sigma has created one of most insane lenses i've ever seen. the specs are pretty astronomical. this lens is not for someone like you or me. in fact, this piece of glass probably needs to come along with a dolly. they might as well put some wheels on it.
so why is it so unique? well for starters, it's very fast. there are not too many lenses that can shoot at F2.8 at 500mm. F2.8 means it's going to be able to shoot in very dark settings. For example, when you shoot a picture of bird flying in a low-light setting, you'll get blur. with a F2.8 lens, you'll freeze the action, resulting in sharper photos with little to no motion blur. the other advantage of having such a fast lens is that there will be less distortion, and the colors will usually be better.
on top of it being fast and super-telephoto, it's a zoom lens. how it operates exactly is still a mystery to me. which one is the zoom ring, and which on is the focus? technically, it is extremely difficult to create a lens that can zoom in the range, let along a range this extreme. this photo, to the left, is actually a smaller lens than sigma.
yes folks... the sigma will probably be 3 times the size of these two lenses.
these are 500mm F4 lenses. These are typically used for sports photography as well as portraits for super models.
if you use lens extender (something that magnifies the image so you can see further), you can actually use these lenses as telescopes. so yes, this is an incredible piece of modern technology. it's something i'll never buy... but it's entertaining to think about how they engineered such a lens. anyway, this is not practical solution for anyone.
what kind of camera do you have? what's your dream glass? read more: http://www.dpreview.com/news/0801/08013101sigma250500.asp