What causes auto-focus “freezing”?

Question

On occasion, the auto-focus system in my camera and/or lens just stops working. I'll be holding down the shutter release halfway and tracking a subject and I can feel/hear the auto-focus system making adjustments, and then it just stops. If I release the button and try to re-activate, nothing happens. Essentially, auto-focus is dead until I turn the camera off, and then turn it back on again.

I am using a Nikon D90 with a Sigma 70-200mm F2.8. Is the problem likely to be in the camera or the lens? I also have a Nikon 18-105mm lens, but don't use it enough that I can recall experiencing the same problem. I have taken over 10,000 shots with the D90/Sigma combo, and have had this problem occur 10-15 times.

There might be a correlation with how long I've been shooting. Often I will be at an event where I might shoot 500-800 shots in a day of shooting (say 6-8 hours), and it seems like maybe this happens more often later in the day than earlier.

Both the camera and lens are still under warranty, but I'm not sure whether it is the camera or the lens, and don't want Nikon and Sigma pointing fingers at each other, and don't know who to contact first.

Has anybody experienced a similar problem? or have any suggestions on a course of action?

Answer

Everything I've read in the past and a quick Google indicates it's been known to happen with some specific Sigma lenses, yours in particular. But it's nothing more than anecdotal.

If it only happens with that lens and internet anecdotes suggest it's the lens, send the lens in under warranty while you still can.

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