Autofocus Modes – Wildlife Photography Tips #10

AUTOFOCUS MODES - WILDLIFE PHOTOGRAPHY TIPS #10

All cameras manufactured in the last 30 years, with the exception of a few highly-specialized models, are capable of automatic focusing or “autofocus”.[1] Many camera models now include a range of options that can be used to optimize the autofocus effect for specific situations. This article will describe autofocus servo modes and area modes, and recommend the best choices for wildlife photographers.

Note: Different camera manufacturers use different terms to describe what are essentially the same capabilities. Because I am a Nikon user I will use Nikon terminology throughout.

On basic cameras like those in mobile phones, autofocus is always enabled. For bridge cameras, DSLRs and mirrorless cameras there is usually a switch so that users can choose whether to use automated or manual focus. Your lens or camera body should have a switch labelled AF-M or M/A-M. AF stands for autofocus and M for manual focus. Ensure that the AF-M switch is set to AF or M/A.

AF switch on Nikon 24-70mm lens.

M/A, by the way, means that the autofocus is activated but you can override it manually. This is a useful option to have when your subject is obscured, for example by foliage. The autofocus system may have a hard time finding the desired point of focus. (Remember that modern autofocus systems are “smart” to a degree, but not smart enough to understand that you want to photograph the bird and not some random twig or leaf). If you have the option to do so I recommend setting the camera to M/A so you can override the autofocus when necessary.

Once autofocus is enabled, you activate the focus system by depressing the shutter release halfway or, if you are using back-button focus, depressing the AF-On “back-button.”

There are two key sets of choices that you can make to optimize autofocus. Somewhat confusingly Nikon calls them Autofocus modes and Autofocus area modes. For the sake of clarity I will refer to them as Autofocus servo modes and Autofocus area modes.

Autofocus servo modes

There are two basic choices here. You can either lock focus on a single point, or use a tracking capability that automatically refocuses as the subject moves.

AF-S: Single servo AF

… or “One-Shot AF” for Canon cameras: This system is primarily used for stationary or slow-moving subjects. It uses a single point at the centre of the frame. When the focus system is activated – by half-pressing the shutter release or pressing the back-button – the camera will achieve and lock focus on that point.

Key point. If the camera and/or the subject moves, even if you hold the shutter release halfway down, the camera will not refocus. So small movements can lead to out-of-focus or not optimally focused images.

Note that you can customize this function using the camera menu system. There are two main options:  (a) the camera will record an image when you press the shutter release (whether it is in focus or not), or (b) the shutter will not release if the image is not in focus.

Cunning photographers (I’m looking at you Phil) have learned that you can use option b to your advantage when photographing small, fast-moving birds. If you track the bird while holding the shutter release down, an image will be recorded at the fleeting moment when the camera has the bird in focus.

AF-C: Continuous servo AF

… or AI Servo in Canon-speak: This mode is optimized for moving subjects, and is the optimal choice for moving targets. While the focus system is activated the camera will continuously refocus until the shutter is released.

Key point. With AF-C activated, when you press the shutter release the camera will attempt to record an image whether it is in focus or not. So you do need to wait a moment for autofocus to find a solution. This wait time ranges from very short (less than a second) with high-end lenses at wide aperture, to several seconds with basic telephoto lenses.

One drawback of AF-C mode is that sometimes it is not as precise as AF-S, so you have to watch carefully to ensure you don’t end up with slightly out-of-focus shots.

AF-A: Hybrid mode

This is a third option often seen on lower-end DSLRs. In this mode the camera switches between AF-S and AF-C depending on whether it thinks the subject is moving or not. AF-A is intended to give a beginner the best chance of getting a good photo. The mode is not present on higher-end camera bodies because the manufacturer assumes that users will not want to cede control of such an important decision to the camera.

So to summarize, AF-S is best for stationary targets, and AF-C for moving targets.

To choose the autofocus servo mode you should consult the manual for your camera. Nikon DSLRs are controlled by depressing the AF button on the front of the camera and then moving a control wheel until the right symbol appears in the viewfinder.

AUTOFOCUS MODES - WILDLIFE PHOTOGRAPHY TIPS #10
AF mode button on Nikon D850.

Autofocus area modes

These options allow you to choose how the focus point is selected. In the Nikon world there are up to ten options depending on your camera body. But don’t despair: only  a few of these are really useful for wildlife photography.

  • Pinpoint
  • Single point
  • Dynamic area modes (x4)
  • 3D tracking
  • Face detection
  • Group
  • Auto-area

Pinpoint

This is a unique-to-Nikon mode that allows precise focus on a very specific point of the target. At the moment only the D850 DSLR and the Z6 and Z7 mirrorless cameras have this feature. It is not likely to be useful for wildlife photography in general, particularly since it is relatively slow to achieve focus. But it is worth considering for specific outdoor applications, e.g.  macro photography of flowers, mosses and stationary (i.e. dead) insects. Note that it is only available with AF-S mode.

