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I am currently a Nikon D50 user and have the following lenses : Kit lenses 18-55 and 70-300. Tamron 18-200 Nikon 50mm 1.8 Sigma Macro 50mm and 105 mm also wide angle 10-20.
I am dithering at the moment about purchasing either the Nikon D90 or 300D although am not too bothered about having video.
Also would like to buy a walk about lens, not too heavy for general use and am considering the Tamron 28-75 2.8 which I beleive is really good or do I go for something like the Nikon 16-85. Am a tadge confused!
any suggestins would be gratefully accepted.

Hi Lynne, welcome to the forum

Isn't the Tamron 18-200 the stereotypical "walk-about" lens? To me that seems the obvious choice considering your kit.

If you do want a new lens, I think it depends on what your priority is in terms of focal length - wide or long? Budget is also something of a consideration. The Nikon 16-85 is around £450, the Tamron 28-75 is around £300 so the Tamron is certainly cheaper but 28mm isn't very wide at all, especially with the 1.5x FoV multiplier.

The other obvious options are the 18-105 VR (£220) (which comes as the kit lens with the D90) and the Sigma 18-125 OS (£250) which is the direct competitor. I've got the 18-105 and am generally very impressed with it.
In a recent copy of AP they compared the 2 lenses and the Sigma won mainly on build quality - there really wasn't anything in it in terms of image quality.

I have to second what Chris says. Generally an 18-200 range lens is called a "walk around" lens. However something in the 18-100 range might be lighter. I am not sure your idea of 28 - 70 is a broad enough range, not wide enough and not enough zoom.