I'm an atypical case, as I entered med school to do ophthalmology and retina, in particular. Most schools don't give you good exposure to ophthalmology, so the onus is on you to investigate the field. If you haven't already, I would contact some local docs and try to set up some shadowing, so you can get an idea of what a typical day is like. Don't just wait for the rotation. I started shadowing my first year just to confirm my interest.