Sigma 17-70 os

This is my first post, and this is one of the first shots I took with my new lens. I just got the Sigma 17-70 os, and I believe the lens is nice and sharp, but I'm a newbie, and I just wanted your opinion if this is a good lens to judge by in the future. I'm not taking photos for a living, just a hobby. The shot was taking by the reflecting pool on the National Mall in DC last weekend, with my T1i set to 1/125 second shutter and f8.0 at ISO 100. I haven't modified anything other than cropping.

Honestly, did it do pretty good?

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Looks good to me. Initially I thought that it's a little soft, but compared to my Tamron's shots it looks good.

To tell if it is sharp or not, a 100% crop of the in focus area of the image is needed. The area in focus here seems to be the ducks head and that seems to be alright.

Also be aware that this lens is not really that sharp wide open.