Just a quick testimony to old Pentax primes… This is a quick comparison of center and and edge… ( Click on the images to see them full sized. ) First here is the target:
Here are 100% crops:
Wide open there is a lot of color fringing at the edges and PF centrally which reduce sharpness. But this improves at f/5.0. By f/5.6 we find the A* 300′s sweet spot, both in terms of sharpness and CA.