Question on AF

[maxxie]

I have a question on how AF accuracy works between lens and camera body.

For a given lens, I might read that AF is not accurate, that there is front focus, etc.

Since the AF sensor is in the camera, how does this work?  I have always thought that the lens responsibility was mainly to respond to "focus in" and "focus out" types of commands from the camera, and if it stops, it is because the camera tells it to. 

Thanks!

[Bob Atkins]

That's not quite how it works. Very basically the AF system can tell how far out of focus a lens is and the body then drives to lens to the predicted focus point.

It doesn't go back and forth forever adjusting focus until it gets it perfectly correct (or at least good enough).

Here's a quote from part of the EOS FAQ which deals with some issues of AF (see http://www.bobatkins.com/photography/eosfaq/eosfaq24/9miscellany.html#q27)

"...One of the fundamental principles of phase detection autofocus is that the AF system uses subject contrast to detect and calculate a defocus amount at the film plane. Another way of saying this would be that the AF system detects whether the accurate focusing plane is in front of or behind the focal plane, and by how much. The precision of this system is more than adequate for highly accurate AF calculations..."