Advise on canon lenses.

Hi,

I am a developing amateur photographer. I have a canon 350D and am looking to purchase some new lenses. I don't have alot of money to spend so am opting for the EF series 50mm f/1.8 II lens - What do you think of this lens as just a standard lens for stills and portraits?

I am also contemplating purchasing an EF-S series 18-135mm f/3.5-5.6 IS or the EF-S 18-200mm f/3.5-5.6 IS can't decide between these two. Should I consider the Sigma and Tamron versions of these also. Does any onyone know of a lens with this zoom range in a macro version?

Is it worthwhile looking at the STM (stepper motor) version of the 18-135mm if I am not shooting video as I can't find any information on whether this system focuses faster when in AF mode than the standard EF-S lens using a 350D (as it is designed for the improved AF system of the 650D). I understand how the focus works and that it is a less noisy motor, but is it quieter and will it work adequately on a 350D?

I already have a EF-S 18-55mm f/3.5-5.6 and EF 70-300mm f/4-5.6 and am considering purchasing a 2x teleconverter also - are these worthwhile?

Thanks so much.

Lincoln.

Lots of questions I know but if you can answer any or all of them I would greatly appreciate it.