Some things to consider when using the 50mm 1:1.2 or any other really fast prime (1.4/1.7 etc.)
Do not use it at 1:1.2, as a general rule of thumb, stop down the fast primes at least one stop.
Why ?
It will reduce the lens aberations significantly.
It will reduce light fallof (although this is generally less of a concern with aps-c sensor and a FF lens)
And in the case of the 1:1.2, it will prevent silly star shapes.
Here are 3 shots of the star Deneb, relatively close to the center of the frame with the K 50mm 1:1.2.
Because I tried to capture the NA nebula (without luck), Deneb is highly overexposed.
IST-D 100% crop
K10D 100% crop
K20D 100% crop
The root cause of these strange square/rectangular shapes are not really clear to me, but are probably caused by the combination of
the anti alias filters and specific pixel/micro lens designs.
The 'V' like distortions are most likely caused by the mirror box, which is too small
See
this link