Canon EF 70-200 mm f/2.8L USM
7. Coma and astigmatism
The astigmatism is a different matter, though. Its average value is not high, amounting to just 4.5 %, so overall the lens fares well but when we start looking at different focal lengths more closely it turns out that the good result is spoiled by the 200 mm. In the 70-135 mm range the astigmatism is just 2.7% but it increases to 8.4% at 200 mm focal length.
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A converter is an additional optical element mounted mechanically between a camera and a lens. In our set it is an additional source of potential slacks and inclination of elements toward the optical axis. Speaking from experience we know that it almost always influences the image quality and it is the most felt in the astigmatism category. The Canon EF 70-200 mm f/2.8 USM connected with the Canon 1.4x converter is not an exception to that rule -the astigmatism increases to as much as 19% and it becomes one of the most serious reasons of the resolution decrease.
