D800 focus with 1.4 prime lenses is very variable and inaccurate

Started Jan 20, 2013 | Discussions thread
Leonard Shepherd
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Re: this is an interesting thread
In reply to primeshooter, Jan 21, 2013

primeshooter wrote:

the D800 has poor AF accuracy when shooting with a 24-85mm focal length and wide open fast (f/1.4) primes at moderate to far distances, with low contrast subjects.



This is an interesting thread - for the wrong reasons

You later confirm autofocus is good in high contrast light.

Page 100 of your D800 instructions say autofocus may not be goods with a low contrast subject, or when subjects which are small in the viewfinder are in front of a distant object. This is probably why autofocus was not as good as you expected with this subject.

The guidance also suggests when auto focus is not accurate, consider switching to manual focus.

If it is any consolation there are seven other images in this thread where Nikon's guidance is autofocus may not be accurate. Three of these seem to fall into the "no hope of good auto focus" category.

For those asking why Live View contrast detect autofocus is sometimes better - they would not ask the question with a moving subject where contrast detect AF can rarely keep up.

Phase detect looks for detail along detection lines reaching beyond the boundaries of the viewfinder mark, and can sometimes detect detail much smaller than the viewfinder boundary, helping to explain why with some difficult subjects focus may not be on what is at the centre of the viewfinder mark.

LiveView which reads from a very small part of the picture area can be better with the type of low contrast small targets you are having problems with. Ordinary autofocus is much better with moving subjects. Either is very good in most high contrast situations with static subjects.

Auto area AF explained on page 92 can help where there is a single face in the picture area. When there is more than one face it may choose the closest face - which is not always what you want at f1.4

Back to the page 100 guidance, this confirms autofocus is not infallible. As you are discovering Nikon are right to give this guidance.

Although some have some focus issues with some current cameras, my experience is generally autofocus is much improved compared to my first F100 bought in 1999

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Leonard Shepherd
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