The way I understand it is that the AF system generates a double
image for the active focus point, just like a rangefinder, at least
with the phase systems like the D200. It is set up so there is never
a zero even at perfect focus. The distances between the double images
are mapped in the controlling chip to distance. So in this case the
camera always knows whether it is in focus or out. If the images of
the fence post are x microns apart the lens is focused at distance y.
It is more like the early rangefinders that read a distance that you
set on the camera by turning the focusing ring except that here the
controlling chip, not you, is doing it.
Winsor
Long Beach, California, USA
On / June 21, 2007 CE, at 3:49 PM, Wayne Harridge wrote:
> I don't think there is any way for the camera to determine if
> something is in focus without first having it out of focus, so no
> way for it to "double check" without moving the lens.
>
> ...Wayne
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