I used two steppers to turn the aperture and the focal length rings of the lens independently. Then, you can set the aperture ring to 1.4, 2.8, 5.6, 8, or 16 and then focus the image with the other ring/belt. Both rings or belts move independently. In the beginning, I used a calibration routine to identify the maximal or minimal position of the rings. Both limits are detected by moving the rings in both directions and measuring the stepper currents. When the stepper reaches a limit, a current peak is measured, and that gives me the minimal o maximal angle.