I have this. Always have. I can make it happen at will and it also happens automatically when I try to focus on somthing narrow (such as the end of a ruler) placed 10-15 inches in front of the bridge of my nose.
I am told that this condition, in the absence of other, related symptoms, is benign.
It has occasionally been useful, such as a time in High School in which the P.E. Coach hit a volleyball with a baseball bat and it hit me right in the head, knocking me over. It didn't hurt that bad, but when he bent over me, my eyes focused on his whistle and started shaking. He practically had a heart attack, thinking he'd damaged my brain or something. Priceless.