Pentax smc DA 17-70 mm f/4.0 AL [IF] SDM
7. Coma and astigmatism
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Coma is not an exceptionally big problem for Pentax 17-70 mm. It’s influence is seen for small focal lengths, what can be observed in the frame corner pictures at 17 and 30 mm. At 50 mm the situation is ameliorating, at 70 mm it’s even better. It was a similar case with Sigma 17-70 mm – there coma was also more visible in the 17-30 mm range than in 50-70 mm.
![Pentax smc DA 17-70 mm f/4.0 AL [IF] SDM - Coma and astigmatism](upload3/2622_pen17-70_koma.jpg "Pentax smc DA 17-70 mm f/4.0 AL [IF] SDM - Coma and astigmatism")
Astigmatism reaches medium level; the average difference between vertical and horizontal MTF20 function values was 9.6%. It’s a tad more than in the case of Sigma, where the astigmatism level was 7.3%.
