Pentax smc DA 21 mm f/3.2 AL Limited
7. 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.
