I own this Sigma 100-300 f/4 EX IF HSM APO for a while now. But before I did by one, I was asking myself: "What lens to buy for nature "all around" photography?" I owned Nikkor 180mm f/2,8. It is an exceptional lens, but 180mm it's a little bit short. There are few options, but what to choose? I didn't want to go on 
"consumer tele " path, since on 300mm I want to have f/4. So all those 70-300 were out. Yes, f/2,8 is better option, but we are talking budget here! At the end, there were only 2 lenses left. Nikkor 300mm f/4 and Sigma 100-300mm f/4. Original or third party lens? Prime or zoom? I was comparing photos and specifications for both those lenses on the net. I called some people, checked prices and finally I decided. Sigma 100-300mm f/4. Or Sigma 100-300mm F4 APO EX IF HSM . This is how it's called officially. It's very well build (EX), focus and zoom rings run smoothly, front element does not rotate (IF), focusing is fast, silent and you can override AF manually whenever you want (HSM). At last but not least, length of this lens does not change. Nice, ha? Optically it's very good, it's sharp at f/4 from 100-270mm. Between 270-300mm sharpness and contrast get a little bit worse, but usable without any hesitation. It come with a removable mount tripod collar, nice padded bag for carrying around and huge lens hood. Size of front glass? 82mm! It's large! Polarizing filter cost a fortune. It is quite a big lens too. It weights 1,4kg. It has 16 elements in 14 groups. I
use lens hood all the time. 
Not because of flare, but for protection. Hey, the front glass is big! And it does reduce flare. I do not know for how much, but lens hoods do that on all lenses! More on technicalities here Sigma 100-300. How does it perform? Very good! This is MHO, but even other friends of mine agree, that this Sigma 100-300mm and Nikon D80 (my review of Nikon D80) are very well matched. It's sharp, contrast is good and it's fast. There is no competition for Sigma 100-300 in this zoom range regarding price/performance. I liked it a lot and would happily recommend it.Click on 1:1 crop below, to see it full size.
300mm - f/5,6 - ISO 100 - 1/400sec
270mm - f/4 - ISO 100 - 1/400sec
I am satisfied with this lens. Would you be? Try it and see for yourself.
