Down Autofocus Issues
the obvious and the solution is unknown*
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:
it clean? Eliminate dirt and dust. At the bottom
of the mirror box (underneath the mirror) is where the autofocus
sensors live. 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. Does the camera focus every time now?
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
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,
and 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
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:
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 use a 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, D700, D3, and D3x) 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
However, note that a true "backfocus test" is not nearly so 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:
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...
still haven't found your problem, you probably need to send your
camera (and preferably at least one lens that exhibits the problem
with that body) to Nikon 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.
to Sir Arthur Conan Doyle.
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