Sigma 120-300 mm f/2.8 APO EX DG OS HSM
7. Coma, astigmatism and bokeh
The change appears after using the teleconverter. The coma is perhaps not very pronounced but it becomes noticeable and, what’s interesting, its shapes in the APS-C/DX sensor corner and in the full frame corner are very similar.
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The astigmatism, understood as an average difference between vertical and horizontal MTF50 values, amounts to 14% - it is a medium value. That aberration is the lowest at 120 mm and a bit higher at longer end being the main reason of weaker maximum performance of the lens. It becomes especially bothersome after attaching the converter and at the maximum relative aperture it can reach even 30%. Fortunately on stopping down the aperture the situation improves quickly – by f/5.6 you get an average value of 13% and by f/8.0 only 6%.
The crops below show a defocused image of a diode at 200 mm for different frame areas. We can practically repeat the same things we wrote in the chapter about spherical aberration. The light in the circles is very evenly spread, there are no sharp edges or noticeable concentric rims which makes us hope the images behind the focus will be smooth and pleasant to the eye.
