improve eyesight for what? seeing spirits or just better vision? i've heard carrots help, but that won't give you perfect vision, just turn you orange. i don't really know of many that help you see spirits better. i came across a chant one day that i use a lot, but doesn't do much, i normally saying while meditating and i can here something in the room, though it never helps me see them, just focus in on where they are. another one i've never used helps for a couple hours. other than that, i've never come across one, and i wouldn't be sure how to make one.
if you do mean poor vision, i know of a couple vitamins and foods that help ones vision, but no 100% fixes.