about the "sweet spot

[YongLee]

i have heard about the "sweet spot" of camera lens when using the iris. who can tell me what is the "sweet spot"? is it right about "the sweet spot is the biggest iris reduce three ”？thanks!

[Stuart Brereton]

All lenses perform better when stopped down at least 2 stops from wide open. The sweet spot would normally be considered to be f5.6

[YongLee]

thank you