What lens is better to purchase

[machangezi]

Sometimes I think I should replace my Canon 18-135 f/3.5-5.6 with SIGMA 17-50 MM. F/2.8 EX DG OS HSM.

Is Sigma 17-50 f/2.8 a better lens than Canon 18-135 f/3.5-5.6?

Thanking you in advance for any/all suggestions.

 

[chuasam]

duh...of course it is! f/2.8!!!
Sigma has some really good stuff.

 

[Ido]

One isn't better than the other, they serve different purposes. You should not choose between them based in sharpness and the likes — you should choose the lens that suits your needs best.
So you need a moderate zoom lens to use in low light, or do you need a high-zoom lens that will be an all-around lens (just not nearly as good for low light)?
The best thing you can do, is read this article and then think what lens would be better for you: Understanding Camera Lenses

 

[Overread]

The 17-50mm f2.8 Sigma offers you increased maximum aperture along with a constant maximum aperture. That gives you more light gathering ability which is helpful in lower light conditions (for both exposure and viewfinder image and AF reliability). The constant f2.8 also means that you won't find the aperture shifting as you zoom on the lens.

It is likely that in a higher level of lens and with less focal length coverage it will also be sharper when you compare it wide open (max aperture) to your current 18-135mm

The 18-135mm really only one bonus in the 50-135mm range it covers.

Whilst focal length values between extremes on a zoom lens are not always exact, dry your zoom in the 18-50mm range; see if its suitable and what you like to use a lot of the time. If you find that its really not where you're using it all that often then its telling you that, whilst the 17-50mm is a good lens, its not going to give you something that you "want/need". On the flipside if those are focal ranges that you're focusing on in a big way then upgrading would make more sense.