Originally Posted by wallyb 
These are all 100% crops I assume? Otherwise you're only testing for distortion which is moot since you use different focal lengths..
They seem like they have too much distortion to be 100% crops. I wrote an online tool
here that helps to compare crops from different lenses. It would be interesting to see 100% crops from the images. I'm surprised the Tamron looks less sharp than the Sigma when they're downsized so much.