Pentax smc DA 21 mm f/3.2 AL Limited review - Image resolution

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Pentax smc DA 21 mm f/3.2 AL Limited

15 September 2011

Arkadiusz Olech

4. Image resolution

The resolution test of the Pentax smc DA 21 mm f/3.2 AL Limited was based on RAW files from the Pentax K-5 reflex camera. In its case the best “primes” can reach results on the level of 52-53 lpmm and the decency level is situated near 34-35 lpmm. Let’s check how the tested lens compares here. A graph below presents its achievements in the frame centre and on the edge of the APS-C/DX sensor.

Pentax smc DA 21 mm f/3.2 AL Limited - Image resolution

As you see the frame centre performance is beyond reproach. Already at the maximum relative aperture the lens exceeds 40 lpmm so it is able to generate good quality pictures. You can have reservations about high astigmatism (but we will elaborate on it in the appropriate chapter) so, depending what is in the frame, the images might appear more or less fuzzy. Fortunately the astigmatism decreases very quickly on stopping down and, as a result, the MTF50 values very quickly increase. By f/4.0 the lens gets a very good value near 46 lpmm and by f/5.6 it reaches the peak of its possibilities with a result of a bit less than 51 lpmm.

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When it comes to the edge of the frame there are no sensation there but, after all, nobody should expect it because “pancake” constructions are supposed to be small by definition and it forces some compromises, most keenly felt on the edges of the frame. Near the maximum relative aperture the image on the very edge can’t be called fully useful but, fortunately, on stopping down by about 1 EV the problems in this category disappear completely. Taking into account the dimensions of the lens we shouldn’t complain almost at all.