Every lens is different, if you wanna be really anal about it you can look up specific MTF charts online that show the optimal F stop sharpness of a lens. But generally the rule of thumb is...Whatever the open F stop of a lens is add 2 to it and that should be around optimal.
obviously this rule doesn't work for everything. macro for instance needs a very high F stop to get a clear photo since you're focusing so close. But that's the rule I follow for most lenses and it generally works.
For example... My 1.8 prime would be optimal around 4.. etc
|