The Focus Target has 2 critical functions. The first is that the focus target provides the high contrast area that the camera's autofocus system will latch onto and lock. An improper target can cause the autofocus system to set the cameras focus on a different plane than that of the target itself. So the choice of target is critical.
1 - Allows the autofocus system to latch onto proper focus lock.
In order for the autofocus system to work properly, a well lit, high contrast target must be provided. In real life photography the target will vary with every shot you take. It might be the eye of the subject in a portrait, or the pedal of a flower in a still life. When testing an autofocus system, as we do when using LensAlign and FocusTune, a proper generic focus target must be provided so that the autofocus system is operating in a nominal mode. By this we mean that the focus target should not cause the autofocus system to make errors that are caused by the target itself.
2 - Allows FocusTune to determine the sharpness a test image.
When FocusTune analyzes a set of test images it uses the focus target graphics to determine the relative sharpness of each test image. By later comparing the results of this analysis, it can be determined which test images are the sharpest and which are the most blurred.