The Art of Autofocus

This article was originally written for Photo Safari India in February 2006 and has been updated for modern times.    The intended audience for this article would be people that shoot wildlife, sports and other other moving objects, but the concepts here hold true for any kind of photography.   Also, while i use the term DSLR out of habit, the same concepts also hold true for MLCs.

For something that is supposed to make life easier, autofocus seems to stymie a lot of people, especially those who are starting out with DSLRs or MLCs.  And usually, it is the poor, innocent camera that takes the blame, while all the crotchety old-timers sit in the corner and shake their heads knowingly, muttering something about how things were better in their days.

For starters, autofocus, like any other camera feature, only works effectively when you, the user, control how it works. Letting the camera decide where to focus is like taking relationship advice from the Penthouse forums – it may sound really tempting but may not be the best thing in the long run.

So the first step in focusing properly is to take control. Your camera most likely has between 3 and 45 focusing points. It also likely has a mode whereby all the focusing points are active, and the camera selects where to focus from one of these points. This is the mode you NEVER want to be in. For starters, your camera doesn’t know where you want the focus to be (it cannot read minds yet, although the next Canon DSLR is rumored to…).  Second, maybe your desired point of focus is not where the sensors lie.    So you need to actively tell the camera where to focus.

So  begin by pulling out you camera manual, opening it to the page where it talks about focus point selection, and read up on how to manually select focusing points. Usually, this involves pressing a button and/or moving a joystick or arrow keys (or, if you are lucky, eye-controlled focus). Now go ahead, practice.    If you have a choice of settings, choose the option that lets you move the AF point directly, without needing to faff around with pressing other buttons first.    Practice enough till you have it down pat,  and are able to select and change between focusing points without fumbling around or taking your eye off the viewfinder.

Once you have that down, let’s move on. For the rest of this chapter, whenever we talk about focusing, we are referring to you, the user, actively choosing an autofocus point, placing it on top of your subject and then choosing to focus on that spot.

The Basic Skill: Focusing on stationary objects

Most cameras have at least 2 autofocus modes – in the Canon world, these are “One Shot AF” and “Continuous AF” (or Predictive AF). The exact term may vary from camera to camera but the concepts are same.  For now, where you are shooting stationary objects, One Shot mode is the one you want.

In this mode, select the AF that you want, put it on top of the subject and half-press the shutter – this causes the camera to focus on the part of the image on the selected AF point is resting. Now, as long as you keep the shutter button pressed, the focus stays locked on that point. This lets you move the camera around and recompose the scene if need be, until you get it exactly the way you want it. Thus, you can put your subject wherever you want – even someplace that is not covered by an AF sensor.

Once you have the composition the way you want it, press the shutter and you’re done.

Sounds easy, doesn’t it? Well, it is. With practice, you’ll find it takes a fraction of a second to lock focus, recompose and shoot.

One side benefit of this is that you no longer need a lot of AF points. All you need is 1 AF point – this “focus-lock-recompose” method then lets you put the subject wherever you want. A lot of people – myself included – don’t need anything more than a single, central AF point when shooting this way.

Do note that in the One Shot mode, the camera will not let you shoot until the focus is locked. Also, if the subject moves once you have locked focus, you will need to re-focus. Lastly, if the subject is close to the camera and you are using a large aperture, the act of recomposing may cause the subject to move out of the plane of focus. However, in the vast majority of cases, this approach works very well and has stood the test of time.

The Advanced Skill: Focusing on moving objects

Well, if the first part was easy, the second part is a fair bit harder. It is like going from a bicycle with training wheels straight to a 950CC MotoGP superbike. Ok, maybe it is not that bad, but it is a bit of a jump nonetheless.

As mentioned earlier, all modern DSLRs and MLCs  have a “continuous autofocus” mode (or whatever it may be called). Here, half-pressing the shutter and keeping the sensor on the subject causes the camera to continuously keep focusing on the subject, tracking it even as it moves. Not only that, the camera even takes into account the time delay between the pressing of the shutter and the taking of the picture, and adjusts the focus to account for the subject movement in that fraction of a second. How’s that for cool?

