Most digital cameras have either a passive auto-focus or an active auto-focus function. Some cameras offer a combination of both.
Passive auto focus determines the exact focus much the same as you would manually. It typically focuses on and around the area at the center of the image. The camera then directs the lens to turn slightly in both directions to find the best focus point. During manual focus, you do the same: determine a preliminary choice of focus, and then turn the lens to confirm your choice or to find another.
The active auto-focus function bounces an invisible beam of light off the primary object or focal plane of the image you’ve composed in the camera. By measuring the precise distance between the camera and the object, the lens is automatically set for the correct focus.
The combination passive/active auto-focus function is typically found on more expensive DSLR cameras. The technology selects whichever function (passive or active) will provide the best focus, based on the conditions of the scene. They can also work together, with the active auto-focus selecting what appears to be the correct focus and then the passive auto-focus checking that selection.
System Crash Challenges
Now that you understand how the auto-focus functions work, it’s also important to learn what types of conditions limit their capabilities.
The first of these is when the light is low and the scene is dark. The passive auto-focus system “crashes” because there isn’t enough light for the camera to see the primary object and move the focus of the lens to the correct position. The passive auto-focus feature on some digital cameras will trigger the flash unit to bounce light off the scene, so the camera can see and focus. This is not a perfect solution, however, since the primary object must be in the focal plane. Those multiple flashes may also distract some subjects in the digital photos you’re trying to take, a small child or a pet, for example.
Although a digital camera with an active auto-focus feature is one solution, since it doesn’t need available light to focus, it can crash too. Various objects in the scene can absorb the invisible, infrared light that the camera emits to determine the distance to the desired focus point. There may also be other light or heat sources in the scene, such as candles, that release infrared light. In either case, the active system can become confused and fail to choose the correct focus point.
The second challenge for digital camera’s auto-focus functions is objects with low contrast. This means the surface is lit evenly or is of one color, such as a white fence or a tropical blue sky. There is no contrast, or light and dark areas, which passive auto-focus systems must analyze to work properly. Without contrast, passive auto-focus is unable to find the best focus setting. Here is where a digital camera with active auto-focus is very helpful. It will, of course, determine the exact distance to the subject and focus point with its infrared technology. Active auto-focus may also find an object with high contrast in the same focal plane as the primary object. That focus point will remain locked as you slowly move the camera to center the low-contrast primary object in the frame before releasing the shutter.
Improving your digital photography skills also includes understanding your camera’s focusing limitations because the perfect picture you thought you shot may not be perfectly in focus.