Sigma EX 70-200 mm f/2.8 AF APO HSM Lens

[Dao]

From the test you did, for some reason, the f/3.5 shot were worst then the f/2.8 one (motion blur??). I assumed photo were taken with camera on tripod. Did you use a remote shutter or time delay? Do you have a flash? Maybe shot it with mirror lock up just to make sure if you are not using flash.

(By the way, the Sigma lens I used for that bird shot was a older generation of Sigma 70-200mm f/2.8 non-OS). It is really not bad at all once stop down a little bit from f/2.8 @200mm