OFF= Iris closed, ON= Wide open (not so intuitive to me)
It is actually convenient for non-G Nikkors too, since you can focus at wide open,
and rotate OFF-ON ring to OFF position to close to set aperture when you shoot.
Nikkor lenses become preset aperture lenses, you can use old pre-AI lenses too.
was $19 :), but shipping $10 :(