Sigma 8-16 mm f/4.5-5.6 DC HSM
7. Coma and astigmatism
When it comes to the astigmatism, we see the situation of the Sigma 1.4/30 repeated. The level of this aberration is so high that it requires in fact a separate screed and explanation how important it is to measure the resolution of a lens on vertical and horizontal borders of white and black, not only on vertical borders, as it is done on most of Internet websites and in editorial offices which test photographic equipment. Then in such tests a lens with huge astigmatism level is groundlessly acclaimed as sensation.
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The measurements based on RAW files and on both vertical and horizontal MTF50 values allow us to get a graph like the one presented below. It shows the resolution we can reach on vertical black and white borders (sagittal focus) and horizontal (tangential focus) – in this case at 8 mm focal length, where the astigmatism of the tested lens is the highest.
This aberration is really monstrous here. At the maximum aperture the vertical border generates a value of almost 52 lpmm, sensational for a zoom lens, whereas the value measured for the horizontal border in the same picture generates just less than 38 lpmm so a bit more than the decency level. On stopping down, in accordance with the laws of physics, the astigmatism decreases. It is very distinct by f/8.0, by f/11 it nears the margin of error level and it disappears completely on further stopping down.
The fact that the astigmatism decreases sharply with lengthening of focal lengths is very encouraging. As at 8 mm its official value amounts to 21%, at 12 mm it is already 12% and at 16 mm we reach a very decent level of 6%.
It is interesting that if the astigmatism was significantly lower, the lens would achieve at all apertures excellent results of about 50 lpmm and these would be had near the maximum relative aperture to boot. Then it would deserve the title of the sensation of the year, a lens blazing a trail of new standards.
