Most cameras have two or three main autofocus modes. Like with everything else, the different manufacturers have given different names to these functions. Canon’s main autofocus modes are One Shot, AI Servo and AI Focus.
These names aren’t particularly helpful, so I’m often asked what they are and what’s the difference between them.
What is One Shot focus?
One Shot autofocus is the same as single servo autofocus in other camera brands. Use One Shot focus when neither you nor the subject is moving. To use One Shot AF:
- Half-press the shutter button
- The camera will focus on the subject and lock on
- until you fully depress your finger to take the photo or take your finger off the button
What does AI Servo AF mean?
AI Servo AF stands for Artificial Intelligence Servo Automatic Focusing and is Canon’s name for continuous focus. Because AI Servo AF focuses continuously, it’s ideal for focusing on moving subjects.
Obviously, it’s not going to be perfect 100% of the time, but for moving subjects, AI Servo is the autofocus mode you need.
How do you use AI Servo mode?
With AI Servo autofocus mode, when you depress the shutter button the camera focuses on your subject and tracks their movement. What’s more, it ensures they stay in focus by predicting the subject’s movements. The focus continually changes to keep the subject sharp, even if the subject or the photographer is moving.
In AI Servo some Canon cameras can also automatically move the focus point around the viewfinder to track a subject moving:
- Towards the camera
- Away from the camera
- From one side of the scene to the other
AI Servo and back button focusing
The best way to use AI Servo on a Canon DSLR camera is with back button focus. You just need to set your camera up so that you can use a button at the back for focusing and your shutter button just for capturing the image. In other words, disable the focus function from the shutter button.
The back button focus button, normally the AF-ON button, is conveniently located for your right thumb to easily hold it down while shooting.
One of the advantages of back button focusing is that it enables you to use both AI Servo to track focus and One Shot for still subjects:
- For moving subjects, establish and track focus by holding down the AF-On button, then take your pictures.
- For still subjects, establish focus, remove your thumb from the AF-On button and take pictures. If neither the camera nor the subject move, there’s no need to refocus between shots. You could even walk away, make yourself a coffee and come back to take another shot without needing to refocus, unless something had moved.
This wouldn’t be possible if the camera tried to focus every time you depressed the shutter button. So by restricting the shutter button to simply taking the shot, you won’t need to switch between One Shot and AI Servo.
One Shot AF vs AI Servo AF
Something to bear in mind, however is that AI Servo AF lets you fully depress the shutter button and take the photo, even if it hasn’t completely focused. So you can’t guarantee that your subject will be in sharp focus.
If neither you nor your subject is moving, One Shot AF could be a better option, because it locks focus and AI Servo doesn’t. This is particularly relevant when photographing:
- At close range (because movement is exaggerated at close range)
- Wide open or with a long focal length (because this creates a shallow depth of field)
What is AI Focus AF?
If One Shot AF is for still subjects and AI Servo is for moving subjects, you’d think that everything was all covered.
Well, not quite, that’s where AI Focus AF is supposed to fill the gap. I say supposed, because it really isn’t a good idea to use AI Focus AF. Let me explain what AI Focus is first.
AI Focus AF is a compromise between:
- One Shot AF (focus, shoot, focus again for the next shot)
- and AI Servo AF (continuous focus)
With AI Focus your camera switches between One Shot AF and AI Servo as it deems necessary. So if it thinks the subject is still, it switches to One Shot and if it thinks there’s movement, it will use AI Servo AF.
Canon isn’t the only camera manufacturer that has a hybrid autofocus mode – the Nikon equivalent is called Auto AF.
What is the difference between AI Focus and AI Servo?
With AI Focus AF there’s often a delay between the subject’s movement and the camera focusing, so focus is often missed. With AI Servo, the camera focuses continuously, no stopping and starting, so there’s no delay when you’re ready to take the photo.
If you’re photographing with a narrow depth of field, it’s difficult to maintain sharp focus on a moving subject using AI Focus AF, because it doesn’t focus continuously and there’s minimal room for error.
On the other hand, AI Servo is ideally suited to tracking moving subjects and focusing at very shallow depths of field.
So, my best advice, is just to use AI Servo and forget you even have the AI Focus option.
Selecting your focus point
Very often new photographers get autofocus modes confused with autofocus area modes. These are two separate autofocusing functions and you need to set both options:
- The AF mode tells the camera to focus continuously or once
- The AF area mode tells the camera where to focus
We’ve covered autofocus modes, so let’s look at autofocus area modes briefly.
You can set:
- A single autofocus point
- A group of autofocus points
- Or even make all the autofocus points active
When you look through the viewfinder while setting your autofocus area mode, you’ll see the different autofocus point configurations display as you cycle through your options.
Most of these area modes can be used in both One Shot (aka single servo) or AI Servo (aka continuous autofocus).
What AF area mode should I use?
Remember, don’t be tempted to think that if you select all the AF points, then the camera will do all the work and you’ll have an in focus subject. The camera is not a mind reader.
While it’s clever enough to figure out that the main subject will probably take up most of the frame and might be the object that’s closest to camera, that’s not always the case. For example, if you’re close enough to your subject to photograph their entire face, the camera might focus on their nose instead of their eye, which isn’t helpful.
In most situations I suggest using single point autofocus and moving it around the screen to your subject, and specifically the eye closest to camera.
Further reading: Focus modes and drive modes – the differences
Leave a comment
If you have any questions about One Shot AF vs AI Servo vs AI Focus AF let us know in the comments.
Also, I love good news, so if my Canon focus tips have helped you to understand how to use AI Servo vs One Shot AF, share that too.