Long time Canon user, new Nikon User - Issues!

Hi all,

I recently sold off all my Canon gear (Canon Eos 50D and 17-85mm lens etc) due to a lack of use and feeling a change was in the air. I purchased a new Nikon D5100 body (i wanted a smaller lighter setup etc) and a used Nikon 18-70mm lens. Having read reviews of the camera and lens this seamed an ideal combo. However i am finding the images taken so far to be well exposed and detailed but rather soft or maybe evan a little blurred./out of focus. Two things: Camera or lens?

I am also wondering about the cameras settings, they are largely left as default with the only change being a switch to a centre/single focus point. All my sample shots have been shot at ISO 200 and A mode using F8 in good light. To my knowledge no other settings have been changed but there do seem to be a lot more options than on my Canon 50d, are there any recommended changes i could make?I chose the D5100 as it uses the same sensor/processor as the D7000 which i would guess is a comparable model in the range to a 50D? I hope it's the lens as i bought it from a reseller and could probably return it, there is also a bit of play in the zoom barrel towards the lens, particularly at 18mm. Could this be causing the problem?

Any help and advice would be greatly appreciated, not too happy so far!

Thanks.