WHAT IS FOCUS LOCK?
Let’s start our exploration of the Multi-CAM 2000 system by looking at some basic information that many of us may not fully understand.
One question often asked is, “What does it mean to lock focus?” That's a great question since it involves how the camera decides when a picture can be taken, and what AF modes you’ll find most useful.
Nikon D2X digital SLR camera image by Dr. Jason Odell (DrJay32)
If a subject is moving, the camera will use two technologies to track it. They're called Predictive Focus Tracking® and Focus Tracking with Lock-On®. More about them later.
Using these technologies, the camera detects that the subject is moving in the few milliseconds that autofocus is in action. According to whether it's in “single-focus” AF-S mode (Single Servo AF) or “constant-focus” AF-C mode (Continuous Servo AF) two distinct events will occur.
Single Servo AF: In this case, the autofocus system sees subject movement and does not “lock” the focus until the subject stops moving. When the subject stops the focus “locks.” Once this lock takes place, the little round green light comes on in the viewfinder, and autofocus activity ceases. You must reactivate autofocus by lifting your finger and reapplying pressure. The focus is truly locked and will not try to follow your subject unless you refocus. To follow a moving subject requires you to tap the shutter button as the subject moves.
Continuous Servo AF: When using this mode the autofocus never “locks” at all. Your camera will capture images with three levels of focus accuracy, according to how you have the AF “priority” set. (Priorities: FPS rate, FPS rate + AF, and Focus) We'll discuss these later on.
Many photographers use a method of shooting best called “Focus and Recompose.” A good example of this is photographing a couple of friends who are standing a couple of feet apart. The photographer, using AF-S mode, moves his camera so that the selected AF sensor is pointing at the face of one of the friends. He locks the focus by holding pressure on the shutter button, moves the camera to the composition that looks best, and then snaps the picture.
As long as the photographer holds pressure on the shutter button, the camera will not try to refocus, since the focus is “locked.” When he presses the shutter button the rest of the way, firing the shutter, the camera will not try to refocus on the background between the friends.
How many of us have pictures of a perfectly focused background with two friends blurred and out of focus? I'll never admit it, but I sure do! (huh?) Using AF-S and the “Focus and Recompose method” makes this problem unlikely to happen.
So, remember this. Once autofocus “locks” it stops working until you release pressure from the shutter button. This is perfect for non-moving subjects, or even slowly moving subjects.
If your subject never stops moving, is moving erratically, or only stops briefly, AF-S is probably not the best mode to use. Then AF-C is better, since it never locks focus and you can better follow movement.
So, “locking focus” simply means that the autofocus system is finished doing its job, and is waiting for you to conclude by taking the picture. This only applies to Single Servo AF (AF-S) mode.