what to do when your lens got back focus?

[htthach]

what to do when your lens have back focus problem?
i did notice some thing strange and i did a test on my 17-70 and i found out that it have back focus problem.
especially at the 17mm focal.
it is less noticable at 70mm.

what to do now?
the lens is 3 days old from shop.
what should i tell MS and ask them to do when i bring the lens back?
usually what do you guys do when encounter this problem?

[htthach]

here are some picture to share on the test
original size too big so i put as link
setting: 17mm 1/180s f/2.8 iso 800
all use center focus point

#1 focus is on letter J
http://www.comp.nus.edu.sg/~hinhtinh...ture%20092.jpg


#2 focus is on the blue pen mark
http://www.comp.nus.edu.sg/~hinhtinh...ture%20112.jpg

[htthach]

can anyone confirm if this problem is true before i bring this back to them?

[htthach]

is my test correct or not?

[kietgnoel]

Looks like front focussing problem to me. You can try to shoot a ruler at a 45 degree angle. Focus at any marking on the ruler. If the sharpest point is futher away than where you focussed then it's back focussing. If it's nearer then it's front focussing.

You can bring it back to MS-Color. They may replace it for you since it's only 3 days old. If not, bring it to Sigma Marketing for adjustment - it's a Sigma lens, right?

[htthach]

ya, it's a sigma
maybe i've mistaken back with front focus