Sorry, this isn't exactly astronomy related, but it is optics related.
Scintillation. AKA mirage.
Rifle shooters adjust their magnified optics slightly out of focus with the target in order to observe mirage. This allows them to read the wind, both speed and direction.
Tonight I was observing the city lights about six miles away with an APO refractor. Very clear and cool night, very still air. At least at my location. Mirage was unreal tonight; half the city looked like it was sitting on a lake. Lots of reflected light and mirage. Beautiful. .
I observed a strange effect that I cannot explain. When I moved out of focus one direction, the scintillation effect was very clearly moving left-to-right. Moving out of focus the other direction, it was moving right-to-left.
I have used spotting scopes and rifle scopes alike to focus on mirage and read the wind. I have never seen it change direction based on which direction I was moving the focuser.
Can someone explain this effect to a noob?