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Well I have absolutely no issues with any of my Canon lenses and focusing, but the Sigma 70-200mm has always front focused, more so on portrait type shots and at close range. This is one of the main reasons I wouldnt trade for the Canon lens and the main reason I wanted to! The spyder thingy showed massive front focusing as I knew and was re-calibrated to allow for this. The results are much sharper images, front and back. I will take it to the show on Saturday and have a go with it there, Im pretty sure though its a good tool for the price.

Regardless of whether you feel its nec or not, it cant harm either your camera or lens, the camera stores the lens setting you have calibrated and immediately switches to that setting once that lens it attached, then reverts to the original setting when another is used. Its a simple press of a button that restores it back to its default setting.