Hi ya
First off here are the prices of the 3 lens you have asked about.
Sigma 17-70mm f/2.8-4.5 DC Macro £188.99
Tamron SP AF 17-50mm f/2.8 £192.99
Sigma 18-200mm f/3.5-6.3 DC OS £295.99
One think I thing you got to ask yourself is what you are going to use this new lens for and what is more important to you, zoom range, wider aperture, weather the lens has IS (OS), or a Macro feature. One thing hopefully we all agree on is that all 3 lens are better than the kit lens. I am no expert but I have been through what you are going through not long ago myself, and I will try and give you pro's and con's on each lens. One thing I won't do is try and compare image quality of each lens because this I don't know because I have only owned one of them and that's one I am not allowed to use yet.
Sigma 17-70mm f/2.8-4.5 DC Macro
First off its the cheapest.
Got macro function.
It zoom range is the 2nd longest.
Its Aperture is f/2.8-4.5 Its f2.8 at 17mm, f/4.0 at 50mm, f/4.5 at 70mm so it would be 2nd widest aperture past 17mm.
Doesn't have IS(OS)
Tamron SP AF 17-50mm f/2.8
Doesn't do Macro
Shortest zoom range
Its aperture is f 2.8 17-50mm It has the widest aperture.
Doesn't have IS(OS)
Sigma 18-200mm f/3.5-6.3 DC OS
Its the most expensive.
Doesn't do Macro
Has the longest zoom
Its aperture is the smallest 17mm f/3.5 - 200mm f6.3
It has IS(OS)
I think what you got to decide is between zoom range, and widest aperture.
If you just need one lens and don't want to carrying other lens and keep changing lens you got to look at the Sigma 18-200mm.
If you are not going to shot over 70mm or don't mind changing lens when you do, and want to do a bit of Macro look at the Sigma 17-70mm
If you don't mind change lens and carrying other lens, and are going to do more low light shots look at the Tamron SP AF 17-50mm.
Hope I have help out and not confused you even more.
Cheers Holo
