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zooms and astro
Generally speaking, the more elements a lens has, the more internal reflections it will suffer. Zooms always have more elements, therefore more internal reflections. Optical performance will always be inferior to fixed focal length for the same speed lens.In all my days of testing lenses, I have NEVER seen a zoom that gives sharp images wide open-they need to be stopped down a good 2 stops to get good performance, except the ridiculously slow kit lenses which are way too slow for astro. Same applies to fixed focal lengths, although some of the more expensive fixed focal length lenses perform quite admirably wide open. Internal reflections are best reduced with a lens hood-it is most likely built in to an f/2.8 lens although it is protected just to the 80mm focal length. Make sure all dust has been blown off the front element.
Good luck
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