[Brightcolours]

Sorry, confused numbers. However, it's just the same, and it doesn't matter if this lens is mounted on Nikon, Pentax, Sony, Canon, whatever - 2 stops down on most lenses gives the best resolution. If you doubt that, you can check all PZ tests to show me something else. 

And that is at MTF testing distances. At closer distances (read: higher magnification), you will find that diffraction softening will worsen, so you can get at a point where closing down 2 stops from wide open will get you softer results.