If you shoot Canon digital cameras you may have heard people discussing the back button Auto focus. Most current Canon models now allow you to focus with the button on the back top of the camera within reach of your thumb behind the shutter button, specific for each model camera. Most people use the shutter button and press it down half way to obtain focus, you do have another option that will be useful for a number of situation such as when you focus on something and other things move between you and your subject or if you want to keep focus and recompose the image. Using the specific custom function for your model Canon camera you can activate the back button auto focus button, once activated your shutter button will not act as your focus button, it takes a little getting use to but there are scores of photographers that wouldn't focus any other way. For more detailed information on this helpful control see the Canon Digital Learning Center.