Hello, I'm interested in photography and I'm just learning about cameras/lenses and photography in general before I move onto buying an SLR (if and when I need it).

For example if there was a lens like this; 'Brand' 17-70 F2.8-4.5 could someone describe what it does and how it differs from other lenses? etc.?

Thanks

'Brand' 17-70 F2.8-4.5

Well the brand bit is important, but for starters, the 17-70mm is the focal length.
depending on brand and model of body it is fitted to, this describes the angle or field of view.

The crop factor of the bodies sensor comes into play here
On canon APS-c bodies the focal length is multiplied by 1.6 x to give the old school 35mm equivalent focal length. So it would become a approx 28-110 mm in old money. Roughly a 4x zoom in nonsense speak , but the focal length equivalence tells you that at this lenses wide end is equivalent to 28mm which is termed wide (and wider than most P&S cams which start at the eq of 36mm) and zooms to 112 mm which is low telephoto. So this would be what i call a fairly Normal zoom.
Nikon uses I think a 1.5 x multiplier (as does pentax???)
Olympus uses a 2 x multiplier


the F2.8-4.5 describes the aperture , which describes the amount of light or opening diameter of the lens as a ratio. Rather than me describe this probably incorrectly I point you to wikipedia. But for an easy answer it refers to the speed of the lens.

http://en.wikipedia.org/wiki/Aperture

None of these particulars really describe the quality of the lens however, but generally the lower the F ratio the more expensive the lens becomes, as larger pieces of glass are required

You mean the sigma 17-70 2.8-4.5?
If so, who cares, it's shite. Get the sigma 18-50mm f/2.8...

I didn't mean any particular lens in general, but I was wondering according to your post, why would the 18-55mm lens be better than the 17-70?

Would the smaller range of numbers be used for less zoom/closer up shots? etc?

constant aperture is alot better than when it moves with the focal distance. Also, glass that is used in the contruction of the lens is VERY important as this gives you your colours and saturation

NOT THE 18-55mm!!!!!!
Make sure you avoid that lens like the plague!!

I was talking about the Sigma 18-50mm f/2.8

Reason: What Lance said - sharper, better saturation, more accurate, constant aperture etc

Harry

im also new at lens's and stuff but i was told the reason it is much better than the other is because of less glass and there for can let in allot more light, hence the aperture ( f/2.8 ) the lower the better see compare these 2 lenses

Just means you can focus easier, and use higher shutter speeds in a dark area.

and that means sharper image right.

Thats a plus side to expensive glass, its sharper, yes.