Unassisted vision (5,3)
I believe the answer is:
naked eye
'unassisted vision' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for naked eye that I've seen before include "Unenhanced vision" , "Unassisted vision" , "Unassisted sight" , "With which to observe" , "Unaided vision" .)