Digital SLR Autofocus Systems Explained

Digital SLR cameras have an autofocus system which has various focus points programmed into the area of the viewfinder. There are a variety of arrangements of these focus points, as well as different autofocus modes to choose from when selecting a camera.

An autofocus point is a small area in the viewfinder which is where the camera will focus when taking a picture. Older generations of SLR cameras had a single point in the center of the viewfinder. This meant that you would center the subject of the picture in the center of the viewfinder so the camera could focus on it. Otherwise the camera would focus on the background and the subject may not be in focus.

If the subject was not going to be in the center of the picture, then you would have to trick the camera into focusing on the subject. This was done by first moving the camera so the focus point was on the subject. Then you would hold the shutter button halfway down until the subject was focused. Next you would move the camera over to frame the picture so the subject was off center and then push the shutter button completely. Hopefully the camera would not refocus before taking the picture. This problem has been solved by using multiple focus points.

The most common arrangements of autofocus points are as follows: 

• 3 points, with one in the center, and one to each side of the center point
• 5 points, with the 3 points as above with an additional point above and below the center point
• 9 points arranged in a diamond pattern around the center point
• 11 points spread across the viewfinder

The purpose of multiple focus points is so your main subject will be in focus even if it is not in the center of the picture. A nice feature is if the autofocus point the camera is using lights up so you will know where the camera is focusing. If you are taking pictures of moving subjects which may be in random places in the viewfinder, then the more points the better to help insure your subject is always in focus. If your subject is in an area of the viewfinder without an autofocus point, then the subject will likely be blurry.

Digital SLR cameras also have different autofocus modes. One shot and continuous are the two most common types.

One shot mode works great for still subjects such as portraits, landscapes, etc. This works by pressing the shutter button halfway down, and letting the camera autofocus prior to pushing the button completely down. You can change what the camera is focusing on by releasing the button and moving the camera slightly so it will focus on something else in the picture. The camera will not take a picture unless the focus is set on a subject.

Continuous mode works better when the subject is moving across the viewfinder. You can still hold the shutter button halfway down and as you hold it there, the camera will continuously autofocus as your subject moves around the viewfinder. You can press the button to complete taking a picture in this mode even if the subject is not in focus at the moment you finish pushing the button. You may get some pictures with the subject blurry, and some with the subject in focus. At least you should get some good pictures.