Panasonic G 20 mm f/1.7 ASPH.
7. Coma and astigmatism
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In the case of fast standard devices, and the tested Panasonic should be considered as such, the coma correction in the frame corners is not easy. As you see in the crops below the Panasonic didn’t manage it well. At the maximum relative aperture the coma is very distinct indeed. The fact that on stopping down by 1 EV the aberration decreases significantly is encouraging though.
The astigmatism is well-corrected but not in a remarkably good way. The average difference between vertical and horizontal MTF50 function values amounts to 9%. What’s more, this aberration is quite pronounced by f/1.7 and f/2.0 and it disappears completely by f/2.8.
