Understanding Autofocus

You probably use the autofocus (AF) mode all the time, but did you know that your camera has multiple focus modes that perform different functions? This article will help you understand the digital camera basics behind focus.

There are three focal modes on most DSLR cameras: manual focus, one shot, and continuous. Knowing which type of focus to use will help you get pictures that are sharp and clear. We'll discuss manual focus some other time. On this page I want to concentrate on the two autofocus (AF) modes, but first let's look at how AF works.

Your digital camera's AF mode is controlled by an array of senors that measure contrast to figure out if the image is in focus. The areas where there is the most contrast are considered the areas with the sharpest focus. While the sensors are measuring contrast they are also measuring distance to help the camera focus. These determinations are all made in fractions of a second. Once the camera makes these decisions tiny motors in the lens adjust to put the subject in focus.

Sometimes the camera has difficulty determining what to focus on. Sometimes a cluster of objects in the shot, motion, or lack of contrast in the photo will cause your camera to start to continuously hunt for a focal point. That's why it's good that there are different focal options on your camera. Let's look at the two AF modes you can use.

One-Shot

One-shot or single-shot mode locks your camera in focus on a single object in view. The sensors won't allow the camera to take a picture until the focus is found. This is the only AF option that most compact cameras have. It is usually one of two options on a DSLR camera.

When you use this mode you should put your subject in the middle of your viewfinder and allow the camera to focus on it. Once that is done push the shutter button halfway down to lock in the focus. Once the focus is locked you can recompose your picture to have your subject anywhere you want in the frame.

This AF mode is great for taking pictures of still objects.

Continuous

Continuous mode allows your camera to refocus continuously while you take pictures. The camera does this by trying to predict the movement of the object and focusing according to that prediction.

This AF mode is most useful when taking pictures of moving subjects because if you use the one-shot mode for these subjects by the time you take the picture the subject will have moved and will be out of focus.

Third Mode

Some cameras actually have a third autofocus mode. This mode allows the camera to choose whether to use one-shot or continuous AF modes. As with other auto-modes on your camera, if you use this mode you run the risk of your camera making the wrong decision and ending up with a badly focused picture.

Related Articles

What is Aperture?--Understand what aperture is and how to adjust it on your camera.

What is ISO?--Discover how adjusting your ISO can improve the quality of your pictures.

Using Manual Focus--Find out when to use manual focus. This page also contains tips that will help you get the best focus in manual.