Preface: This will not be a technical review, such as the ones Ken Rockwell writes, but simply my experience using this great lens. Unfortunately, Ken did not write about this lens. There are some tech specs at Sigma Imaging UK.
I was fortunate and able to purchase a truly mint condition used Sigma AF 170-500mm f/5-6.3 APO Aspherical lens for use with my Nikon D80 10.2MP Digital SLR camera at, what I consider, an exceptional price. It came in the box with everything one would receive when purchasing a new piece of glass, for only $500. It has been much more than just an excellent investment. It is a joy to utilize this lens due to the fantastic focal range, ease of use, and quality of results.
As a note, I have not tried any other brand of lens in the 300mm to 500mm range, where I use this lens the most (primarily for wildlife and nature), so I cannot compare one versus the other.
What can be said is that, when using a tripod with a remote shutter release, the results of images – throughout the focal range – has far surpassed my expectations, especially considering what I paid.
Even hand-held, the percentage of shots that were perfectly in focus was pretty good, though nowhere near the almost 100% when using my tripod. For information, this lens with my D80 and Multi-Power Battery Pack is weighty, so I use a Manfrotto Bogen 3221W tripod with 804RC2 pan/tilt head. On that subject, the Sigma tripod collar is excellent and operates smoothly.
Admittedly, at times, the lens can be a bit slow focusing, when compared to, say, my Nikon AF-S 55-300mm f/4.5-5.6G ED VR DX lens, but the difference has not been that much to make me dislike the lens in the least. I am quite sure that the Nikon AF-S 200-400mm f/4G ED IF VR II lens would surpass this Sigma lens in every way, but for the price I would expect that to be the case. and, the Nikon still only gets to 400mm.
The colors rendered are very nice and crisp. There is a good contrast in the images. The depth of field produces a sharp and clear main object and a great Bokeh (blur) in the background.
The above photo was taken from a distance of approximatelty 30 feet (9.14 meters) at 500mm, sitting on the Manfrotto tripod, using a remote.
The zoom operates very smooth throughout, no stickiness or hang-ups. It can suffer from chromatic aberration (CA), though that usually occurs only in very high-contrast situations where the background is quite bright and foreground objects are dark.
There is definitely lens creep, no way around that unless you decide to use a band. For me, I simply have the strap around my neck and hold the camera in my palm, with the lens pointed up, when walking. Keep in mind that this is not a light weight lens.
This lens will work in both AF (auto focus) and MF (manual focus) modes with Nikon camera bodies that have the motor to drive the lens, such as a D80, D100, D3, etc. It will only manual focus on entry-level models such as the D40, D60, D3000, D5100, etc.
I cannot give it 5 stars for honest all-out performance, only 4.5.
For the money, however, I am very pleased with my Sigma 170-500 and would recommend it to anyone who needs (or wants) to have more than a 300mm focal range and does not want to, or cannot, lay out four figures for a solid performing lens with this focal range.