Wide angle lens advice saught..

Hi I am considering ditching my kit wide angle for a sigma 17-70mm 2.8
now heres the question.. Having a lens covering the 50mm focal length in the sigma if I get it, would that render my other lens the 50mm 1.8 pretty worthless, or would I be silly selling that. Just when im on a shoot having 3 lenses shooting portraits gets annoying swaping them over. What do you think.

My lenses are
currently
85mm 1.8
50 1.8
17-55 kit

Keep the fifty-- 1 1/2 stops is a lot of light.

Add the Sigma, shoot with it, see whether or not you still need the 50. If you don't need the 50 get rid of it.

tsaraleksi wrote in post #2093624
Keep the fifty-- 1 1/2 stops is a lot of light.

Can you explain how the f-stop math works? How do you know that is 1 1/2 stops? I don't understand the relationship.

TMR Design wrote in post #2093648
Can you explain how the f-stop math works? How do you know that is 1 1/2 stops? I don't understand the relationship.

I'm not if this is 100% correct, but it should be close anyway. The stops go 1.4, 2.0, 2.8, 4.0. Each stop lets in twice as much light as the next, thereby giving you 2x the shutter speed in a given situation. So if you can shoot at f/4 1/250, then you can shoot f/2.8 1/500 and so on. The f/1.8 is a half stop, hence, the 1/2 stop figure.

Keep the nifty, it'll be sharper, the f/1.8 can be useful and it can easily double up as a cheap macro with tubes.

What are peoples opinions of the sigma 17-70 though any and all views would be apreciated.