眼睛看得清楚
這是字面的用法，數字直接來自視力檢查。
noun · B2
Normal, clear eyesight: the ability to see things at a distance as clearly as most people can, with no glasses or contact lenses.
標準視力、不用戴眼鏡
核心畫面
Think of an eye chart twenty feet away, and a person reading the small line without trouble. That is the picture the two numbers carry: sight that is completely clear. English then takes that clearness and points it in a second direction, backwards in time.
可以想成二十英尺外的一張視力表，有個人輕鬆讀出小字那一行。這兩個數字帶著的就是這個畫面——看得完全清楚。英文後來把這…更多收起
可以想成二十英尺外的一張視力表，有個人輕鬆讀出小字那一行。這兩個數字帶著的就是這個畫面——看得完全清楚。英文後來把這個「清楚」轉了一個方向：不是往前看得清楚，而是往回看得清楚。From the word’s history
From a method of testing vision by reading charts at a distance of 20 feet.
這個說法來自視力檢查的方法：站在二十英尺外讀視力表。讀得和一般眼睛一樣清楚，就記成 20/20，所以兩個二十一個是距離，一個是比較的標準。
Wiktionary entry for twenty-twenty (kaikki:3f6f69a190036ae4:786810027), etymology_text supplied in the evidence packet
Two branches
回頭看才清楚
把「清楚」從眼睛移到時間上：事情過了以後，一切看起來都很清楚。
noun · B2
Used to say that it is easy to see what the right choice was after something has already happened, even though nobody could see it clearly at the time.
事後諸葛、馬後炮