It is when the plane of focus in the final image is different that
where you focused with the camera. Lots of times it is not a quality
control issue. Many times it is user error with fairly complex focus
systems. Focus systems are designed to lock onto high contrast. So if
you are photographing a low contrast subject in front of a high
contrast background and you are not controlling the focus point with
the 4 way selector you may be responsible for back focus.
Coming under the quality control issue is lack of correspondence
between the focus points on the LCD overlay in the viewfinder and the
actual focus sensors in the bottom of the mirror box. It is not a bad
idea to check that with some test shots. You may think you have the
focus on the right eye, but it may be on the wall behind or maybe the
ear. I don't think there is anyway to get that adjusted. Just learn
that the actual spot is maybe slightly to the right or above or where
ever your test finds it.
Winsor
Long Beach, California, USA
On / June 21, 2007 CE, at 1:54 PM, Ali Shah wrote:
> Just wondered. When speaking of an AF lens - people
> talk about backfocus problems and other issues. Mainly
> quality control related things. What is backfocus and
> what other issues should be considered with AF lenses?
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