Nikon 18-200 Autofocus Hunts

[Neil Laubenthal]

My 18-200 Nikon zoom recently started hunting during autofocus. I use BBF and normally have AFS-C and d21 set. It seems to focus then either hunt back and forth slightly around the correct focus point…even for stationary subjects like tree stumps. Alternatively…sometimes instead of hunting it zooms to minimum focus distance then back to approximately correct…then either hunts around that point or goes to minimum distance.

Neither my Sigma 10-20 or Sigma 50-500 exhibit the same symptoms; although the 50-500 did until I locked the mirror up and used a can of air to blast off the autofocus sensors. Mirror is clean, and I have sensor cleaning set for both startup and shutdown. The 18-200 exhibits the same symptoms when moved to my wife's D7000…leading me to think it's the lens and not the body.

I've tried selecting AF-S, AF-A and various other focus point selection options including single but that doesn't fix the issue.

I've cleaned the front and rear lens surfaces…as well as the UV filter it's always got on it…and even tried leaving the filter off in case that was it as it is a couple years old and is showing some minor scratching on the filter outside surface…but they're small enough that I wouldn't think they're any issue. Internally…at least as well as I can tell holding the lens up to the light…I don't see anything to cause a problem.

Is this a sign that there's something wrong with the lens…or does it just need a good internal cleaning at the shop…or possibly something else? Trying to figure out where to troubleshoot next. How about needing a professional cleaning on the body to get the autofocus sensors cleaned…as I said I blew any dust out but if there's something adhering to them it might not have gotten blown off and you really can't see down into the autofocus sensor well very well. Both cameras…along with lenses and other gear…are 5-6 years old and have traveled extensively to AK and the west where it's dusty…but there isn't any indication on photos of any dust buildup that I can see.

Thanks.

 

[Orangecroc]

My 18-200 Nikon zoom recently started hunting during autofocus. I use BBF and normally have AFS-C and d21 set. It seems to focus then either hunt back and forth slightly around the correct focus point…even for stationary subjects like tree stumps. Alternatively…sometimes instead of hunting it zooms to minimum focus distance then back to approximately correct…then either hunts around that point or goes to minimum distance.

Neither my Sigma 10-20 or Sigma 50-500 exhibit the same symptoms; although the 50-500 did until I locked the mirror up and used a can of air to blast off the autofocus sensors. Mirror is clean, and I have sensor cleaning set for both startup and shutdown. The 18-200 exhibits the same symptoms when moved to my wife's D7000…leading me to think it's the lens and not the body.

I've tried selecting AF-S, AF-A and various other focus point selection options including single but that doesn't fix the issue.

I've cleaned the front and rear lens surfaces…as well as the UV filter it's always got on it…and even tried leaving the filter off in case that was it as it is a couple years old and is showing some minor scratching on the filter outside surface…but they're small enough that I wouldn't think they're any issue. Internally…at least as well as I can tell holding the lens up to the light…I don't see anything to cause a problem.

Is this a sign that there's something wrong with the lens…or does it just need a good internal cleaning at the shop…or possibly something else? Trying to figure out where to troubleshoot next. How about needing a professional cleaning on the body to get the autofocus sensors cleaned…as I said I blew any dust out but if there's something adhering to them it might not have gotten blown off and you really can't see down into the autofocus sensor well very well. Both cameras…along with lenses and other gear…are 5-6 years old and have traveled extensively to AK and the west where it's dusty…but there isn't any indication on photos of any dust buildup that I can see.

Thanks.

Have you tried it at different apertures?

 

[Nawty]

Have you tried it at different apertures?

It's a G lens so aperture will be wide open until the shutter is released, actually that's true for D lenses too and in fact any Nikon lens that isn't manual aperture.

 

[StewartR]

Have you tried it at different apertures?

That shouldn't make any difference. The autofocus is always performed with the lens wide open, and then the aperture stops down when you take the photo.

HOWEVER, it would be checking if the aperture is opening up fully. If there's something sticking in the diaphragm mechanism, the camera might think the lens has opened up to f/5.6 and think it should be able to autofocus properly. But if the lens has actually only opened up to (say) f/8 or f/9 there won't be enough light reaching the autofocus sensor and it will tend to hunt.