Ok, let's get nit picky. If you want a reference B&H says, generally the sharpest aperture is 2 to 3 stops down from max open. Sharpness for a good quality lens is dependent on lens diameter and wavelength and f-stop. At wide open resolution is dependent on lens quality, and lens diameter. But at wide open aperture lens aberrations are at their worst. The difference between design challenges by f1.2 and f1.8 is huge. Increasing the f stop reduces lens aberrations. Resolution increases with f-stop until diffraction errors become the dominant factor. For a good quality prime lens at say, 50mm focal length peak sharpness that is typically f4, f5.6 or f8. At f4 to f8 the MTF (Modulation Transfer Function) is relatively flat. But then drops off steeply. A cheap lens will perform as good as a top of the line lens at f16 and up. So if I spent the money for a good lens, I want to squeeze the performance out of it. I used to test all my lenses.