I like to have the versatility of a zoom lens within this magnification range, so, based on several good reviews, I decided to upgrade my old Sigma 70-300mm APO macro zoom.
In some ways, I wish I hadn`t bothered! (I got many more sharp flight-shots with the older lens.)
The lens is reasonably sharp, and, at least with my Sigma tele-extender, works surprisingly well (optically) with a 2x converter. The lens makes a good walk-around lens, perhaps when combined with a wider-angle zoom. I have found the zoom range useful for precise in-camera cropping on long-range landscape shots.
However, the major reason I bought the lens was for in-flight photography of relatively large and approachable birds, such as gulls. For that, the lens (or at least my copy of it) stinks!
For instance, I took the 100-300mm, along with a Sigma 500mm f/4.5 lens, on a day-trip to Gigrin Farm, to photograph the Red Kites. Using a Canon 20D, I figured I had a number of good shots taken with both lenses. But, on examing them in detail on my computer when I got home, every shot taken with the 100-300mm was out-of-focus! On the other hand, virtually every shot taken handheld with the much heavier Sigma 500mm f/4.5 was pin-sharp.
I have since replicated these disappointing results several times.
To cap it all, the autofocus mechanism recently stopped working entirely, and the lens had to be sent off for an expensive repair.
To sum up: for static subjects, I would recemmend this lens. If you plan to photograph birds in flight (or any other fast-moving subjects), I wouldn`t! Instead, try something in the Canon range, like the 100-400, or the 400mm f/5.6. (I don`t know anything about Nikons!)