In theory, this whole process is easy enough: simply keep the selected sensor on the subject and keep the shutter half-pressed, and shoot when you are ready.

But this is where the complication arises. Because the subject is moving, you have to follow the motion with your camera. The faster or more erratic the movement of your object, the harder it is to accomplish this. Your goal is to try to pan with the subject in a smooth, continuous motion – and gaining the ability to do so requires practice.

For the most effective results, continue panning not only until you’ve pressed the shutter but till a little after that (till you can see the subject again in the viewfinder) – this ensures that you don’t break your rhythm at the time of taking the shot. Also, in order to increase your chances of an in-focus image, track the subject for at least a couple of seconds before clicking the shutter.

Panning is but one aspect of shooting moving objects. In continuous AF mode, “lock and recompose” does not work, so you have to activate the appropriate auto-focus point and keep it on the subject. That, of course, makes composition harder – if you use the central AF point, you are putting your subject dead center: usually a compositional boo-boo. On the other hand, if you move your subject off-center, you run the risk of not having an AF point at the appropriate place in the frame. And of course, your subject has to fit the frame as well.

So now let’s break the first rule we laid down while discussing autofocus.

If your subject is against a uniform background (like a flying bird against the sky), or is large enough to cover a significant portion of the frame, select all the autofocus points and let the camera pick (this is the one time it is ok). This makes it a lot easier to keep focus on the subject and also makes it easier for the camera to track the focus. However, you run the risk of the camera selecting the wrong point, true – for example, focusing on the bird’s body instead of its head and eyes. If the subject is some distance away, this usually isn’t a problem. Also, if you are shooting against a uniform background like the sky, you can also stop down a little more for additional depth of field to compensate for this.

Do note that this above technique only works properly against a uniform background – try it against, say, foliage, and you’re in for some serious frustration. If your sensor comes off the subject, your camera will try to refocus (and out of sheer spite, will choose the wrong focusing spot, causing your subject to blur out and your lens to start hunting).

In such cases, it is often easier to select the central focusing point and keep it on the subject. This gives you room in the frame and prevents accidental cropping of the subject. You can always modify the composition by cropping the final image later. As you get better, you can move on to selecting an off-center point and putting it on the subject – but this only works if you can be sure of not accidentally cropping off part of the subject.

The secret to successfully shooting moving objects lies in one word: practice. There are no short-cuts – this is simply a matter of keeping at it. If you have a DSLR, you are lucky – practice is free. If you have a film SLR, buy large quantities of the cheapest film you can find… or initially, practice without any film in your body. Start out with easy to track subjects – like people walking. Then follow faster objects, but with predictable movements. And then move on to tricky stuff – like flying birds, playing puppies, etc. The good news is that modern AF systems have become so good that this skill is within grasp of most of us.

This mode is best used with your camera’s drive mode set to continuous shooting. Most SLRs these days can chug along at a very respectable 3-4fps at the very least – that gives you a fairly good chance of capturing exciting action sequence. You’ll never get all shots sharp and in focus, but with practice, you’ll get enough keepers.

The Expert Skill: Advanced focusing tricks

Ok, so now you know how to focus stationary objects and moving objects. But what if you don’t know whether your subject will stay still or move? If your subject was stationary, you’d want to lock focus and recompose for the best composition. On the other hand, if your subject suddenly started moving, then you’d have to pull your camera down, change focusing modes and then re-shoot. By then, you’ve lost your shot. On the other hand, if you stay in continuous focus mode, you wouldn’t be able to recompose – the very act of pressing the shutter to take the shot would cause the camera to recalculate focus. That’s quite a dilemma, innit?

Well, technology and intelligent design to the rescue.  Most DSLRs and MLCs have a button in the back called “Autofocus Lock” (AFL) – pressing this locks focus.     And most cameras also have a setting which lets you assign only “Autoexposure Lock” when you half-press the shutter button.   That’s how you want to set your camera up (and you may have to read the manual on how to achieve this, as it varies significantly from camera to camera).

This way, when you want the camera to achieve autofocus, you put your desired AF point on the subject and press the AFL key – the camera achieves auto focus.   When you want to shoot, press the shutter – the half-press will lock the exposure, and the full press will take the shot.

