Though most professional filmmakers or camera operators will tell you about the horrors of auto focus, there are times when its use makes sense. If you are trying to maintain a constant filming pattern, as is usually the case when filming real world events that continue independent of your control, you may want to choose an auto setting rather than having to stop and calibrate your focus. This choice is usually the result of a cost benefit analysis where versatility is outweighing perfect imaging. If you are going to use auto focus then here are a few tips to help it work for, rather than against, your final video.
One of the main issues with auto focus on your digital video camera is that it will find and focus in on objects that are not your main focus. If a person passes by your lens that is in between you and the object you are filming then the camera can refocus on that object briefly before focusing back on the main object. In another case it can simply automatically focus in on an object that is closer to you than the object. If you are going to use auto focus try and stay away from too many objects in between you and the focus of your video. The same is true of people or objects passing by your frame. Try to avoid anybody passing through the direct line of film between your camera and your subject.
Though you are going to be using the auto focus setting on your camera you may still want to start with focus calibration. This would be good for identifying the image and singling it out so that you can at least start with correct focus. This will then feed in to the auto focus function that will pick up where the manual focus left off.
You are likely to be in motion while you are using the auto focus. Here you are going to have to accept a certain amount of focus shift throughout. The best way to avoid this is to follow a set subject, trying to keep them the focus of the video the entire time. If you can remain close to them then you have an extra advantage.