Canon EF 90-300 mm f/4.5-5.6 USM
7. Coma and astigmatism
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The coma is easily noticeable at 300 mm focal length, which results in a worsening of the picture quality at the edges of the frame. The level of the coma at the 90-200 mm focal length range is smaller, but it is still visible.
Generally speaking, lenses with plastic elements in their body are more vulnerable to extreme astigmatism. In such cases, even at the beginning of the lens’s use, the slight looseness sets the optical elements in an incorrect position relative to the optical axis. This situation causes serious off-axis defects. In the case of the Canon EF 90-300 mm USM the astigmatism reaches 10% at the beginning of the lens’s performance. We consider this an average to somewhat large level. As time passes, the situation will get worse.
