Where the lower ranks are harder to read? (3,5)

I believe the answer is:

eye chart

I'm a little stuck... Click here to teach me more about this clue!

If this is the answer I cannot explain how the clue works.

(Another definition for eye chart that I've seen is " reading test".)

I've seen this clue in The Times and The Sunday Times.

Want a hint initially instead of a full solution? Install my app