Nikon 70-200 focusing issues

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ted21

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Hello all,

I've noticed that my new-ish 70-200 has a tendency in AF-C mode to jump around the desired focus point - i.e. jump back and forth between slightly in front of target and slightly behind target. It seems like the minimum focus step is large enough that the shallow depth of field doesn't quite hit where it's supposed to be.

The only thing stopping me from sending this to Nikon is that the lens seems to be an unusually sharp sample - I also have the 200mm f2.0G and I have to say there is very little in it. That, and I can get away with a 2xTC more often than not with the 70-200 where many tests I have seen show trouble with this combination.

What I'm getting at is that one way of 'fixing' my problem is to soften the optics so that there isn't such a difference between bang in focus and not... which would cause the AF to stop hunting.

Does anyone know if Nikkors can have the focus step size adjusted smaller? Or has anyone a similar experience, maybe it is just a common behaviour of the lens.

Thanks for any help...
 
What body are you using it on? What sort of AF setup are you using?

I've got to say the 70-200 is pretty damned accurate with focus and rarely misses for me - and I shoot motorsport with it!
 
It could do this if there was a lack of contrast in the subject.

I sometimes get hunting if there are bright highlights on the subject - like polished frame tubes on motorcycles. But it doesn't hunt more than a couple of inches, it knows where it wants to be, it is just being blinded. Move the focus point slightly and it is fine.

I have never had an issue over sharpness either.
 
do you have 3D tracking enabled on your camera, it jumps about on the D3 if 3D tracking is enabled
 
Thanks for your replies - sorry, I should have been more specific. It's in use on a D300 and I often use 3D tracking... will have a test without and report back. There may also be something in reflections as I was last shooting around waxy shiny foliage reflecting at me, again worthy of investigation.

It's not a lack of detail/contrast to focus on (like trying to lock on a plain white wall) which tends to make AF rack from near to far across the complete range, this is jumping about an exceedingly small range.

And just to make myself clear, it's definitely not a lens sharpness issue - if anything the lens is too good!

Thanks
 
3D tracking is a little random... you are far better off taking the decision away from the software and whacking it into 9 focus point mode then selecting the single point with focus assist points (the middle position of the switch). Then you can select any of the nine central cross point sensors with the d-pad and get exactly the result you want.

The D300's focus system is a major technological miracle, if you understand how it works :D
 
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