lens repair

[Curtis]

My lens has recently developed problems with autofocus. My camera is a D750 and the lens is AF-S Nikkor 70-200 mm f/2.8G ED VRII. The camera works just fine with the other lens that I use AF Zoom-Nikkor 28-80 mm f/3.3-5.6G. The problem lens autofocuses just fine much of the time but sometimes in the middle of shooting subjects the lens is completely out of focus and won't focus when I press the shutter release button. Removing the lens and replacing it numerous times makes no difference so I don't suspect a problem with the connections to the lens. I accidentally discovered a temporary fix, however. While the camera is set on autofocus, if I manually focus my long lens a little past the top setting, i.e., turning the ring all the way clockwise, then the lens starts working again. It will randomly stop working again at some point later.
I'd like to hear any suggestions for me to fix the problem, however, if that's not possible then I'd like to know about a repair shop that I could send it to.
Thanks to anyone for helpful information.

 

[Woodyg3]

When you say that it doesn't auto-focus at all, do you mean that it moves back and forth but does not focus, or that the focus doesn't change at all? When you are focused at a long distance, it is not unusual for autofocus to have a problem finding something close and focusing on it. It can sometimes rack the focus back and forth and not settle on a subject. In this case, pre-focusing on something close will aid the AF in finding the subject.

 

[hark]

If you are zoomed out on the long end towards 80mm, your aperture will be at f/5.6. Are you focusing on something with contrast? If not, the camera might not be able to secure focus.

Have you cleaned the lens' contacts?

 

[BF Hammer]

@hark : I read this as his 70-200mm f/2.8 lens as the problem. Aperture opening should not be the issue.

It's worth trying to clean the electrical contacts in the mount of the lens and camera body. Use cleaner applied to a swab if you can. Lint-free cloth otherwise.

But I also suspect it could be a small crack in a gear for the autofocus drive. As in the motor can torque past the crack when it is in motion but if the focus parks on the crack itself the motor cannot overcome the drag. In that situation, buying a used replacement could be in the neighborhood of the cost of getting this one repaired.

 

[hark]

@hark : I read this as his 70-200mm f/2.8 lens as the problem. Aperture opening should not be the issue.

I mistook the affected lens for the other one. And I agree an f/2.8 lens shouldn't have a focus issue unless the scene is pretty dark with low contrast. Thanks for catching my mistake!