Digital Camera Focus Mode: Continuous Auto Focus
All digital cameras come with different focus modes. The most common mode used is continuous auto focus. Your camera will more than likely be set to this when you use it for the first time.
Understanding Continuous Auto Focus
What continuous auto focus does is it takes the most prominent object in your shots composition and it uses this as its focal point. However, the system is not always perfect at guessing what you want that object to be. You can compensate for this by zooming into the object you want to be in focus, hold that zoom until the object is sharp, and then slowly zoom out.
The mechanics behind continuous auto focus are complex. There are two different types of auto focus. Passive auto focus uses sensors to estimate the contrast in an image and determine what is most prominent. Active auto focus estimates the distance of the subject from the camera to the subject.
Because of how auto focus works, it is affected by both distance and lighting contrast. The less contrast in an image, the harder time the camera has focusing. This can cause the focus to constantly change and will render it ineffective. Active auto focus is dependent on distance, so if a subject moves out of the depth of field, they will become blurry.