sigma 70-300 4-5.6 calibration?

i have a sigma 70-300 4-5.6http://www.sacamera.co​.za …ro-%28Canon-mount%29.html,i use it on a 400d and i find the images are slightly soft
i read about getting lens's calibrated(the first time i ever heard of this was today), should i try get my lens calibrated?
or could the problem be from a slightly dirty sensor.
any help would be great!

A dirty sensor would result in spots on your images, unless it actually has a film of dirt (cigarette smoke etc) I would say its more likely the lens.

When you say "soft" how soft exactly? that lens is not known to be tack sharp, especially more so with the non APO version.

Have a look at these, if yours is worse then yes it might be time to get it calibrated.

http://www.sdeklerk.co​m/photography/index.ht​ml