Pentax smc FA 50 mm f/1.4
7. Coma and astigmatism
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At the maximum relative aperture the axis aberrations are set loose as you can notice by looking at the shape of the diode both in the frame centre and at the edge of the frame. On stopping down by one stop the situation in the centre improves a lot but at the edge the coma still deforms distinctly the circular diode image.
The astigmatism is on the medium and low borderline, for a change. Its value, understood as an average difference between vertical and horizontal MTF50 function values, amounts to 7%.
