Auto Focus in Digital Photography

Auto Focus (AF) is a technology which automatically brings the intended area into focus, there is passive autofocus and active autofocus. In this article, we will discuss about the basics how auto focus works and its usages.

Types and Technology of  Auto Focus

Auto focus in this method is done by measuring the image contrast and is basically the same as eye. Since the camera in contrast to a human has no idea how far the subject is, from contrast measurement it is not possible to do the auto focus. At least two measurements are needed for proper auto focus. Due to the computational complexity in practice usually only the relative method of auto focus is mainly used.

The processor of the camera computes the frequency distribution in the image. Greater the proportion of high frequencies, sharper the image is. The term Relative process in case of Auto Focus means, the need of multiple shots with different focus to identify and do improvement in the sharpness of the necessary focus. The disadvantages of this method of Auto Focus are the required energy for computational effort and driving the motor, which exerts a negative effect on the battery life.

The older method of Auto Focus is the passive phase comparison. This method of Auto Focus is complex and requires a special sensor, but in principle it requires no great computing power and the focus direction can be determined with the first measurement. This method of Auto Focus is derived from the purely visual image rangefinder.

General Factors and Special extensions of Auto Focus

In the case of Auto Focus, the camera periodically checks whether an object is located in the focal plane, once an object is within the focal length, the camera will shoot the photo. This is known as Focus event. Some cameras can recognize the speed of a moving object and can adjust automatically to make the image sharper, this is known as predictive auto focus.