Focus Issue

[CBA100]

I received my A700 afew days ago, and only today have been able to try afew test shots. I'm using a Sigma 18-50mm F2.8.


I took a quick shot and noticed (what i was focused on) WASNT in focus??

I focused on the writing of my A700 box, and again out of focus. I tried manual focus and the shot came out perfectly? I then tried my Tamron 90mm 2.8 on the A700, again AF on the writing and shot was perfectly sharp meaning it seems to be a fault with the Sigma and not the A700 body.


I tried the Sigma on my brothers A200 and once again, the focus was out? I bought the Sigma off here 4 weeks ago for my A200, but due to constant work i never got around to using it so i didnt notice any focus issue.


What could be the problem?

 

[CBA100]

Just tried afew more shots and the Sigma is absolutely terrible. My Tamron 90mm has out done it on every shot. Could it be a front/back focus issue? How much would Sigma charge to rectify this, if this is the case?