8 out of 10 points and recommendedvalue priced, solid, well designed lensNot the best choice for wide field astro work
I purchased this lens as a midrange widefield astro-photography accessory to my Canon 40D. At lower zoom, it works well. At full 500 mm zoom, there are minor chromatic and coma, which I don't get with less expensive fixed lens like the now eol'd Orion 80ED 500mm F/6.25, which has similar glass quality and "APO" rating as this lens. The lens is still useful for selecting and framing a shot before activating a fixed-zoom much-hard-to-focus telescope setup.reviewed August 29th, 2008 (purchased for $975)
For the much cheaper manufacturing of the 80ED that originally turned me off on that tube, I would have preferred that the Sigma had the better night performance.
Either way, I still love using the Sigma for what it was designed for, daytime nature and event photography. It is my first real telephoto, and I am happy I picked that one for it's range and other features.
I would have liked to know if the Canon equivalent lens was any better in this astro application, but the price pushes it over competitive high-end designed-for-astro tubes.
Overall though this Sigma lens is solid, very portable and quick to set up.