How accurate should one expect autofocus to be?

Started Mar 30, 2013 | Questions thread
Brandon birder
Senior MemberPosts: 2,142
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Re: How accurate should one expect autofocus to be?
In reply to Anker BergSonne, Apr 1, 2013

Anker BergSonne wrote:

The outer focus points work with targets that have good vertical contrast. A vertical bar doesn't have that attribute and it will have difficulty focusing on it. To use a vertical bar as a focus target you should use one of the inner cross-focus points.

Anker

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.

For clarity that means a contrasting horizontal bar is a good target for the outer focu points.

Vertical contrast  equals horizontal lines.

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