So I recently decided to purchase my first high-end camera/lens since my old Nikon F4 in the film days. I am now using a Nikon D810 and a Sigma 50mm f/1.4 Art lens.
According to DPReview/DXOMark, the Sigma lens is most sharp at f/2.8 (specifically at the center of the frame); however, I am finding the lens sharper at f/4 - f/11? I've taken various photos to compare, but I suppose my most reliable test would be when I placed my camera on a flat and hard table (to keep it perfectly still) and focused on a hardcover book about 1.5 meters away. I used the 10s timer to prevent camera shake from my pressing the shutter release. Higher f-stop photos were clearly sharper and with less magenta fringing.
Is someone able to shine some light on these findings for me?