To measure corner sharpness we photographed
the Enhanced Digital Camera Resolution Chart with each lens using a Nikon
D2X in Aperture Priority mode. The D2X was set to Large Fine JPEG mode,
optimised for quality rather than size, and with A for Image Sharpening
and Tone Compensation.
Each lens was tested at its widest and longest focal lengths and with various
apertures using Aperture Priority mode. The crops are taken from the lower
left corner of the chart and presented here at 100%. The 18mm crops show
a larger area of the chart because minimum focusing distances forced us
to shoot it from further away.
At the 18mm wide angle end, the Sigma visibly delivers the poorest result
here, although to be fair both it and the Tamron samples improved considerably
just a short distance in from the extremes. The story is also the same
at the 200mm telephoto end, with both the Tamron and Nikkor out-performing