Sigma AF 500mm f/4.5 APO

Amazing This lens earns the Bigma name. I am surprised at how sharp it is at 500mm on a K-3. A prime would probably do a little better, but for birds it is definitely more than sharp enough. I cannot think of a more versatile lens for nature shooting. The range is totally amazing and it is very sharp at the widest settings and holds good edge to edge sharpness up to 200mm where the corners start to fall away. Even wide open this lens is very usable and you will need it as it is pretty slow optically. The stabilization is amazingly good and can give you sharp handheld shots. I only use a tripod with this lens for long exposures. Everything else is purely hand held. It is heavy but it is not had to hold it stable. I would highly recommend this lens to anyone that wants reach for birding or nature. It also has pretty amazing close focus capabilities considering.Read full review

Great lens, Big and heavy but manageable, Great zoom I bought this lens to use on my Pentax K1. Very fast focus and the OS works very good all the to 500mm I saved $400 buying this lens from Japan with shipping and no US warranty. I had another Sigma lens that stopped working and it only cost me $100 with shipping to get fixed since it was out of warranty so I think this was the way to go on this lens.

Impressive lens, heavy but with tremendous reach. Another option would be a 300mm f4 + TC. Fast focusing, much better for AFC than the Pentax 55-300. Quite sharp for this type of lens. Impressive stabilization. Heavy and quite big, not for anyone. Really interesting to use, an incredible piece of equipment. Works perfectly with Pentax K3II.