Involuntary Eye Movement

Your eyes move on their own, giving you double vision, shaky vision, crossed eyes, amblyopia, and involuntary eye movements. You have to use a bonus action to focus your eyes (DC 5 + (IE) Constitution check) before making any action requiring visual focus. Once you succeed, you can retain this focus until either the situation changes (e.g., an encounter begins or ends) or you take damage.

Real-world Examples

Strabismus, Nystagmus, Diplopia, Amblyopia

Assistive Options

An eye patch can correct for all but shaky vision but gives disadvantage on all Wisdom (Perception) checks involving seeing on the patched side.