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Fine-Tuning the Focus of Your Lenses
In Chapter 10, I introduced you to the D7000’s AF Fine tune capability. In this sec-
tion, I’ll show you how to calibrate your lenses using this feature.
Why is the focus “off ” for some lenses in the first place? There are lots of factors, includ-
ing the age of the lens (an older lens may focus slightly differently), temperature effects
on certain types of glass, humidity, and tolerances built into a lens’s design that all add
up to a slight misadjustment, even though the components themselves are, strictly speak-
ing, within specs. A very slight variation in your lens’s mount can cause focus to vary
slightly. With any luck (if you can call it that) a lens that doesn’t focus exactly right will
at least be consistent. If a lens always focuses a bit behind the subject, the symptom is
back focus. If it focuses in front of the subject, it’s called front focus.
As I noted, you’re almost always better off sending such a lens in to Nikon to have them
make it right. But that’s not always possible. Perhaps you need your lens recalibrated
right now, or you purchased a gray market lens that Nikon isn’t willing to fix. If you
want to do it yourself, the first thing to do is determine whether or not your lens has a
back focus or front focus problem.
For a quick-and-dirty diagnosis (not a calibration; you’ll use a different target for that),
lay down a piece of graph paper on a flat surface, and place an object on the line at the
middle, which will represent the point of focus (we hope). Then, shoot the target at an
angle using your lens’s widest aperture and the autofocus mode you want to test. Mount
the camera on a tripod so you can get accurate, repeatable results.
If your camera/lens combination doesn’t suffer from front or back focus, the point of
sharpest focus will be the center line of the chart, as you can see in Figure 11.20. If you
do have a problem, one of the other lines will be sharply focused instead. Should you
discover that your lens consistently front or back focuses, it needs to be recalibrated.
Unfortunately, it’s only possible to calibrate a lens for a single focusing distance. So, if
you use a particular lens (such as a macro lens) for close-focusing, calibrate for that. If
you use a lens primarily for middle distances, calibrate for that. Close-to-middle dis-
tances are most likely to cause focus problems, anyway, because as you get closer to infin-
ity, small changes in focus are less likely to have an effect.
Chapter 11 ■ Working with Lenses 385