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 Just a Photographer, Mar 31, 2013

I was down at the race track (horse) yesterday.  I have always used AF-C 9 point when shooting horses galloping and that works fine.  I also usually shoot at about f8.  Back at the barn I tired an experiment.  I shot the horses first with 9 point and then with AF-C single point. I focus using the AF-on and not the shutter release.

One gray horses or horses with a white stripe, star, etc. the 9 point works fine.  On the bays and chestnuts with no white markings, the 9 point is always out of focus from where I pointed the selected (center) point.  The AF-C single point is dead on.  So the manual is a bit misleading or else I did not interpret it correctly. In reality, if there is not much contrast like a dark horse under a shed row then it will find the best point (which in my case was usually someone's garment, the white wall of the barn, etc. ) for it to focus.

So of these types of subjects AF-C single is the best option by far.  It is not really a fault of the camera - it is doing what it was programmed to do and in many cases that is the right thing to do like galloping horses.  However, the camera cannot read you mind and if you give it an option it will take it.  I expect that outdoors in good light it won't matter.  It would nail the focus in the selected point and not need to look around.  In marginal light - that is a different story.

On day I stayed for the races and photographed people both outside and inside.  I tried AF-C Automatic.  It worked great outside.  It grabbed faces, etc. and the exposure was great.  Inside it did not work so well.  Of course it told me where it was focusing and where it was focusing was not the face I wanted.  It was the highest contrast area in the scene which could have been a shirt, etc.  It was doing what it was programmed to do but it was not reading my mind.

It gets down to a matter of working with the camera and understanding what it will do in particular situations and not expect the camera to read your mind about the focus point.  This is particularly true in more difficult environments like indoors or otherwise lower ambient light. If in doubt use AF-C S or AF-S.

Truman

Just a Photographer wrote:

PHXAZCRAIG wrote:

Another suggestion - try a multitude of different AF settings.   What worked on previous cameras may not be the best choice on a D800.    Might as well try 51-pt 3d and see if the face recognition tech in the camera works better than single point...

He is not using single point mode as he has set his camera to AF-C d9. Which means there are 8 active focus points around the selected middle focus point available, making 9 active focus points.

The camera can pick any of those 9 AF points and will then take the AF point with the greatest difference in contrast when he presses the shutter. This biggest contrast difference might be something else then the eye when pressing the shutter with moving objects.

Single point AF would probably work better for him and only if the subject moves in strange patterns (like birds or dancers) I would advise to use 51-pt 3d.

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Truman
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