Servo AF is an autofocus mode in which the camera never stops trying to acquire focus. Wherever you point the camera for any amount of time the camera will keep trying to acquire focus there and it will adjust automatically if it loses focus.
One Shot AF
One Shot AF works differently in that once the focus is acquired the camera will usually beep to notify you that it has acquired focus, and then it will stop trying to acquire focus. At this point the camera will not adjust to any kind of movement of the photographer or the subject.
Differences in practice
It becomes clear then that Servo AF is great for moving subjects since it will try to track the movement of the subject. While One Shot AF is better for static subjects or landscapes.
However, one issue with Servo AF is that you can’t focus and recompose if you’re using the shutter button to AF with. If you like to shoot using rule of 3rds or you’re just getting creative with your framing this can be an annoying limitation. However there are ways around it.
Back button AF for Servo AF
To get past the issue of focus and recompose when using Servo AF you can use back button AF instead of the shutter half press. To do this you will go into your camera settings menu and turn off the AF function for the shutter button. Now you will only be able to AF by using the back button. What this allows you to do is acquire focus with Servo AF by holding down the back AF button, then you can release the back button to lock your focus, recompose your shot however you want, and press the shutter button without changing your focus.
AF Stop function
Another option for using Servo AF is to set your AF back button to be an AF stop button instead. Now when you hold the shutter half way the camera will focus with Servo AF like normal. But when you’ve acquired focus you press and hold the back AF button to stop focus from happening. At that point you can recompose your shot and fully depress the shutter button without changing your focus. Simply release the back AF button to start Servo AF again.
AI Focus AF
Yet another option for your AF is not available on all cameras. It is called AI Focus mode. In this mode the camera will try to intelligently determine whether it should use Servo mode or One Shot mode to acquire focus. If the camera thinks the subject is moving it will use Servo AF, if it thinks the subject is stationary, it will switch to One Shot AF. My Canon 6D II has AI Focus AF and it works pretty well, but you have to pay attention to what the camera is doing to make sure you know the focus mode it is using is the correct one for what you want to do.
I personally don’t like to use the back AF button, but that’s just me. I find it a bit tricky to maintain a good steady grip while freeing up my thumb to press buttons on the camera. I prefer to use AI Focus mode or just shoot wider than I need to and move my focus points around so I can frame my shots the right way while using Servo AF. On most modern cameras this works pretty well.
Know any other focus tricks? Let us know about them in the comments below.