Since the 1970’s, there have been attempts to develop cameras with autofocus capability. Besides camera shake, using the wrong focus is a main cause of a photo appearing blurry. Since about 1985, automatic SLR cameras have played an important role in the world of photography.
Currently, the speed and precision of the autofocus is so technically advanced that it comes very close to being able to do the same things as a manual focus. The highest quality digital cameras available have up to 51 image sensors that are responsible for correctly capturing the object being focused on. For cameras with a touch screen, the photographer can use their fingers to select the area they want to focus on, similar to the way focusing works on a smartphone.
Basically, there are three kinds of autofocus modes:
AF.A (Autofocus Automatic): the camera alone is responsible for deciding if the image is already focused or if the focusing needs some adjustments, depending on how the targeted object appears when the shutter release is pressed.
AF.S (Autofocus Single): even if the object being photographed moves between the time the shutter release is pressed and the actual photo is taken, the camera uses the focus settings set when the shutter release was pressed. Thus, if the object moves further away from you during this time, it can cause your photo to appear blurry.
AF.C (Autofocus Continuous): the camera justifies the focus continuously until the actual image is recorded. This is dependent on the relative movement of the object to the lens, i.e. if the object is moving closer to or farther away from the photographer.
There are different kinds of autofocus systems. Roughly speaking, they’re divided into either passive or active autofocuses. The passive autofocus tends to be the most popular. Here, the camera basically finds the optimal focus after a “series of tests.” The two most reliable methods are called contrast detection and phase detection. The camera’s sensor is instructed to find the optimal object contrast and adequate lighting.
On the other hand, you have the option of using an active autofocus. This focus even works in the dark. One method uses ultrasound to measure the distance directly; here, the time it takes for the ultrasound to reach the object and return is measured. Thereafter, the camera can calculate the optimal focus to use. This method is used in Polaroid cameras. Passive autofocusing is supported in poor lighting conditions when the object is illuminated. When this is the case, the object is illuminated by auxiliary light from the camera and can therefore be better focused.
The use of autofocus is very simple. The photographer can point the camera at something and press the shutter release down halfway. The camera will then try to find the correct focus. If the camera is able to find the correct focus for the photo, the photographer can press the shutter release down completely and take the photo.