Why reduce f stop will sharper

not all lens sharp at wide open

簡單的講同「球面像差」有關：
Spherical aberration
http://en.wikipedia.org/wiki/Spherical_aberration

先理解一下「球面像差」再想想收細光圈就是把"焦平面"孔徑縮少,使偏差了的光路阻擋從而減少了光暈,成像就會更為清晰!
http://article.xitek.com/showarticle.php?id=2346

光圈收得太細又會出現 繞射/衍射 Diffraction 影響成像!
http://hk.knowledge.yahoo.com/question/?qid=7006071102340&others=1
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