The answer to this question is related to how you have the custom setting called "lock-on" set. Focus tracking with lock-on allows you to select the length of time that your camera will ignore an intruding object that blocks your subject. It is found in the Custom Setting Menu under a Autofocus.
How does it work? Let’s say you are focused on a bird flying past you. As you pan the camera with the bird’s movement, the autofocus system tracks it and keeps it in good focus. As the bird flies by, a road sign briefly interrupts the focus tracking as the bird moves behind it and then re-emerges. How would you feel if the bright, high-contrast road sign grabbed the camera’s attention and you lost tracking on the bird? That would be quite aggravating, wouldn’t it?
Nikon provides Focus tracking with lock-on to prevent this from happening. The “lock-on” portion of this function helps your camera keep its focus on your subject, even if something briefly comes between the camera and subject. The camera locks on to your subject doggedly if this function is enabled. Without Focus tracking with lock-on, any bright object that gets between you and your subject may draw the camera’s attention and cause you to lose focus on the subject.
The camera provides a variable time-out period for the lock-on functionality. Lock-on time-out allows an object that stays between the camera and your subject for a predetermined length of time to attract the camera’s attention. You can adjust the length of this time-out with a time period from Short to Long.
You’ll need to test the time-out length to see which works best for you. You might start with the factory default Normal and let something get between you and your subject. If you’d like the camera to ignore an intruding subject for a longer time, move the setting toward Long, or for less time, toward Short.
I wouldn’t suggest turning it Off unless you fully understand how it works and do not need focus tracking that locks on to your subject. Following are the screens to configure Focus tracking with lock-on:
Figure 1 – Focus tracking with lock-on configuration
With the variable timeout period (figure 1, screen 3) you can fine-tune how you want Focus tracking with lock-on to work. The camera can ignore an intruding subject for up to several seconds.
With Single-point AF, the camera will start the lock-on time-out as soon as the single AF point is unable to detect the subject.
With Dynamic-area AF or Auto-area AF and Focus tracking with lock-on enabled, I was amused at how adamant the camera was about staying with the current subject. I’d focus on a map on the wall and then cover most of the focusing points with the user’s manual. As long as I allowed at least one or two AF points to remain uncovered so it could see the map, the focus did not switch to the manual. I could just hear the camera 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 camera's manual completely in front of the lens, covering all the AF points, did the camera decide to start timing the Focus tracking with lock-on time-out. After a few seconds, the camera would give up on the map and focus on the manual instead.
Try this yourself! It’s quite fun and will teach you something about the power of your camera’s AF system. It will also let you see how long each setting causes the timeout to last, so that you can choose your favorite.
Does Lock-On Cause Autofocus to Slow Down?
Focus tracking with lock-on is an autofocus algorithm that allows your camera to maintain focus on a subject and ignore anything that comes between the camera and the subject for a period of time. It will “lock-on” that subject and track where it is on the array of AF points in the Viewfinder. Focus tracking with lock-on is controlled by configuring Custom setting a3 or a4 (per camera) to a duration period or to Off.
Some misunderstanding surrounds this technology. Since it is designed to cause the autofocus to hesitate for a variable time period before seeking a new subject, it may make the camera seem sluggish to some users.
But, this “sluggishness” is really a feature designed to keep you from losing your subject’s tracked focus. Once the camera locks on to a subject’s area of focus, it tries its best to stay with that subject even if it briefly loses the subject. This keeps the lens from racking in and out and searching for a new subject as soon as the previous subject is no longer under an AF point.
It also causes the camera to ignore other higher-contrast or closer subjects while it follows your original subject. You will have to judge the usefulness of this technology for yourself. I suggest that you go to some event, or down to the lake, and track moving objects with and without lock-on enabled. Your style of photography has a strong bearing on how you’ll use—or whether you’ll use—Focus tracking with lock-on.
Focus tracking with lock-on has little to do with how well the camera focuses. Instead, it is concerned with what it is focused on. There are several good reasons to leave Focus tracking with lock-on enabled in your camera.
If Focus tracking with lock-on is set to Off, Dynamic-area AF and Auto-area AF will instantly react to something coming between your subject and the camera. When you enable Focus tracking with lock-on, the camera will ignore anything that briefly gets between you and your subject. If you turn it off, 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 may instantly switch focus to the intruding subject.
If you turn off Focus tracking with lock-on, you’ll have a camera that doesn’t know how to keep its attention on the subject you are trying to photograph if something interferes. When using Dynamic-area AF or Auto-area AF modes, I call turning off Focus tracking with lock-on “focus roulette!”
Configuring Focus tracking with lock-on is not difficult. However, you’ll need to decide just how long you want your camera to lock on to a subject before it decides that the subject is no longer available.
Should I Use Focus Tracking with Lock-On?
I leave Focus tracking with lock-on enabled at all times. When I’m tracking a moving subject, I don’t want my camera to be distracted by every bright object that gets in between me and the subject. Nikon gives us variable focus lock time-outs so we can change how long the camera will keep seeking the old subject, when we switch to a new one. I suggest you play around with this function until you fully understand how it works. Watch how long the camera stays locked on one subject’s area before an intruding object grabs its attention. This is one of those functions that people either love or hate. Personally, I find it quite useful for my type of photography. Try it and see what it does for you.
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