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Lens review

Pentax smc DA 21 mm f/3.2 AL Limited

15 September 2011
Arkadiusz Olech

7. Coma, astigmatism and bokeh

The coma in the tested lens is quite noticeable. At the maximum relative aperture, the image of the diode in the frame corner is heavily deformed. Fortunately on stopping down by one exposure value you can make the level of this aberration decrease significantly.

Pentax smc DA 21 mm f/3.2 AL Limited - Coma, astigmatism and bokeh

The astigmatism is a serious problem of the tested lens. Its value, understood as a mean difference between vertical and horizontal MTF50 function values and averaged in the distance of 2 EV near the maximum relative aperture gets to a high value of 20.1%. This number in itself doesn’t reflect what really happens at the maximum aperture, where the lens broke perhaps a disreputable record for all our tests. Whereas the MTF50 value, determined for the vertical border, can reach a sensational level of 51 lpmm the value of the horizontal border is only 31 lpmm so definitely below the decency level! A huge difference! Fortunately, stopping down helps fighting this aberration a lot.


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For those who like assessing bokeh looking at blurry pictures of the diode, we present some appropriate crops below.

Pentax smc DA 21 mm f/3.2 AL Limited - Coma, astigmatism and bokeh