Saturday, May 12, 2012

Effective Autofocus Use: The AF-ON Technique

Autofocus is a great feature, without which the photographic experience would be so different. Modern DSLRs feature very complex autofocus systems that provide good result in an extremely wide range of situations. Although not very difficult to master, sometimes they can be a nuisance if you find yourself configuring the autofocus system shot by shot. Although many cameras provide ways to alleviate this problems, such as physical knobs, custom menus and recallable configuration banks, it's not so difficult to miss a shot just because the autofocus system didn't take the right decision.

If you're worrying about this, chances are you're willing to take some more "responsibilities" in order to get better and more predictable results. Moreover, the technique I suggest aims to reduce at a minimum the reconfiguration steps you've got to perform while shooting. On one hand, you, the photographer, will have more control over your end results, and on the other hand you won't find yourself fiddling with your camera buttons and menus so often as you're used to.

Common Problems

Common problems arise because of the very nature of the autofocus system. Basically, modern autofocus systems work this way:
  • You choose a focus point.
  • You choose an autofocus mode.
  • You let the autofocus system do its job while the system is engaged (which usually occurs when the shutter release button is halfway down).

Most problems arise when using an autofocus mode other than static or single servo. According to the settings and the capabilities of the autofocus system, the camera will track the movement of the subject using a variety of information, including: distance from the subject (in case your lens can provide it), direction of the movement, information from surrounding focus points and information from the metering system. Most of this information is inferred by the autofocus system algorithms which may work most of the time but not always.

While you're tracking a moving subject, for example, some obstacles between you and the subject (such as people passing by) may deceive the tracking system. On other occasions, the subject isn't lit properly and the autofocus system may have a bad time locking on it. There are lots of situations that may deceive the autofocus system: just read your camera manual and you'll find plenty of them. You'll discover many more while learning to master your camera.

You should not put the blame on the autofocus system itself: it's an ancillary system that greatly helps us, but it's the photographer who's missing a shot, not the camera. As a photographer, however, it's up to you knowing your system and take advantage of it the best you can.

Recomposing

The interesting thing, which drove me to write this post, is what people ask me about most of the time: focusing problems while recomposing. Unless the subject in focus is exactly on the chosen focus point, people usually have to pick their favorite out of the following options:
  • Using the single servo autofocus mode (AF-S).
  • Using the continuos servo autofocus mode (AF-C) and the autofocus lock (AF-L).
  • Choosing the appropriate focus point (if possible).
  • Have the camera track the subject while recomposing with an appropriate AF area mode (dynamic or 3D) and width (the number of focus points used while tracking).

The first option consists in using the single servo mode to lock on the subject and then recompose. This option leaves little space to error unless you move the focal plane too much while recomposing, which is an issue especially in macro photography or when you're opening wide your aperture. In those cases, however, I would suggest you use manual focus instead. The problem with this option is that it requires an autofocus reconfiguration. Often you don't have the time, or simply forget, to reconfigure the autofocus mode and switch to single servo.

The autofocus lock button (usually AE-L/AF-L) exist just to solve this problem. Often, the default behavior of this button (AE-L/AF-L) locks both the focus and the exposure: sometimes it's a good idea, sometimes it's not, depending on the shooting conditions. In my case, I always prefer to separate those functions and assign them to different buttons. Also, if you want to take a new lock if the camera loses it, you've got to release the AE-L/AF-L and the shutter release button and start over. Personally, I consider the AF-ON technique more versatile than using the autofocus lock. Read on and decide for yourself.

The third option implies that you've got a suitable focus point to use in order to avoid recomposing completely. Although this technique works, it has its own drawbacks. First of all, you may not have a suitable focus point, to begin with. And if you do, you have to set it every shot you take (which is something photographers often forget). Moreover, as you know, focus points aren't all equal: some work better than others and the former are usually called "cross-type" focus points. The centre focus point is a centre-type focus point on practically every camera. Since lenses usually have better resolution at the center as well, if you want good focus, it's a good idea to use a cross type focus point and focusing on an area of the lens with good resolution. That's why, if you can, I suggest you use the centre focus point when you're taking shots relatively quickly or you're not in a controlled environment (such as a studio). In this case, too, you will need to reconfigure your camera, which is something we want to avoid.

Your mileage may vary if you choose the fourth option. The quality of the results will depend on your camera autofocus system, on the number of autofocus points chosen and some other option offered by your camera software. The biggest drawback, in my opinion, is having to choose a sufficiently high number of focus points that will assist the tracking system. That's another knob to turn and will slow you down while shooting. And the more focus points you'll use:
  • The more freedom you'll have while recomposing.
  • The longer will take the camera to have a lock on the subject.
  • The less stable it will be.
This option is widely used, probably because it offers a great deal of flexibility and requires less camera reconfiguration than other options do. However, you're "at the mercy" of your autofocus system and you will potentially suffer from focusing issues such as the above mentioned.

A Solution: Engaging Autofocus On Demand (AF-ON)

There's an effective and somehow obvious solution to this problem. Sit down and think about it. Many focusing issues described above would be mitigated if you could "disengage" the autofocus system at will, while framing your photo and with the shutter release halfway down, waiting to take your shot. If you could do that, you wouldn't need switching between single servo and the other autofocus modes. If you could do that, you could simply focus on the subject using a focus point of your choice and decide whether to track the subject or not. Basically you need:
  • A way to engage the autofocus system at will.
  • A way to disengage the autofocus system when the shutter release button is pressed.

But hey, you can do that! That's what the button AF-ON, found on many prosumer and professional cameras, is supposed to do, together with some additional autofocus options. Let's see how.

Nikon D800 - AF-ON button

The AF-ON button is used to engage the autofocus system of your camera and keep it engaged while the button is pressed. On the other hand, cameras equipped with this button can be configured not to engage the autofocus system while the shutter release button is pressed. If you leave the autofocus system configured with an automatic mode, such as AF-C on Nikon cameras, you can decide whether to just focus on a subject and keep a lock or track your subject movement:
  • Push the AF-ON button to focus on the subject.
  • Keep the AF-ON button pressed if you want the camera to track the subject or release it otherwise.

The major advantage of using this technique is that you can quickly switch between the two different patterns of autofocus use (single servo and continuous) without any camera reconfiguration.
Using this technique, you'll limit the number of autofocus reconfigurations you'll need to perform while shooting, in most cases down to zero. Perhaps, you will only need to change the number of focus points used to track your subject. However, since you can disengage the AF system at will, the overhead introduced by a relatively excessive number of autofocus points used will be mitigated since the AF system will be engaged only during the strictly minimum amount of time.

There's indeed an additional advantage using this technique: once you get a lock to a subject, you don't need to engage the AF system until the subject goes out of focus. In many circumstances, you will make a great deal of shots without engaging the AF system: your battery will last more time, and you won't be subject to any kind of AF deception.

What If My Camera Does Not Have an AF-ON Button?

If your camera isn't equipped with an AF-ON button, as the Nikon D7000, you can use this technique provided you can remap the AF-ON functionality to another button and the target button is easy to use while shooting: ergonomics is really important in this case because while shooting you will be using that button all the time with another finger on the shutter release button.

At least on Nikon cameras, the best candidate is the AE-L/AF-L button. First of all because it's located in a comfortable position to be pressed while shooting. On the other hand, you won't need autofocus-lock any longer (AF-L) when using this technique. As far as exposure lock (AE-L) is concerned, you can remap that function to another key, such as the Fn key.

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