To measure lens vignetting and light fall-off we photographed a white target with a highly diffused custom filter. The lenses were tested at their widest focal length with the aperture wide open using a Nikon D2X in Aperture Priority mode. The images were analysed with Imatest and the full areas presented here at a reduced resolution. Bigger percentages are better.

The Sigma and Tamron lenses perform similarly here, with each losing almost half their light by the time you reach the corners. Falling off by around 10% less in the corners, the Nikkor 18-200mm VR comfortably out-performs its cheaper rivals in terms of vignetting at wide angle.