Pentax HD DA 20-40 mm f/2.8-4.0 ED Limited DC WR review - Coma, astigmatism and bokeh

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Pentax HD DA 20-40 mm f/2.8-4.0 ED Limited DC WR

8 February 2014

Arkadiusz Olech

7. Coma, astigmatism and bokeh

It would be difficult not to have any reservations here – the coma is visible at every focal length. Perhaps its level is not especially high or bothersome but, with such parameters and at such price point, a better performance should be expected.

Pentax HD DA 20-40 mm f/2.8-4.0 ED Limited DC WR - Coma, astigmatism and bokeh

In the case of astigmatism the situation looks a bit better. The average difference between horizontal and vertical MTF50 function values amounted to 6% which is low.

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The appearance of defocused light points is quite interesting. The light doesn’t spread evenly there, with the inner part of the circle featuring a distinct tufty structure. The intensity of the outer rim increases with the stopping down. Vignetting seems to be another serious problem – even after stopping down to f/8.0 the circle in the corner of the frame remains truncated.