By Ranga Burada
Autofocus plays a major role in many camera system applications with variable focus, including consumer electronic devices. Camera systems must be able to focus at a variety of distances. Optical systems on cameras only allow a certain range of distances from the camera to be in focus at once (this is often known as the depth of field, or depth of focus). The distance from the camera where objects will be most in focus, effectively the center of this range, is the focus distance—the role of the autofocus system in a camera is to set this point accurately every time.
We refer to autofocus consistency as the ability of a camera to focus on a given point correctly, repeatedly. To determine if a point is in focus, we measure the sharpness of an object (specifically, a test chart) at that distance. By taking many images of the chart—and letting the autofocus system reset each time and try to focus on the chart anew—we can tell if the camera system is focusing consistently or not. By examining the MTF50 values calculated from these images—a common objective image quality metric which correlates well with perceived sharpness—we can tell if sharpness varied between captures, and thus if focus accuracy on the chart varied. (more…)