my observation comes from comparing multiple copies of the K an A series 50mm f/1.2 lenses I picked the one that showed the greatest sharpness...a I returned the rest to my collection. I test lenses on the Pentax LX with ilford PAN-F 50 and Kodak Tech-pan both exposed at Iso 25 - under a meticulous development regime and I compared the results under a high magnification precision schneider loupe.
here's a sample image at f/1.2, people often moan that the Pentax 50mm f/1.2 lenses aren't contrasty enough wide open. the Leica Noctilux f/1.0 isn't particularly contrasty lens wide open either niether, is the Voigtlander nokton 35mm f/1.2. that's the price you pay for extremities of lens design, but consider what the f/1.2 lens is for...low light, making images of usually high contrast scenes. You don't want to use a lens that has tons of contrast for scenes that are already too contrasy?
and a 100% crop - unsharpened.
these were shot hand held, 1/8 second @ISO1000 f/1.2