Sigma 8-16mm sharpness tests
Outdoors on my tripod, I thought center sharpness was quite good at all apertures. Corner sharpness was good, and even better at F8 and F11. Lab tests on various websites indicate that the Sigma 8-16mm lens has excellent sharpness in the center & corners.
Center and Corner Tests at 8mm
At 8mm, F4.5, center, 100% crop
At 8mm, F5.6, center, 100% crop
At 8mm, F8, center, 100% crop
At 8mm, F11, center, 100% crop
At 8mm, F4.5, corner, 100% crop
At 8mm, F5.6, corner, 100% crop
At 8mm, F8, corner, 100% crop
At 8mm, F11, corner, 100% crop
Center and Corner tests at 16mm
Test photos will be up shortly. The lens was tested at 16mm at F5.6, F8, F11, F16. Center sharpness was good and I could see little difference at different apertures. Corner sharpness was also very good, maybe F8 & F11 were slightly better. Lab tests report very excellent center sharpness, esp. at F5.6, and very good corner sharpness. It's safe to say the lens will not really limit you, only your technique will.
Strange exposure results
In manual mode, at any focal length, shooting the lens wide-open gave the same exact exposure as stopping down 1/3 or 2/3rd's of a stop. Does this mean the lens cheated me out of 2/3rd's of a stop, and was really a F5.6.-F7.1 lens? It would appear so. I replicated this test several times! I'd be curious to see if anyone else saw the same results. It could be I just had a faulty copy.