AI Servo (or AF-C on Nikon) is basically the continuous auto-focus mode for your camera. Meaning, in this mode, when you hold down your focus button (whether it be back button or shutter button) your camera is constantly looking for a change in the subject where your focal point is landing, and it's updating your focal plane as the change is happening! So if you have your focal point directly on a subject, hold down your focus button, and your subject takes a step forward, your camera makes the adjustment for you.
The alternative to this continuous auto-focus mode is, duh, the single focus mode (know as One-Shot on Canon and AF-S on Nikon). This is pretty self explanatory. When you hold down your focus button, your camera will only focus once. If your subject takes a step back in this mode after you've pressed your focus button, your subject will likely be out of focus. You’ll need to do the work of refocusing yourself by letting off of the button and pressing again.
How can you use this information in real life shooting?
You'll want to understand these modes to determine the best use for them. When your subject is moving quickly, your focal plane will need to continuous-ly update to keep up. Al-Servo (AF-S) is going to give you the sharpest images in this instance. For example, when my bride and groom are walking toward me during the processional and recessional, I always make sure to shoot in Al-Servo!
In this instance, with them moving toward me or away from me, they will be going outside of the original focal plane (or the area that I want to keep in focus!). But no worries, because Al- Servo mode will keep my camera quickly updating to the new focal plane. Using this mode, I should manage to keep my subject in focus!
So, Why can’t you just keep your camera in Al-Servo all the time?
It sounds like it could be helpful because most of what we shoot moves right? Yes and no. Movement is okay as long as the area that I want in focus never leave my focal plane. If I have a wide enough focal plane, a little movement (especially side to side) is no problem. Now, let's say you want to follow the rule of thirds and you need to focus and recompose your image. You simply can not us AI-Servo's when you are trying to focus and recompose. Let's take the photo below as an example. If in AI-Servo mode, the moment I recomposed so that my focal point was now touching the water, I would have lost the focus from my couple and they would have been a blur while the water would have been sharp. This is because the camera was set into a mode where it is constantly updating the focus.
Another example of when you can’t use AI-Servo…bubbles! If anything could get in the way of my focal point, my camera could automatically update to a new focal plane in AI-Servo mode! In the example below, I need my camera to focus on the couple and not the bubbles. This can get tricky if the couple is also walking towards me, so in this instance, its best to keep a wider depth of field (larger f-stop) and keep my camera in One-Shot mode. I can continue resetting my focus manually and as quickly as possible depending on how fast they are moving towards me! It's my job to ask them to walk very slowly and stop in the middle for a smile and a kiss!
Finally, I won’t get into pros and cons of Back Button Focus, but I will just say that if you are using Back Button Focus, you should always keep your camera in Al-Servo mode. Basically just remember that when focus and recomposing, you MUST focus once and the let off of the back button before recomposing. If you don't let off of that focus button, your camera will automatically decide on a new area to focus!
I hope this helps those of you who've been wondering about these modes. Happy shooting!