 1 to be incorrect.

The above is one reason why some of the mirrorless systems such as micro 4/3 can be more accurate focusing on stationary targets. They use contrast detection to determine whether the image is in focus, but in a closed loop manner - the focus is continually rechecked until the point of maximum sharpness is found.

 

[Durbs]

Ah cool, makes sense. Bearing that in mind though, the difference in focussing performance in good/low light would be down to the body more than the lens though wouldn't it?