When I press the red button on my letus extreme I really don't notice a difference in the look whether it's on or off, should there be a noticeable difference whether it's on or off?
If you are wide open. F 1.8 perhaps even 2.8 you will not always notice the GG. Stop down to F 5.6 and test it on and off. You should see the grain disappear when the motor is running.