Sigma 10-20 F4-5.6 focusing on 450D

Started Apr 8, 2012 | Discussions thread
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role_of_72 Junior Member • Posts: 28
Sigma 10-20 F4-5.6 focusing on 450D

Hello Everyone!

I'd like to know your opinion on the followings:

I've recently bought a Sigma 10-20 f4-5.6 EX lens for my EOS 450D camera. One can easily find threads of front/backfocus issues of (third party) lenses on the internet, so I checked my copy carefully and here is my problem:

I have to say that I'm not quite impressed with the focus accuracy but can't find out the reason of it. The lens on my camera exhibits noticeable front focus: with greater distance of the subject this phenomenon becomes more pronounced.

As an example at approx. 20meters of subject distance the focus is locked at about 1m according to the distance scale of the lens, and the resulting image shows front focusing.

Now the thing is when I switch to manual focus, press the shutter halfway, turn the focusing ring and watch for the focus confirmation beam in the bottom right of the viewfinder it shows sharp focus from 40cm almost to infinity..
I'm using the center AF point but at the side points I have better accuracy.

I know that uwa lenses have bigger dof but at F4 I assume it's not that big to cover that much.

Now I don't really know if this comes from the lens or it is the focus system of my 450D that cannot handle this wide focal length?

Please tell me your thoughts/experience on the issue. Every bit of information is greatly appreciated! Thanks.

 role_of_72's gear list:role_of_72's gear list
Canon EOS 450D Canon EOS M Canon EOS M5 Canon EF 50mm f/1.8 II Sigma 10-20mm F4-5.6 EX DC HSM +4 more
Canon EOS 450D (EOS Rebel XSi / EOS Kiss X2)
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