Canon EF-S 15-85 mm f/3.5-5.6 IS USM
7. Coma and astigmatism
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In the case of Canon 15-85 mm coma is the largest at 15 mm. It decreases marginally at 30-50 mm and vanishes almost entirely at the maximum focal length. Even at 15 mm the level of this aberration isn’t really large, making Canon look better than both Zeiss and Nikkor.
However, we have absolutely no reservations to the way Canon 15-85 mm corrects astigmatism. When measured on Canon 20D the average value between the horizontal and vertical MTF50 values reached 3.7% and in the case of 50D just a bit more, 4.7%. Both values are still quite small and to an extent identical, as the error of the average, in this case, amounted to around 0.8%.
