Summary: I bought this zoom for its long reach. I couldn't afford the similar canon zooms so I purchase this one. I am using this zoom with the canon 7d for bird photography. Canon 7d is very fast which is perfect for birding.
Summary: pro's 1) You don't need to constantly change lenses. 2) images are tack sharp 3) image stabilization works well con,s 1) autofocus goes hunting at critical times 2) lens is heavy 3) some vignetting at 500. 4) somewhat stiff when going from 50 mm up.
Excerpt: This is not a lens that can be easily handheld or carried. It is a lens to be taken on excursions devoted to photographing subjects that are appropriate for the long reach of the lens. It is too heavy and bulky to carry in the field on extended hikes just in case you might find a use for it.
Excerpt: If your thing is wildlife and you want to lug only one lens you may consider the Bigma as it's versatility can not be beat. At 200mm its useful for those quick shots of dragonflies and butterflies.
Summary: The Sigma 50-500 F/4.5-6.3 APO DG HSM SLD Ultra is a reasonable compromise between shorter and longer lenses. I use it on both a Canon T4i (using the additional lens hood adapter for the APS-C camera) and a Canon 5DIII using the standard Sigma full-frame lens hood.
Excerpt: This is an affordable long lens which takes decent quality images. It is sharp up to about 400mm, gets soft after that point. The lens, however, is not constructed very well. The auto-focus is clunky, the optical stablization is erratic, and the contact points with the camera body failed for me...
Summary: After reading reviews here and elsewhere, I took the plunge on this lens because I was looking for something a bit longer than Canon's 100mm-400mm lens (which is getting long in the tooth and overdue for a new version).
Excerpt: I bought this specifically for a vacation trip to Alaska, and it stopped auto-focusing two days into the trip. I've just sent it off for repair, so I can evaluate their service too! Even with manual focus the wildlife was brought in sharp and clear.