Advice wanted on how to best use Sigma 24-70 2.8f

I've purchased a Sigma 24-70 2.8f macro lense for my 350D last night.
I bought the lens as a general walkabout lens, to use the macro facility, but specifically to use at a friends christening in a church in September.

I fired off 50 or so shots indoors last night and noticed that it hunted around quite a bit more than my canon lenses and that sometimes it would not focus. Also it seems to focus a little bit before the subject. I'm unsure if it is me or the lens. As I'm new to the 350D I mainly use P mode with the autofocus setting on the central point. I'm going to practice in the meantime and intend to take it outdoors and then into a church to practice for the christening.

I'm looking for advice on how to get the most out of this lense, what to expect (the DOF is different to the kit lense I've been using) and if I can do any tests to ensure it is focusing correctly or if it is my technique. Generally how to get the best out of this lens.

Regards,
Phil B.

first of all, what ISO were you shooting in, and how bad was the lighting?

secondly, i'd try taking a bunch of shots outdoors in bright lighting to test if it's actually front focusing.

thirdly, when focusing, always try aiming for a high contrast area with lines or edges.

DOF depends on your aperture, the sigma has a constant maximum aperture of 2.8 which is larger than the maximum aperture of 3.5 on the kit lens. larger apertures reduce depth of field. for more, read up on the basics at dpreview

I was shooting at ISO 400/800/1600 to see what effect each level had on the quality. The lighting was not good - curtains partially closed and a 40W table light. I think I was expecting more out of my first 2.8f.

I took the lense out yesterday for a try on the roof garden at work - great skyline of London, Butlers Wharf and Tower Bridge. I was a lot happier as the lense was more responsive and locked on to the subjects a lot better than the previous evening. It still hunted a little bit, but I guess I'll have to practise a bit more. It seems a lot better at focusing on distant objects.

The Sigma 24-70 f2.8 was the first lens I bought with my 350D. It does take some time to figure out what it can and can't do, and while you can't expect L quality from it I've gotten some pretty good shots, and you probably will too. Out of my current three lenses it's the one I use the least, but it's one I enjoy having around. As a walkaround lens (which I bought it for) I found it to be fairly big, heavy and noisy. Since buying it I've grown to prefer primes, but until I can afford more of them I'll definitely keep the Sigma around.