Despite its ubiquitousness, autofocus has only been around in mainstream consumer photography for around 25 years. Before then, it was the preserve of top-end pro cameras, and before that, we all focused our cameras manually. These days, autofocus is just an underlying technology that works silently with all the other functions on our camera to aid us in creating superb images. However, like all underlying technology, it is not infallible. Even these days it can get fooled or simply not focus on what you want to focus on. Understanding how autofocus works will help you overcome these problems and increase the number of tack-sharp images that you take.
Autofocus is not infallible.
Image via Flickr
Before we go into the various modes of autofocus, let’s have a simple, layman’s look at how it works on most modern cameras. Typically, autofocus is calculated using a sensor near the main camera sensor. It works by looking at the contrast in a scene. For example, if you have a black line on a white background, and it is out of focus in the camera, there will be some grey contrast between the pure white and the pure black. The camera will focus the lens until that contrast is reduced to its minimum. This is the point of sharpest focus.
Most modern DLSRs have two distinct autofocus mode types: area modes and shooting modes. By understanding the differences between the two and the various selections within these modes we can start to master the art of autofocus.
When you look through your viewfinder, you will probably see a series of small translucent boxes arranged in a pattern in the viewfinder screen. Each of these boxes represents a focus point, which is an area where there is an autofocus sensor. Some cameras may only have 3-5 of these points. Others can have up to 50 of them.
Your camera will have a number of modes that define how you use these autofocus points. The default mode is very often what is known as multi-point dynamic mode. In this mode, the camera uses a number of focus points across the screen based on what it believes the scene to be. It then analyses that scene, and attempts to identify the required focus point. For general subjects this is usually very accurate. This mode also has the ability to track moving subjects across the scene. But if you are using focus as part of your composition; for example, when using a shallow depth of field, multi-point autofocus mode can very often get it wrong.
A typical multi area focus mode
Image via Flickr
To counter this, the best mode to use is single-point autofocus. In this mode, you select one of the points within your viewfinder, and only this point will focus. For example, if you are shooting portraits in portrait format, you might select a focus point in the upper part of the viewfinder, allowing you to move that point quickly to the subject’s eyes. Because it allows you to define a single focus point, it is perhaps the best mode for an experienced photographer, as you can quickly change focus point positions to suit your scenes.
One other focus mode is area mode. This is in some ways similar to multi-point dynamic, except that it utilizes information from all the focus points within the camera and makes a judgment on where it thinks focus should be. This is fine for snapshooting, but may be restrictive for more creative shooting.
These determine how your camera’s autofocus reacts when you are pressing the shutter. The default mode is usually single-shot servo mode. Here, when you press the shutter, the camera will obtain focus and fire a single image only once focus is achieved. This is the ideal mode for general photography such as landscapes and portraiture, as all shots will be in focus.
Next we have continuous servo mode. This is the mode to use for moving subjects, because when you half-press the shutter, it will focus on the subject. When the subject is moving, it will continue to track its position, ensuring a high chance of good focus when the shutter is finally pressed. When combined with your camera’s high-speed shooting modes, you can shoot sequences of images; for example, a bird in flight, with a high rate of successful focus.
This kind of shot is ideally suited to continuous servo mode
Image via Flickr
Much-underused manual focus.
As we said at the top, autofocus works by detecting contrast. This means that autofocus can sometimes have issues in low-contrast scenes such as a misty day, or in low light. For these times it’s worth learning the art of manual focus, a technique that can really improve your photography. Most modern cameras will use their autofocus systems to aid manual focus, showing focus points and even telling you which way to turn the lens ring to obtain focus.
Understanding the autofocus on your camera and its many modes of operation will open the door to more creative photography. By adapting your autofocus to the scene in front of you, you will find yourself getting a much higher percentage of in-focus shots.