Thank you so much. Would the sigma also work for taking pictures of my kids at home? I was looking into it and just want to make sure it's good for other scenarios. Which would you recommend as best for fireworks and parades?
Yes, you absolutely can use the Sigma for indoor family pictures. You can also use the mentioned Tamron 17-50 2.8 easily for indoor family pictures without flash under most lighting situations.
I only switch to a fast prime (mine is only 35 1.8) for candle-light type pictures indoors. The 2.8 is fast enough for most indoor lighting situations.
For fireworks -- the lens is not your most important piece of equipment. See the recent thread about how to takes fireworks shots. You can use any lens -- as long as you are happy with the focal length. Even the basic kit lens would work great. The important equipment for fireworks are: Tripod, remote shutter release, and possibly ND filter.
Parades -- Depends how close you are to the parade, day or night, etc. Hand-held in low light, would be another time to use that Sigma.
But in regular day light, along the parade route.. the basic kit lens, the 17-50.. those would all be fine.
The reality is, where most lenses typically differ, beyond focal length, is in the aperture. But even cheap lenses perform pretty well when the aperture is stopped down to f8...
So what's the different between the Canon kit 18-55 lens, versus something like the Tamron 17-50?
Well, when zoomed to 50mm.... the Canon aperture is about 5.6, while the upgraded lens can still be used at 2.8. Meaning, you have the potential to gather much more light by upgrading the lens.
Furthermore, the upgraded lens should work better at 5.6, than the kit lens at 5.6.
But if you are using both lenses at.. for example... f11 --- Which is what you would often use in bright day light, both lenses should perform very similarly to the naked eye.
Many people both underrate and overrate the kit lenses -- An upgraded lens does contain many advantages, but under many photographic situations, the kit lens will perform very well, and there will be no noticeable difference between the basic kit lens and an upgraded lens.
If you are serious about taking indoor photographs without flash, that is the biggest reason to upgrade from the kit lens, right from the start.