Lenses are optimised for what aperture they are optimised. This might
be even wide open.
In theory, lens is sharpest wide open, and then looses sharpness as it
is stopped down.
In real world, that is rarely achievable, perhaps with the computer
chip engraving lenses (and usually only for single wavelength of
light, using laser). That's because of lens aberrations. If they were
perfectly neutralised the lens would be sharpest wide open. But
they usually can't be neutralised perfectly for all conditions. Some
of the aberrations improve by stopping down. But stopping down
increases difraction which limits resolution. Stopping down also may
introduce other defects apart from difraction - flare, and focus
shift. Especially focus shift can cause problems with lenses that are
focused wide open and stopped down for shooting.
Thus for some conditions, you can indeed make the lens sharpest wide
open.
Hope this helps.
Good light!
fra
