If you’re just getting familiar with your camera autofocus settings, you’ve likely encountered the big question:
Should I shoot with One-Shot AF? Or should I use AI-Servo AF? (Some cameras also offer a third option, called AI Focus, but it doesn’t work well and so I recommend you ignore it completely.)
Unfortunately, there’s no easy answer, because One-Shot and AI-Servo are both useful depending on the type of photos you shoot.
So in this article, I’ll explain everything you need to know about these two autofocus modes. And by the time you’re done, you’ll know which option is right for you and how you can use it for amazing images.
Let’s dive right in, starting with the basics:
What is One-Shot AF?
One-Shot AF is the most commonly used autofocus method and is likely the default setting on your camera. (Note: On many non-Canon cameras, it’s known as AF-S.)
When you half-press your shutter button, the camera sets focus once (hence the term “one-shot”) and holds that focus until the picture is taken.
As you can imagine, this is hugely helpful if you want to focus then recompose. You can focus on one subject, then maintain focus while adjusting your composition to include other elements in the shot. For instance, if you’re photographing a tree next to a fountain, you could set focus on the tree, then freely move your camera toward the fountain to create the best possible composition – while the focus stays locked on the tree.
On the other hand, One-Shot AF isn’t so great if you’re trying to track moving subjects. Lock focus on a biker riding toward you, the focus will remain locked on the original spot, even as the biker zooms forward. Pretty soon, your subject will be out of focus, and you’ll be forced to lock focus again and again as the biker changes position.
What is AI-Servo AF?
AI-Servo AF tracks moving subjects. You half-press the shutter button to tell your lens to start focusing – then, if the area under your point of focus changes, your lens refocuses continuously until you take the photo.
(On non-Canon cameras, this is often known as AF-C.)
AI-Servo AF is perfect for scenes with moving subjects. Bringing back the biker example from above, if you placed your AF point over the biker and half-pressed the shutter button as she rode forward, the autofocus would do its best to keep the biker in focus.
Unfortunately, AI-Servo isn’t flawless; sometimes, if your subject is moving quickly, it will lag behind and you’ll end up with slightly out-of-focus images. Or if your subject moves out from under your autofocus point (and you’re not using some form of broad AF tracking), the AF will focus on the background instead.
But AI-Servo is certainly better for moving subjects than One-Shot AF, which consistently focus behind the subject.
Of course, AI-Servo has its drawbacks. You can’t use the focus-and-recompose technique mentioned above – as soon as you take your AF point off your subject to recompose, your lens will refocus and lose critical sharpness. And if you try to focus on an unmoving subject, it can change focus at the last second and cause you to miss the shot.
When should you use One-Shot AF?
One-Shot AF is the way to go when focusing on still subjects, such as landscapes, still lifes, some portraits (assuming your subject isn’t jumping around, running, or dancing), and flowers. Here’s a more complete list of genres that rely heavily on One-Shot autofocus:
- Landscape photography
- Architectural photography
- Cityscape photography
- Still life photography
- Macro photography
Of course, there will be times when you’ll want to switch over to AI-Servo AF – for instance, if your macro scene includes a fast-moving dragonfly – but for the most part, you can use the list to guide your choices.
And here’s a list of genres that use One-Shot AF some of the time:
- Portrait photography
- Street photography
For portrait photography, you must consider the type of photos you’re taking. Will you be doing a fast-paced portrait session with a lot of movement? Or will your subject be sitting or standing in place? For still subjects, One-Shot AF is a good idea, but for moving subjects, go with AI-Servo.
As for street photography, some photographers use One-Shot AF to prefocus on specific compositions then wait until a subject walks into the scene. Other photographers use AI-Servo AF constantly and snap shots as people move toward them. It all depends on your style!
When should you use AI-Servo AF?
Use AI-Servo AF whenever your subject is moving (especially if you’re working with a shallow depth of field).
So if you’re shooting sports players in action, birds in flight, or cars on the move, you should definitely use AI-Servo nearly all the time.
Here’s a list of photography genres that rely heavily on AI-Servo autofocus:
- Bird photography
- Wildlife photography
- Sports photography
- Car photography
- Wedding/event photography
- Underwater photography
So if you’re photographing birds, for instance, unless you see an obvious reason to switch to One-Shot AF, I’d recommend you set your camera to AI-Servo AF and keep it there.
Which autofocus mode is best?
As I mentioned at the beginning of this article, this question has no real answer. In some situations, One-Shot AF is the best option, but in other situations, you’ll want to use AI-Servo AF.
In fact, there are even times when manual focus is the better bet. If you’re doing ultra-high magnification macro photography or you’re shooting in the dark, your lens will likely fail to autofocus, so manual focus will be your only good option.
I do have a personal recommendation, though, and it’s what I use for most of my photos:
…but not with the normal half-press focusing method.
You see, most cameras let you set your focus via a button on the back of your camera, which gives rise to a technique called back-button autofocusing.
When used with AI-Servo AF, back-button focusing gives you the best of both worlds. Here’s how it works:
Start by programming a button on the back of your camera to autofocus when pressed (and make sure AI-Servo is activated). Then, when you hold down the special back button, your camera will focus continuously, just as if you half-pressed the shutter button.
And when you let go of the back button, focusing will lock. You can recompose all you like, you can take shots with the shutter button, etc., without worrying about losing focus as you shift your AF point.
So if you have a biker riding toward you, simply keep the back AF button held down. Then, if the biker stops and you want to position them in the corner of the frame, you can let go of the back button and change your composition – while your focus remains locked in place.
Thanks to its versatility, I use back-button AF 90% of the time. It’ll save you plenty of headache trying to switch back and forth between AI-Servo AF and One-Shot AF!
One-Shot vs AI-Servo: final words
Hopefully, you now know whether to use One-Shot AF or AI-Servo (and you’ve hopefully also been inspired to try out back-button AF). Learn to master your camera’s autofocus capabilities, and you’ll be unstoppable!
Now over to you:
What do you think about these two focusing modes? Which do you plan to use in your photography? Do you think you’ll try back-button focusing? Share your thoughts in the comments below!