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.
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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 If you do not release shutter button pressure, and your subject starts moving, your focus will be out of date and useless.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!
AF-C (CONTINUOUS SERVO AF MODE)
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.
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!”
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