USING FOCUS PRIORITY

It is also very important to set Focus Priority (custom setting a1 or a2) so that the camera won't fire when out of focus.

This can be frustrating if one's panning technique is not so good, since the camera will refuse to take an image unless it can focus on the plane. But, why take dozens of unfocused images to grieve over later at home?

USING "FOCUS TRACKING WITH LOCK-ON®"

I also feel that turning "Focus Tracking with Lock-On" (Custom setting a4) ON is somewhat important in shooting airplanes, since it allows the camera to stay with the target plane even if a bright cloud or another airplane gets near. Leaving custom setting a4 on is not absolutely necessary if the sky is clear and blue, since nothing will distract the autofocus system.

Custom setting a4 (Lock-On) will make the camera feel slower since it does not react quickly to focus changes if it loses the subject it was tracking. It will keep focused on the "area" of the subject in hopes of finding it again.
	By Nikonian Steve Lamb (Novar)

It takes about two or three seconds before the Multi-CAM 2000 AF module will start seeking a new focus subject once it has lost an old one. This may seem like a speed reduction, but in fact it is not.

The AF module is just trying to keep its focus on ONE subject only, instead of being influenced by other objects that might be picked up by an alternate AF sensor. If you lose the airplane briefly in the viewfinder, and custom setting a4 is enabled, your camera will still be focused on the airplane when you find it again. It won't go searching for a new subject for several seconds.

One Nikon D2x user, Patrick Godfrey, reported that he had his best results with Lock-On enabled. He said that the planes would often fly through smoke from the airplane smoke trails, and the smoke would tend to grab the camera's attention with Lock-On disabled.

After he turned Lock-On to the ON selection, he said that the camera would stay locked on the airplane and ignore the smoke trails sorrounding it. Take note of some of Patrick's images illustrating this article.

YOU will have to be the judge about whether Focus Tracking with Lock-On is beneficial to you. It has a lot to do with sky conditions. If there are clouds, smoke, and other planes around, leave a4 on. If not, set it off and see how it does. I have had better results with it on, myself. I find that any autofocus camera shooting against a plain sky will "search" while seeking something to focus on.
	By Nikonian Patrick Godfrey (pateod)

With Lock-On turned off, if one loses the airplane for but a moment, the autofocus will start racking in an out looking for a subject. With it turned on the D2x will not start searching for a new subject until about three seconds have elapsed. This gives one plenty of time to re-acquire the subject. Test for yourself!