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I have had the 70 - 200L IS F4 the 300L IS F4 and now the 100 - 400 , I used the 70 - 200 L for astro briefly and the 300 quite a lot the 300 was very good for astro at F5 , I have never used the 100 - 400 for astro and probably never will I doubt it has the quality and its also slow even wide open.
The 100-400 is very handy for birds and othe photography having IS and closer focus than the 400 5.6 L but the 400 F5.6 is said to be sharper.
So I'm keeping the 100-400 and may buy a Sigma 150 F2.8 OS macro for astro and macro stuff , the test reports show the sigma to be excellent wide open .
Its hard to find a zoom that really performs well for astro so maybe you need 2 lenses a long zoom for regular ( the 70 - 300 L sounds good ) and a fast prime ie the Sigma 150mm or the canon 200 2.8 L .
Zane
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