While reading Thom Hogan’s detailed review of the Nikon D300 today, I came across this interesting little bit in his discussion of the autofocus system on that camera:
The camera detects the current focus situation, calculates where focus should be (yes, it knows not only how much out of focus, but which direction, the error is when you initiate a focus operation), then tells the lens to move a set amount. To prevent hunting, if the new error after the focus is performed is within a reasonable tolerance, focus isn’t attempted again. Tolerances are the bane of accuracy…
In other words, what I was trying to explain last week, in my entry on using a superzoom lens as a focusing tool in wide angle shots where deep depth of field is needed, appears to be correct: If the camera thinks it has turned the lens far enough, i.e. “within a reasonable tolerance”, then that’s where it stays, rather than trying to find the absolute optimal position. And this would be why I was sometimes getting shots where the farthest background or closest foreground details were out of focus, even though the depth-of-field at my chosen aperture should have been adequate to prevent that from happening.
And what I secretly love about this is that I managed to reason it out just from using the camera. Heheh. ;)