In continuous auto focus drive, the camera starts trying to focus when you half press the shutter button. To compound the problem, if you are shooting with a shallow depth of field (a small f-number like f/2), even TINY movements like your or your subjects breathing will move them from the plane of focus. If you are photographing moving subjects, such as wildlife or children, they will likely move out of your plane of focus before you take the picture. There are a number of major catches with single focus drive. Single Focus Drive has different names on different camera systems: To get it to focus again, you have to release the shutter button and start over. As soon as it thinks it has focused, it stops trying. When your camera is in single focus drive, and you half press the shutter button (tell the camera to focus), the auto focus system will try to focus. In fact, if you are making portraits and using a shallow depth of field, your subject’s and your breathing is enough to loose focus in single focus drive. It works with moving subjects like flying birds or running children, but also in stationary situations like portraits. Move your focus point where you need it and use continuous focus drive. Modern focus systems have a lot of points spread out. That was needed because early auto focus systems had very few focus points clustered around the center of the frame. Single auto focus drive was created to allow photographers to focus (half press the shutter button) and then recompose their photo.
Most cameras have 2 options: Single and Continuous. Besides options like using area vs single point focus, cameras allow you to control whether the focus system tries to keep focusing or stops after it achieves focus. Most new photographers don’t realize that their auto focus system is configurable. This mistake manifests itself as slightly front or back focused images, usually with quickly moving subjects. Topping this list is the most common problem I see: using the wrong focus drive mode.
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