[Brightcolours]

When the light gets low, your ability to detect contrast is worse because of the noisy results from the sensor. Is there more contrast, or just a too light pixel (due to noise) you are looking at? And the following frame things can look different again.

I do not know if and (if so) why the 56mm f1.2 APD would be worse with AF in lower light than for instance a n f2.8 lens.