Sigma 600mm f/8.0 for Nikon AI and AF

Pros: Impressive build quality-600mm reach (most others are 500mm)
Cons: Circle shaped doughnut highlights- typical of all mirror lenses present.

The Sigma 600mm is a unique lens in a unique lens category. There are no other fixed mount (camera specific) 600mm mirror lenses that I am aware of. If you understand the inherant optical traits of mirror lenses they can be a great "nature shooter" lens...

Pros: Good quality if you have plenty of light.
Cons: Heavy. Manual focus

Don't want to spend thousands of dollars on a high power telephoto lens that you might use once in a while. Sigma produces high quality lens that you can afford. The 600mm mirrored lens might be what you are looking for. If you are looking for the same quality as what you view on television or in the movies, then it is time to get a large loan from the bank.

For that once in a while, when you need to get that close up shot, this lens will make you happy, without making you poor.