Nikon D200 Digital SLR

These two tracking systems can work together to help you get well focused images. What are the differences between them?

Predictive Focus Tracking …

This relies on 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 is a mere 50-millisecond delay which 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:

Subject IS moving: Predictive Focus Tracking figures out how far the subject will move before the shutter fires. Once you’ve 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 camera has time to move the mirror and get the shutter blades out of the way.

It takes 50 milliseconds for the camera to respond to pressing the shutter release. If you are shooting an Airshow, for instance, in 50 milliseconds a fast moving airplane can move enough to slightly change the focus area 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 mirror moves up and the shutter starts opening. That takes the previously mentioned 50 milliseconds. In the time it takes for the camera to respond to your shutter release press, the airplane has moved slightly, which just barely throws the autofocus off. The camera’s computer predicts where the airplane will be when the image is actually exposed, and adjusts the focus accordingly.

Think about this illustration: Let’s say you were playing a ball game and you threw the ball to a running player. You would have to throw the ball slightly in front of the receiving player so that the ball and he arrive in the same place at the same time.

Predictive Focus Tracking does that for you so that you don’t have to focus your camera in front of your subject and wait 50 milliseconds for it to arrive. That would be a bit hard to time!

AF-C (Continuous Servo AF Mode)

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.

Lens movement, especially with long lenses, can be interpreted by the camera as subject movement. Predictive Focus Tracking in this 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.

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. If the subject is moving toward or away from the camera, it will track focus while attempting to predict where the subject will be when the shutter is released. Predictive Focus Tracking is not activated by the D200 for sideways subject movement or panning.

Focus tracking with Lock-On® (custom setting a5) …

This technology is designed with a completely different purpose in mind. It’s a focus algorithm that allows your D200 to lock focus on a subject and ignore anything that comes between the camera and the subject for a period of time. It will “Lock On” and track where that subject is on the array of focus sensors.

AF-S and Single AF mode is not as accurate when using Focus Tracking with Lock-On since the D200 is using only one selected sensor.

The “Lock On” part will work with one sensor in use. But how will the “Focus Tracking” work since there is only one sensor in use?

Dynamic Area AF will give you more accurate tracking of moving subjects, since it uses an array of sensors. 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.

Custom setting a5 (Lock On) has little to do with how well the Nikon D200 focuses. Instead, it is concerned with what it is focused on. Here are some good reasons to leave Custom Setting a5, “or Lock-On, enabled in your D200.

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 a5, the camera will ignore anything that briefly gets between you and your subjects. If you turn a5 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 or brighter object enters the edge of the frame and is picked up by an outside sensor. The camera will instantly switch focus to the intruding subject. If you turn off Custom Setting a5 you’ll get a camera that doesn’t know how to keep its attention on the subject you are trying to photograph.

When using Dynamic Focus modes, I call turning off custom setting a5, “focus roulette!”

Custom Setting a5 also allows you to select the length of time that your camera will ignore an intruding object that blocks your subject. The a5 setting specifies the following:

Short (about 1 second)
Normal (about 2 seconds)
Long (about 3 seconds)

This allows you to fine tune how you want Focus Tracking with Lock On to work. It can ignore an intruding subject for a few seconds.

In testing the Nikon D200, with Lock On enabled, I was amused at how adamant it was about staying with the current subject. I’d focus on a map on the wall, and then cover the focusing sensor with the Nikon D200 manual. The Nikon D200 would quickly switch to another sensor that detected the map. When I covered the new sensor with the manual, it would pop to another sensor and keep the focus on the map. I could just hear the Nikon D200 muttering, “Hah, you can’t fool me, I can still see a little edge of that map there, so I’m not changing focus!”

Only when I stuck the Nikon D200 manual completely in front of the lens, covering all the sensors, did the Nikon D200 decide to start timing the “a5 Lock On” time-out. After a second or three the Nikon D200 would give up on the map and focus on the manual instead.

In fact, I think I could hear a tiny little voice saying, “Hmm, I can’t see the map because of that stupid manual in front of my lens. Okay, 1-second…still no map…2 seconds…no map yet…3 seconds…well, I guess the map’s gone, I’ll focus on the manual now.”

Try this yourself! It’s quite fun, and will teach you something about the power of your camera’s AF system. But, if you actually do hear voices - consult a doctor. (I did!)

Below is a series of Nikon D200 menu selections showing how to set up Custom Setting a5 and its “Lock On” time-out period.