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I use the 17-40L and 70-200L a lot for astro work, they're my two main lenses in general.
I find that both are fairly good but I still do get distortions on the edge corners, especially if slightly out of focus. I do find that both are very susceptible to being slightly out of focus - that little bit, and I see significantly more elongated stars.
I have wanted to get something in the 20 - 50mm range, fixed focal length, large aperture, and compare them. I would be hoping for something fixed focal length being less susceptible to distortions. I feel I used to more easily get sharp flat images from my old pentax 50mm.
Both the 17-40 and 70-200 are very smooth focusers, very well built, relatively light lenses. There is little comparison between them and cheaper non-L canon lenses.
Roger.
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