This is mostly true, but our eyes can also do "smooth pursuit"[0]. You can lock on to a moving object and follow it smoothly. I've trained myself to be able to

This is mostly true, but our eyes can also do "smooth pursuit"[0]. You can lock on to a moving object and follow it smoothly. I've trained myself to be able to do this with vertical lines, like window blinds. [0]: http://en.wikipedia.org/wiki/Smooth_pursuit http://en.wikipedia.org/wiki/Smooth_pursuit