PREDICTIVE
FOCUS TRACKING VS. FOCUS TRACKING WITH LOCK-ON
(Custom Setting a4)
There is some confusion about the differences
between Predictive Focus Tracking (manual page 73) and Focus
Tracking with Lock-On (manual page 185). In fact, these are
not the same technologies, but do work together to help you
get well focused images.
PREDICTIVE FOCUS TRACKING …
…is
a technology designed to help in instances when your subject
is moving as you press the shutter button to actually take
the picture. There’s a delay in the shutter actuation
time of only a few milliseconds. This delay, though small,
could tend to cause fast moving subjects to go out of focus
by the time the shutter actually fires.
When
you press the shutter button for autofocus the camera’s
computer asks, “Is this subject moving?” Here’s
what happens next:
AF-S (SINGLE SERVO AF MODE)
Subject
is NOT moving: If not moving,
it instantly LOCKS the focus on your subject,
and waits for you to fire the shutter. If you do not release
shutter button pressure, and your subject starts moving, your
focus will be out of date and useless. Once you have focus
lock, take the picture quickly.
Subject
is moving: Predictive Focus
Tracking figures out how far the subject will move before
the shutter fires. Once you have pressed the shutter button
all the way down it moves the lens elements slightly to
correspond to where the subject should be when the shutter
fires a few milliseconds later. In other words, it focuses
slightly in front of your subject so that the shutter has
time to open and get the shutter blades out of the way.
It
takes 37 milliseconds for the camera to respond to pressing
the shutter release. In 37 milliseconds a fast moving race
car can slightly blur the focus by the time the shutter
opens. If you press the shutter in one smooth motion all
the way to shutter release, first autofocus occurs, then
the shutter starts opening. In the time it takes for the
camera to respond to your shutter release press, the car
has moved slightly, which just barely throws the autofocus
off. The camera’s computer predicts where the car
will be when the picture is actually taken, and adjusts
the focus accordingly.
A slightly gross comparison would be a hunter stalking a
deer. Good hunters have learned to aim and shoot the gun
slightly in front of the running deer so that the bullet
and deer arrive in the same place at the same time. Predictive
Focus Tracking does it for you so that you don’t have
to focus your camera in front of your subject and wait 37
milliseconds for it to arrive. That would be a bit hard
to time!
Since
AF-C mode never “locks” the focus, it’s
always ready to take a picture. It will focus on the subject
as long as you hold the button down, but even small camera
or subject movements will make it refocus over and over. You’ll
hear the lens chatter as the focus stays on your subject,
and constantly makes small adjustments. When you press the
shutter button fully, the picture is taken in whatever the
last focus position was. If you have Focus Priority set (custom
settings a1 and a2), the image will be in focus, if you do
not, it may not be.
Predictive
Focus Tracking cannot be disabled by changing Custom Setting
“a4” to off. That custom setting disables “Focus
tracking with Lock-On”, a completely different technology.
According to Nikon, Predictive Focus Tracking cannot be disabled
... period.
Lens
movement, especially with long lenses, can be interpreted
by the camera as subject movement. Predictive Focus Tracking,
in that case, is tracking your camera movement while simultaneously
trying to track your subject. Attempting to handhold a long
lens will drive your camera NUTS, as it will you, when you
later view the shaky pictures. Use a vibration reduction (VR)
lens or a tripod for best results with Predictive Focus Tracking.
Nikon says that there are special algorithms in Predictive
Focus Tracking that notice sideways movement, realize that
you are panning, and shut down Predictive Focus tracking.
In
fact, page 73 of the Nikon D2x manual says, "If
the subject is moving toward or away from the camera,
the camera will track focus while attempting to predict where
the subject will be when the shutter is released."
(italics mine)
Notice it says “toward or away,” which means Predictive
Focus Tracking is not the best technology for sideways movement
or panning.
FOCUS TRACKING WITH Lock-On® (custom setting a4) …
…
is a technology designed with a completely different purpose
in mind. It’s a focus algorithm that allows your D2x
to lock focus on a subject and IGNORE ANYTHING THAT
COMES BETWEEN THE CAMERA AND THE SUBJECT, while tracking
where that subject is on the array of focus sensors. It’s
best to use more than one sensor when using Focus Tracking
with Lock-On. Dynamic Area AF will give you more accurate
tracking of moving subjects. When you switch to AF-C mode,
also get in the habit of switching to one of the Dynamic Area
or Group Dynamic focusing modes.
Should
I turn off custom setting a4, which disables the “Lock-On”
functionality?
Some
have claimed that this will improve the autofocus on
the Nikon D2x. But, in my opinion this may not be entirely
true! Custom setting a4 has little to do with HOW WELL
the Nikon D2x focuses. Instead it is concerned with
WHAT it is focused on.
We
will discuss some of the controversial issues surrounding
a4 towards the end of this article.
In
the meantime, below are some good reasons to leave Custom
Setting a4, “or Lock-On, enabled in your Nikon
D2x. |
 |
Nikon
F6 sample by jrp |
As
we will consider below, Dynamic Area AF with Closest Subject
Priority, with Lock-On disabled, will instantly react to something
coming between your subject and the camera. By enabling custom
setting a4, the camera will ignore anything that briefly gets
between you and your subjects. If you turn a4 off and use
Closest Subject Priority, your camera will happily switch
focus to a closer subject, even if it only appears in the
frame for a moment.
A
good example of this is when you are tracking a moving subject,
and just as you are about to snap the picture a closer object
enters the edge of the frame and is picked up by an outside
sensor. The camera will instantly switch focus to the intruding
closer subject. If you turn off Custom Setting a4, that’s
exactly what you’ll get; a camera that doesn't know
how to keep its attention on the subject you are trying to
photograph. I call turning off custom setting a4, “focus
roulette!”
-
