Hi Vasco and all the others,
I have different lenses. A Sigma 2,8-4/28-105mm, a Nikon 4,5,-5,6/75-300, a Nikon 2,8/80-200 and a Sigma 5,0-6,3/170-500mm.
The Nikon 80-200 is by far my best lens, as range is not always the most important thing. I have so many different lenses as a "multi-zoom" like Vascos 28-300 lens, brings not the best results compared to other lenses as vignetting is the problem and the sharpness in the edges.
The Sigma 170-500 is a good lens, but with 500mm you need at least 1/500 sec to make a sharp shot and don't forget with that lens haze is becoming a problem for you, as you will shoot while with your smaller lens you won't.
I am doing slides and there you see every unsharpness on the slide. I don't know how it is, when you make color prints.