Eliminate the obvious and the solution is unknown
It seems a lot of folk end up reporting autofocus problems (very few of which turn out to be real component failures), so I'm going to provide you a troubleshooting checklist to run through before you run off screaming to the manufacturer. Do these steps, in this order:
- Is it clean? Eliminate dirt and dust. At the bottom of the mirror box (underneath the mirror) is where the autofocus sensors live on DSLRs. Get dust balls down there and autofocus will be problematic. Clean using a Speckgrabber or blower brush. While you've got your French Maid's outfit on, check the contact nubs on the lens and the contact areas in the lens mount. The nubs should all have spring to them and be in the fully extended position, and both areas should be clean. Clean with a contact cleaning solution like DeOxit. Does the camera focus every time now?
Yes? Then the problem was your lack of maintenance. Have your camera and lenses CLA'd regularly by a service center, or learn how to do it yourself to avoid repeats.
No? If the nubs aren't all fully out or don't have good spring, send the lens in for CLA, otherwise continue on:
- Do you know where they are? The brackets in the viewfinder are not the size and shape of the actual autofocus sensors. Put the camera in Dynamic Area AF, select each sensor individually, then find a place where you have a near, sharp edge that can be placed against a far background (like a pole for a near street sign with a distant background). Activate autofocus and watch where the edge snaps into focus relative to the bracket. Note that in low light the cross-hatched autofocus sensors (center on CAM530, CAM900 and CAM1000 cameras, left/center/right on CAM1300 cameras, all but the two outer sensors on the CAM2000 cameras, middle group of sensors on the CAM3500) shift (usually) down and to one side (it varies, unfortunately), and are wider! Now, with that knowledge in place, does the camera focus every time now?
Yes? Then the problem was related to your trusting the brackets. Read Issue #1 of my Nikon Digital SLR Report if you want to know more. (Since that issue of the newsletter is no longer available, you can also get that article at Luminous Landscape).
No? If the sensors don't seem to be at least reasonably centered on the brackets, the camera probably should go back to Nikon for service with a note to that effect. Sometimes the solution is just a simple realignment of the focus screen, sometimes it is more serious misalignment of the sensors themselves. Otherwise continue on:
- Is it settings related? Put the camera into Single Area AF and select the central autofocus sensor. Make sure that Closest Subject Priority, Auto Area, or 3D (if available on your body) is not enabled. Does the camera focus every time now?
Yes? Then the problem is most likely related to contrast issues when using multiple sensors. Read the autofocus sections in my books (or the extended discussions I've produced in Nikon Digital SLR Report) carefully--many settings interact and not all autofocus sensors are equal.
No? If changing the settings changed the symptoms, I'd be inclined to think user error is the problem and ask you to repeat Steps 2 and 3 with more diligence. If you think you know what you're doing, continue on:
- Is it a regular error? If you think the camera always focuses in front of or behind the actual subject, you need to test that hypothesis. Take a ruler and place it 45 degrees to film plane and focus at a point on the ruler (or better still use a product like LensAlign). Try this at several distances from the lens (a 12-foot or longer tape measure works best, therefore). Does the resulting picture always focus the same amount in front of or in back of the focus point you selected?
Yes? Then it's possible that the autofocus sensor positions are slightly off from where they should be. If so:
a. Use the AF Fine Tune capability of your camera if it has it (D300, D300s, D700, D800, D7000, D3, D3s, D3x, and D4) and tune your focus.
b. If you tried 4a and got very large errors or don't have a camera that has AF Fine Tune, then the camera and lens need to go back to Nikon for adjustment (send the photos you shot!).
However, note that a true "backfocus test" is not nearly as simplistic as I just outlined. Modern lenses have a number of factors that can also account for small differences in focus position, especially as you move off center. Moreover, the AF sensors themselves are probably larger than the "point" you really want to focus at, and they have some clever contrast management in them that might alter the actual point of focus slightly from where you think they should.
No? Can you quantify how much the focus is off, on average? Is it less than 1cm at 2 meters with a 50mm lens? That would probably be within tolerances of most autofocus systems, including Nikon's, actually. If you see large errors or very inconsistent results, continue on:
- Is it lens related? Does it only happen with one of your lenses? If you've gotten to this step and haven't isolated the problem, there aren't a lot of things it could be, and most of those would involve servicing (mirror misalignment, for example). But if it happens with only one lens, then you need to go a step further: are you turning the camera off before changing lenses?
No? Tsk, tsk. Especially with the VR lenses, but even with some of the other AF lenses, there can be a tendency for "early contact" with the wrong positions (remember, you have to twist the lens in place) can make the camera experience a "senior moment."
Yes? There are other things you can look for, too: is the focus varying from side to side with that lens? Is the problem distance related? Is the reported aperture in EXIF data incorrect? Any of these things would be bad news, but if you go this far..
- ...and still haven't found your problem, you probably need to send your camera to Nikon (and preferably at least one lens that exhibits the problem with that body) for servicing. For what it's worth, it is much better to be as specific about the problem you're experiencing than it is to be vague. For example, writing "autofocus doesn't work right" will usually get a cursory bench test to see if the there's anything obviously out of tolerance. But "With attached lens in Single Area AF mode with the central sensor selected the autofocus point is always at least 1cm in front of the actual point under the sensor (see attached photograph)," you'll force Nikon to make a specific test and, in theory at least, won't get the camera back with a cursory "AF checked OK" statement.