Autofocus is one of the great convenience of modern photography. 18 years ago when I picked up my first camera, autofocus was making a big splash. It had been around for a while, but it wasn’t until that time that it really started to work. Today, we take it for granted. The autofocus systems on today’s cameras are amazing and help us to capture images that would be difficult, if not impossible, to capture without it. Despite the ready acceptance of the technology, most photographers don’t even realize that their cameras have different options when it comes to autofocus. On my canon cameras, I find settings for one shot, AI servo, and AI focus (other manufacturers offer similar settings though the names might be different). What do these settings mean and how do they impact our photography?
Before we get started, we need to understand a term: point of focus. When you tell you camera to focus, what you are really telling it is focus on a single point within the image. This point is the point of focus and it impacts how autofocus works.
(click on image to see larger version)
One Shot Autofocus:
One shot autofocus is what most people think of when they use autofocus. The way one shot works is that once focus is achieved (pressing the shutter button part way), it remains locked on a single point of focus until the shutter button is released. Moving the camera around does not change the original point of focus. This is convenient when you want to recompose the scene after focusing. Place the subject int he middle of the frame, focus, then recompose to take advantage of the rule of thirds.
The image above, for example, shows a mother baboon holding her baby as she eats the fruit from a sausage tree. While there certainly was movement, the baboons were not going anywhere and the baby was fairly still (mother was the only one eating). This was a perfect situation for one shot. To get the shot, I focussed on the baby’s face, recomposed the image so that the baby was in the lower center of the frame, and waited for the pair to assume a nice pose.
(click on image to see larger version)
AI Servo Autofocus:
In the second image (above) one shot would not have worked. The mother was on the move with her baby and locking down a single point of focus would have been problematic because the female’s location was constantly changing. Attempting to use one shot would result in blurry pictures and a lot of frustration. The solution is to switch the setting to AI servo. Unlike one shot, AI servo does not lock once a point of focus has been set. Instead, AI servo tracks the subject as it moves around the frame. As long as you keep the subject in the frame, AI servo will try to keep it in focus. While not perfect, AI servo does work surprisingly well in situations where the subject is mobile, such as flying birds, running cheetahs, or monkeys on the move. The downside of AI servo is that moving camera can cause the point of focus to shift, so it can be difficult to reposition the subject within the image.
AI Focus Autofocus:
In recent years, Canon’s camera’s have offered a third autofocus option: AI focus. In theory, AI focus is the best of both worlds. Using advanced technology, the camera decides if the subject is moving or stationary. Once that decision is made, the camera shifts between one shot and AI servo. When it works, AI focus is a fantastic tool. Unfortunately, I find it unpredictable in the real world, so I tend to stick with either one shot or AI servo for my autofocus needs.
Hopefully this provides you with a better understanding of how the different autofocus settings work and which setting will give you the best results in a given situation. When in doubt, keep it simple: stationary subject = one shot, moving subject = AI servo. Now get out there and give them a try.