ASIAIR uses a pared down version of PHD2. While very simular, it is not fare to compare the two.
Autofocus looks at star size. It takes a shot, calculates the star size, then moves the focuser the number of steps you have set in the autofucus settings screen. It then takes another image and recalculates the same star size. it will do this in a move more steps in the same direction, or move in the opposite diesction unti the smallest star size can be found.
Optics, camera and exposure time will all have an affect on the performance.
