Lens review

Pentax smc DA 35 mm f/2.4 AL

5 April 2011
Arkadiusz Olech

7. Coma and astigmatism

Looking at the crops below you can notice that the coma is corrected in an ideal way. Even at the maximum relative aperture its influence on the circular image of a diode is negligible. It is a better result than in the case of the Nikkor 1.8/35, which, at the maximum relative aperture, showed distinct coma. On the other hand, though, on stopping down to f/2.4 this aberration became negligible too.

Pentax smc DA 35 mm f/2.4 AL - Coma and astigmatism

In order to have low astigmatism two conditions must be met. Firstly the aberration itself should be corrected well by the optical construction of a lens. Secondly, all optical elements of a lens should be put perfectly on optical axis and work well within. Any incorrect slopes make all off-axis aberrations “explode”, especially the astigmatism. In the case of the Pentax 2.4/35 both conditions were satisfied because the average difference between horizontal and vertical MTF50 function values amounted to just 5%.

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You must remember, though, that if many elements of a lens are made of plastics, it bodes ill for long and intensive usage of equipment. Such devices always remind me of a friend of mine who has bought a new Canon EF 50 mm f/1.8 II and for less than a year really enjoyed the quality of images his lens provided. After some time, though, he noticed a significant decrease of resolution which manifested itself as huge coma and astigmatism. He sent the lens to its producer for service and it returned untouched, only with a disarmingly honest annotation: “Backlash appropriate for this class of equipment”. Take this story into consideration if you ever decide to buy “plastic” lenses.