Focal length, build quality, and image quality for price.
f/6.3 at 500mm is limiting. Vignetting might be a problem in some photos.
These are very early impressions after having had my lens for about two hours. I'll update this at least for a few weeks as I get more experience with it.
This link was updated on 3/1/2011 to point to my new photo gallery. There are hundreds of pictures here taken with this...
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