Before sending it in, it is worth doing the reset (they will do this anyway as part of the service). And as several others have suggested, examining the contacts on the camera body flange and perhaps a careful swab of isopropol alcohol on them as well.

Also, take it to the store where you purchased it. They will likely clean it for free.

I don't think so, but I will check the ca,era setting. I do use AF ON from time to time, so it is possible I locked AF while changing lenses. But changing the battery and turning the camera off and on dis not fix the problem, only removing the lens - which I had checked for proper seating.

Thanks for all the good suggestions. If it happens again I guess I'll have to send it in.

Did you accidently turn on the "AF ON" button? There is a setting where the AF is not on until you press that button.

I think that since remounting the lens is the only way that fixes it...there are two things wrong really. It is caused with the hot mounting. And possibly not having the lens completely locked or something of this nature.

Or you have faulty contacts on your body. Assuming the lenses work/worked on a different body always.

hawkdl2 said:

My gut feeling is this is from hot swapping the lens, and sometimes in haste,

I would agree this is a very likely cause

next time, rather than completely removing the lens try giving it a "wriggel" if it moves, it was not locked in, in the first place

The shooting conditions were near perfect on at least several of the occasions: mid-day Africa clear sky with high contrast subjects (bull elephant on a dirt road with green vegetation around 11am). Others were late evening or early morning, but never in what I would consider challenging lighting conditions.

The shots in question were at ISO 100-800, maybe 1600 on one occasion and normal shutter speeds. The lens would not hunt - it did nothing. It was as if the camera was in MF mode. Metering, displays, etc were all fine. I cleaned the camera every night and the last time this happened was in Rio on my hotel balcony shooting the skyline in the evening. Other than this problem, I agree that the D800 is in all focusing and metering areas notably better than my prior D300s. In fact, I am usually amazed at how well it will focus in very low light, but the problem has occurred in fair to excellent lighting.

Battery was 3/4 to full on all occasions.

My gut feeling is this is from hot swapping the lens, and sometimes in haste, but like Msmoto, I did this a lot on my D300s and never had the camera/lens freeze up like I'm experiencing.

You didn't say the shooting conditions you were in.

I find in low light (f4 @ 6400 and darker) you need a straight line running vertical or horizontal for it to see something to AF on. Also If you are in a low contrast setting, it can hunt without a line somewhere. I have not noticed any difference but for the better than my D300.

You mentioned Africa - if you are in various "rough" environments you may try cleaning the mounts and contacts. You could just have some build-up.

As noted by Eric..low light or when the object has no contrast or other focus objects, my D4 will do this. Solution is to find a different target, get it "seeing" again and the problem self corrects.

I am a believer that the mounting of a lens while "hot" is something which can be done thousands of times with no problems. But, it is like hitting two keys at once on the computer. At some point the simultaneous inputs will mess things up. And, the contacts may short out when hot, and this could cause lots of problems with the micro circuitry and the programming as it sends messages to the camera which are bogus.

So, as suggested by Adam... Nikon is most likely the only way to get this repaired, provided you have reset the camera and it still occurs. Battery charge status may be a factor as well.

Do the shooting conditions make any difference? I've had "abandoned focus" with my lenses but usually with low light or extreme focus changes such as from a very near target to a distant target. I simply "help" the focus manually or pick a very easy focus target. I have never had to remove and remount the lens.

If this is a regular issue, you might want to have the lens mount checked. It's possible that you have a bad connection. Take a close look under magnification - you might even use another camera and macro lens to take some images of the mount connections.

IMHO, the only option will be to send it to Nikon. Never had this kind of problem with my D800 nor I have heard anyone who has such a problem.

Over the past several months on a number of occasions my D800 will not AF - it won't even try. No sound from lens or camera and no focus hunting. This has happened with my 20-70, 70-200 and 80-400 on several occasions for each lens, so I am confident the problem is not my lenses. However, it is not a common occurrence, which is why I've waiting several months to post - I wanted to be certain it was real. The only way I can get the system AF-ing again is to remove the lens and remount it.

I will admit that on several occasions I "may" have changed lenses without turning off the camera first, but on my recent trip to Africa, it occurred without any such provocation, and I'm sure I had done the same with my prior D300s and NEVER had the camera fail to AF.

Switching from M/A to M and back, pulling the battery, manual focusing to move the elements, checking the focus lock button, etc. do not resolve the issue. The lens has to be removed and remounted.