Focus shift on a zoom lens

[fijiphil]

I have recently shifted to a 350D with a Sigma 17-70 lens, from a few years using a EOS 100 with a Canon 28-105 lens. I have been struggling with a strangely high number of out of focus shots though. One trick I used to use on the EOS 100 was, I would zoom in to the subject, focus on it, then zoom out to the desired composition and shoot. However when I try that on the Sigma, I find that the shot goes out of focus - I need a different focus for the zoomed compared to the unzoomed shot.

Is this to be expected or does it indicate a dodgy lens? Should I get it checked / replaced? Or is it because of the long extension of the Sigma lens - the lens itself moving closer to / further away from the subject hence requiring refocussing?

[Keitht]

Some lenses retain focus when zoomed, others need to be refocussed. There is no hard and fast rule. If you are using autofocus and still experiencing this problem it may be a faulty lens. There may also be a focus lock option on the 350D which is kicking. Check the manual to exclude that eventuality before taking the lens back in.