How accurate should one expect autofocus to be?

Started Mar 30, 2013 | Questions thread
Truman Prevatt
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Re: How accurate should one expect autofocus to be?
In reply to John4, Mar 31, 2013

There are two comments.  First is cars might have a lot more contrast - spectral reflections - than a target.  The second comment is something pointed out by Thom.  He stated that for best results the focal point should cover the object you wish to focus on.  If there it is too small and the area contains other objects, it can get confused.

John4 wrote:

I have perhaps the same or a similar problem.  When I got my D800 I tested the extreme focus points are they were right on.  However in subsequent photographs of my grandson some photos were were noticeably and consistently soft.

I just did an interesting experiment.   I used extreme left focus point and focused on a bar in a window.  The vertical bar went right through the center of the focus area.  In the background, across the street, were some cars.  The camera focused on the distant cars.  Doesn't this violate Greatest Contrast captures focus, or Nearest Object captures focus?  Yet when I focus on something with texture which occupies the whole focus rectangle, the focus is good.

Advise and opinion welcome.

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Truman
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