Canon EF 300 mm f/4L IS USM
7. Coma and astigmatism
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Such crops, as those shown below, we would like to see more often. They prove that the coma is well corrected, also after connecting to the converter.
When it comes to astigmatism the situation is a bit different, though. As long as we don’t use the converter, the performance is ideal because the average difference between vertical and horizontal MTF50 values amounts to only 3.5%. Unfortunately after connecting to the converter the astigmatism level increases rapidly and reaches as much as 16%. Such a performance is often observed and typical for situations when we have to join two optical systems – such an operation always can result in minimal slacks or deviations of optics elements.
