With an auto focus lens all you have to do is push a button to have the image automatically brought into focus. This can be done as the lens can calculate the distance of the subject in the image by comparing the contrast brightness of two images.
One of them is reflected from a fixed mirror and the other is taken from a movable mirror. When you achieve maximum contrast, an electronic device will convert the brightness data into electronic impulses.
These will activate a motor which will automatically move the lens until it reaches sharp focus.
However, these auto focus systems aren’t always reliable or effective, especially in low lighting conditions, when the image doesn’t contain much contrast, or it is all one color.
Some of the auto focus cameras actually use infrared or sonar systems. These operate when an infrared or sonar signal is emitted to calculate the distance of the subject. The distance is calculated by the amount of time it takes the signal to return from the subject to the camera’s sensor.
This data is then relayed to a motor which will automatically move the lens until it reaches the proper focus point. However, the sonar auto focus systems won’t allow you to focus properly through glass objects as the sonar will reflect off the glass rather than the subject.
You can get auto focus systems for most types of lenses and the majority of them also have a manual focus option.
Remember, you should never try to manually focus the lens when it’s in auto focus mode as you might damage the camera. Auto focus lenses typically range from about $50 to several thousand.
Photo Credit: Trypode