Brightness Control
You can set the amount of brightness you want by turning the aperture adjustment ring when using your lens.
[Aperture adjustment ring markings]
When you turn the aperture adjustment ring between 22, 16, 11, 8, 5.6, 4, 2.8, 2 and 1.4 you are making 1 stop each turn.
5.6 and 8 are 1 stop apart (1EV).