If you are taking a picture of a magnificent church, it’s a completely different thing than taking a picture of someone riding a bike in front of that same building.
You should always think about what it is you want to be in sharp focus in your image, and adjust your camera settings accordingly. In an article over at Digital Photography School this is what photographer James Brandon had to say on the subject.
This is probably the most commonly used method and is likely the ‘standard’ setting on your camera. When you press your shutter half way down (or use your AF-ON button instead) the camera sets focus one time and holds that focal distance until the picture is taken. If you’re shooting still life or a subject that simply isn’t moving much then this is a good method to use. You can even go into your settings and make the camera beep when focus is locked so you know it’s good. This setting is also a must when using flash in low light; for whatever reason (I really wish Canon would change this) the focusing grid on your external flash will only work in One Shot mode. Switching to Servo will disable the grid completely.
Using AI-Servo was really a huge step forward for me . When I discovered it for the first time I was pretty blown away! AI-Servo tracks focus on moving subjects so as your subject moves closer or moves further away your camera constantly updates focus to keep the subject as sharp as possible. This means that a train could be headed straight towards you and you could set your center focus point over the nose of the train and your camera would keep the train in focus as it got close and closer. Eventually the train would be too close for your camera to focus though and at that time it’s best to get off the tracks (protip).
So Which One Is Best?
Well that really is the question isn’t it? Unfortunately there really isn’t a right answer. The better question is which one is better most of the time? And I will wager that the correct answer to that question is AI-Servo.
We found a great example video to show you how changing the focusing mode will make it so much easier to get sharp images of moving subjects.
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Source: Digital Photography School