So how does this work in real life?

Let’s say you have a stationary subject. Point the camera over to it, press the AFL button and your camera autofocuses on the subject. Release the AFL button and you can recompose to your heart’s liking – all the benefits of One Shot Autofocus.   If the subject starts moving, all you have to do is put your selected AF sensor on subject and press the AFL button again to track focus. – all the benefits of Continuous Autofocus.

One of the biggest annoyances of shooting with a big telephoto is its tendency to re-focus everytime you press the shutter button. By using this mode, the camera only focuses when you want it to focus – the rest of the time, it behaves like a manual focus camera.

Yep, you do have the best of both worlds. Ain’t technology great?

Another trick you can try, if you want and your camera lets you, is programming one of the rear buttons on your SLR to switch to a “favorite” focusing point. For example, my default focus point on my SLRs is the center point; however, I have one of the rear buttons programmed to switch to all AF points.

Thus, if I am shooting and find the center point is not enough in helping me track the subject (either it is too small, or moving too fast), then I simply press this rear button and all 45 points on my body come alive and start tracking the subject. I can do this while in the middle of following the subject, and without taking my eye off the camera.

The Technological Short Cut:    Subject Tracking

Modern technology is great – photographers today have the benefit of using something called focus tracking (again, the exact name varies from brand to brand).   Basically, what this does is that once you get your AF point over a subject and lock in focus, when the subject moves within the frame, the camera will automatically shift AF points to keep the subject in focus.

There are variations of this whereby the camera can use all its sensors, or only smaller grids (3×3 or 5×5) but all of them essentially accomplish the same task:  use the camera’s processor to identify the subject and then keep it in focus.

This has the advantage that you no longer have to rely on your own tracking skills (ie, panning the camera along the same arc as the subject’s movement).   The downside, of course, is obvious – your composition changes as the subject is no longer in the same place in the frame.   However, in otherwise difficult conditions, this could be useful tool to add to your repertoire.

Common Focusing Errors

Still having problems? Before accusing your SLR of having faulty focus, going back to manual focus, or worse yet, blaming me, consider the following:

  • If you are panning the camera or your subject is moving, it will appear at least partially blurred, unless you are using a high shutter speed (how high depends on how fast the subject is moving, how far it is from you and its direction of motion). This isn’t always a bad thing – a little bit of blur implies motion and makes the image more dynamic.
  • Even if you focus correctly, if your shutter speed is too low, you will get somewhat blurred results due to camera shake. The old rule for getting sharp results while handholding is to keep the shutter speed equal to, or faster than, the inverse of the focal length. So for a 300mm lens, you’ll want to shoot at 1/350 or faster.
  • If you are using a telephoto, then you don’t have a lot of depth of field to play with. Moving your AF sensor even slightly off your desired point may cause your image to appear improperly focused.
  • Sometimes, if you are too close to the subject, the act of locking focus and recomposing can cause the subject to move out of the plane of focus and result in a loss of sharpness.
  • Do note that camera’s AF system does require some time to acquire AF – that depends on light, the lens/camera combo and the contrast of the subject.   So even if you are doing everything correctly, it may be that you simply haven’t given the AF system enough time to do its thing.   This is especially true with larger telephotos.
  • Small branches and objects in the foreground can occasionally throw off your camera’s autofocus.  So be aware of those!

Furthermore, depending on your subject’s size and how close you are to it, even a properly-focused photo may not yield the entire animal sharp from nose to tail. This is because you don’t have enough depth of field. If that is the case, consider stopping down your aperture to increase your depth of field.

Lastly, if you are shooting macro, don’t bother with AF.  Use manual focus.    The same applies when conditions are too tricky for autofocus (eg, lots of branches, etc between the subject and you).

Even though technology has made getting in-focus shots very easy, you still need to develop your skills on how to utilize it for the best.    With trickier shots, you may still end up with out-of-focus shots, but the trade-off, in the form of a very hard-to-capture-correctly photograph, is worth the effort you put in.

I hope this tutorial is useful.