Sigma 4.5 mm f/2.8 EX DC CIRCULAR FISHEYE HSM
7. Coma and astigmatism
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When you want to measure the value of coma, the hard part begins anew. If we wanted to do it in a traditional way by putting the diode in a limited distance from the lens we would meet problems - the plane of sharpness would move away reacting to the change of the position of the diode from the frame centre to the frame edge. In order to avoid that obstacle we must have placed the diode at infinity. The nature helps us here, providing almost ideal spot light sources, placed almost at infinity – nothing else than the stars. A simple photo of a night sky allows us to assess quickly the level of coma by selecting some bright stars in the field centre and at its edge. We are very satisfied to announce that the coma is controlled in the right way.
When it comes to the astigmatism correction the lens fares a bit worse because its level was assessed as 10%. Once again, though, the result might be influenced by the fact that the lens deforms our measurement areas so you can hardly consider it to be the Sigma’s fault.
