I do exactly the same thing sometimes, My light shade actually dissapears into the ceiling and other really weird stuff happnes, like you said you have control over it so it can be quite fun

As for damage to your eyes I'm not quite sure, your eyes are designed to stand the wear and tear of everyday life for 80-90+ years so I doubt a few twitches and movmements will turn you blind or anything! In my opinion it's all ok and the same thing kinda happens when your sitting up high aswell.

If your eyes ever start to hurt on a regular basis then maybe you could say it's down to cannabis, but aslong as they are healthy don't worrry about it!

Or if your actually seriousley paranoid, see an optician.