Single point

This is one of the most important modes for wildlife. The camera focuses on a single point at the middle of the frame. This is the fastest and most accurate of AF modes, and is particularly good for the following situations:

  • when you have time to compose a shot (and are using back-button focus activation). This mode allows you to zero in on the most important feature – usually the eye of the subject, and then recompose the shot knowing that the focus will remain on the chosen point. This is ideal for large mammals and birds, frogs, turtles etc that are close to the observer.
  • when the lighting is poor (explanation later in this article)
  • when your target is distant and there is a chance that using an area mode (described below) will lead to inadvertently focusing on a nearby twig or stalk of grass instead of the target

9-point Dynamic area

25-point Dynamic area

72-point Dynamic area

153-point Dynamic area

The idea of these dynamic area modes is that you can choose your focus point, and then as the subject moves the camera will “dynamically” keep the subject in focus. This is a very useful capability for photographing birds in flight, as long as you are aware of one limitation. If you wish to choose a specific initial focus point you do so by cycling between available points using a joystick or a rocker wheel on the back of the camera body. Professional photographers apparently perform this juggling act but mere mortals such as myself have enough issues to deal with in tracking a fast-moving bird without trying to manually move the focus point at the same time. Perhaps the professionals are photographing slower birds… 😊

On top of that, in order to use the joystick or rocker switch with my thumb I would have to re-route focus control from the back-button back to the shutter release, and I am unwilling to give up on the advantages of the back-button.

Camera controls – Nikon D850. Note that the rocker switch is in the locked position.

However if you are happy with a random focus point these modes work well. The Sandhill Crane below was taken using 25-point dynamic area mode. These birds were landing in waves at the Lost Mountain wildlife reserve in Saskatchewan and each group passed over very quickly. At my skill level there was no scope for picking a specific focus point but the autofocus mode did a good job selecting a usable one. (FYI I believe it is the left wingtip of the second bird). There is sufficient depth of field at f/8.0 that I’m not sure the image would have been improved if I had chosen a specific focus point.

AUTOFOCUS MODES - WILDLIFE PHOTOGRAPHY TIPS #10

Caveat

One thing to note is that the dynamic modes with larger numbers of focus points take slightly longer to achieve focus, and have an increased chance of focusing on something other than your desired subject.

But overall the dynamic modes are useful tools that are worth familiarizing yourself with. In order to use them the autofocus mode needs to be set to AF-C.

BTW the equivalent Canon term for Nikon’s dynamic modes is AF Point Expansion.

3D-tracking

This mode is a clever bit of technology. Once you have picked a focus point it analyzes colour differences to determine which object you want to focus on, and then uses as many focus points as necessary to keep that moving object in focus. I have not experimented much with this mode but sources I trust suggest that for birds in flight it works well against a clear background such as the sky, and less well when the background is busy. For large mammals, which are generally slower moving, this mode probably works very well. And another source reports that in a large mass of fast moving birds this mode is very good at picking out one bird and tracking it. 3D tracking mode is also only available in AF-C.

Face detection

This mode is self-explanatory. It is very useful for people photography but not really geared towards wildlife.

Group

This mode is one that I use frequently. When aimed at a group – e.g. a small flock of birds – it will focus on and track the closest member of the group. It uses only the centre point and the four others that are close to centre so it achieves focus quickly and holds it well.

Auto-area

This mode takes control of the focusing process and guesses what you wish to focus on. It seems to be intended for neophyte users who want to automate the whole photography process, so that their only responsibility is to point the camera in the general direction and hope for the best. I have not tried this mode but I understand that it is fairly good at finding faces. Otherwise it tends to focus on the nearest object. This mode is not really relevant to the wildlife photographer.

Recommendations

My first recommendation is that if you want to take full advantage of the capabilities of your camera you should learn how to change AF modes and AF area modes quickly and efficiently.

In my view the most useful AF combinations for wildlife are:

For stationary targets , especially if they are distant and/or the light is poor, use AF-S/single servo mode and a single (centre) focus point. Focus on the key feature (usually the closest eye). If you are using back-button focus you can then recompose without refocusing.

For all moving targets, use AF-C mode and one of the autofocus area modes.

Of these, I would experiment with 3D tracking for slow-moving targets (large mammals, e.g.).

For faster-moving targets, use either Group or one of the dynamic area modes. Of these I would stick to 9 or 25 point for most situations.

When I am out birding I will normally have the camera set at AF-C and 25-point area mode as this gives me the best chance of getting onto a fast-moving bird.

AUTOFOCUS MODES - WILDLIFE PHOTOGRAPHY TIPS #10
AF-C, 25-point dynamic area. Ring-billed Gull.

A note on low-light photography

Autofocus systems use areas of contrast to judge focus. If there is not enough detail (clearly defined edges) in the subject then the autofocus system will “hunt” – that is, cycle back and forth trying to establish a focus point. This explains why autofocus doesn’t work well in low light conditions – there is not enough contrast to achieve focus. So though your high-end DSLR can capture an image at astonishing ISO values, you will probably have to focus manually or accept blurry images. Or check your manual to see if your camera has an AF-Assist function that can help out.

The Blue Bill

This article originally appeared in the Blue Bill, the journal of the Kingston Field Naturalists, Volume 68, Number 4, December 2021.



[1] The term is a bit of a misnomer because it’s not really automatic – you still have to activate the autofocus system, or at least ensure that it is focusing on what you want it to. 

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