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Thom Hogan’s Complete Guide to the Nikon D300
Page 527
lock onto some of that black. More likely, it’ll start
hunting for focus. With 3D-tracking turned on, the camera
has a pattern (face/body against background) that it can
use to improve focus.
Now consider the situation where the face moves off the
autofocus sensors but the body doesn’t get closer or
further: focus ought to stay where it was and the 3D
system can actually detect that because the color pattern it
sensed via the meter didn’t change in size and only
moved laterally in the frame.
Finally, consider the case where the pattern gets smaller
(subject moving away from you). The camera knows it
must move focus away from the original point and thus is
looking at the focus sensors that indicate that this has
happened and guessing that it should use those.
Obviously, it’s much more complex than that, but in a
nutshell, that’s the choice you’re making between
51
points
and
51 points (3D-tracking)
. In the former, only
the autofocus sensors are being used, and they may
stumble in low contrast or low light situations. In the latter
setting, the color metering system gives the system more
information that make it much more likely to find the right
spot in challenging situations. The downside, if any, is that
51 points (3D-tracking)
seems to be a little more
processing intensive, and thus tends to take a fraction of a
second longer to lock into focus in the first place. What
I’ve found in testing is that the more advanced 3D
function makes more of a difference on a D3 (where the
autofocus sensing area is smaller) than on a D300 (where
the autofocus sensors cover a larger portion of the frame).
If you have a clear pattern of something in your frame that
you want to keep focus on (the more distinct from the
background the better), it’s probably worth trying
51
points (3D-tracking)
.
