Many people think that stage productions need 6400 ISO, f/1.4 lenses, and so on.
Only partly true. Perhaps in a school gym. But professional stage productions are well lit! Look at this picture of Richard Dawkins, which I took a few years ago:
I took that from the front row (yes, be there early), with a 50mm lens at: f/2.8, 1/100th sec, 400 ISO. Here’s one more, exactly as I shot it, same settings:
So, when shooting people at presentations, a well lit stage is fine with an f/2.8 lens. (Why did I use my 50? Because I was close, and I wasn’t sure how well lit this stage would be).
And if you are in a school gym? Then go up to 1600 ISO or whatever you need. And there, an f/5.6 consumer lens will not cut it.