I've noticed for a while that when I'm really concentrating (not focusing on study or playing music but when my eyes have to concentrate e.g. threading a needle/beadwork/small writing etc.) that my vision blurs. It's almost like my eyes are vibrating but it only lasts for a second. It doesn't always happen and I can't do it at will.
I've heard that some people can vibrate their eyes at will but I looked up what I'm experiencing and couldn't find anything.

Does anyone else experience this, or is it just me?
Do you know what it's called or what causes it?