Olympus M.Zuiko Digital 17 mm f/1.8
7. Coma, astigmatism and bokeh
The astigmatism is a much bigger problem. That aberration exactly is responsible for lacklustre results in the resolution category. Usually the astigmatism stops being bothersome immediately after stopping down by about 2 EV. Here it is a different story because that aberration is visible even by f/5.6. An average difference between vertical and horizontal MTF50 function values amounted to as much as 21% which we consider a high value - nothing to boast about.
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When it comes to defocused images of a diode, once again we can observe something interesting - a feature registered for the first time in the case of the Olympus 2.8/60 Macro. Even at the maximum relative aperture the image is not perfectly round and you can notice the influence of aperture blades. Perhaps indeed it is caused by the fact that the real aperture of the lens is a tad faster than those declared f/1.8 so the diaphragm blades are visible in the field of view and even by f/1.8 they block some surplus light.
The circles are hardly perfect. There are some local extremes in the light spread and in the corner you can additionally see more pronounced edges. Fortunately these effects are not of big calibre.
