dance recital

I am shooting my daughters dance recital this weekend. I was about to next day air Canons 100-400L IS (mostly for sports photography), now I am wondering if I should get Sigmas 120-300 2.8. Any advice?

I assume this will be indoors = low light. You will be a ways away & some action/movement. Both of those 2 lenses you mentioned are really good outdoors, not indoors. You need something along the lines of an 85/1.8, the 135/2 L or even the 100/2.8 macro. If you can get close enough the 50/1.8 would be great and at a reasonable price.

Theatre (which is what I presume this will be like) often has low light most of the time, bright lights in some places or some of the time, and people moving quickly. All that makes taking the photos quite challenging. Have a read of the article below, it's about concerts but it's almost the same thing. I use a 50mm F1.4 lens for theatre photos, and even then and at ISO 1600 I sometimes get blurred shots because shutter speeds are too slow - you need to aim for about 1/125 if they're reasonably still while you're taking the photos, and more like 1/500th if they're running around. Sometimes using a flash is the only option, but often they're not allowed.

http://www.photo.net …rts/mirarchi/co​ncer_1.htm

The 100-400 will probably be too slow. It's great for outdoors with plenty of light. I use to have it, sold it & got the 300 2.8IS.
If you also need the lens for sports, the 120-300 2.8 would be a better choice.
F2.8 will probably be fast enough, depending on the amount of light of course.
I use the 10D+300 2.8IS+monopod+IS & works out well for me.

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