As a baby emerges from the darkness of the womb, its rudimentary sense of vision, which would deemed legally blind for an adult, would be rated about 20/500.

这句话怎么理解啊？

A baby emerges from the darkness of the womb with a rudimentary sense of vision that would be rated about 20/500;an adult with such wision would be deemed leally blind.
一个刚从黑暗的子宫里出生的婴儿拥有一个发育不全（不知道这样解释对不对，反正可以理解）的视觉的几率大约是20/500， 一个拥有一样视觉的成年人肯定会被认为是瞎子。

呵呵，凑活着理解吧