I've got three lenses with my T2i - the kit lens, the Canon EF 50mm II and Canon EF 75-300mm 1:4-5.6 III. I'm going to a high school play and would like to take some stills along with some short videos. It will be a no flash environment, with the stage brightly lit. Any suggestions on lens and/or settings? I've tried all three lens in the past and just put the camera on no-flash mode. Some of the pictures aren't too bad so i can probably do that again.
Your best bet is probably the 50mm. Since the maximum aperture is f/1.8 instead of f/3.5 or f/4, it will work in significantly lower light. This is because it is at least 2 stops faster (meaning 4 times as much light passes through the aperture when wide open).
Also, you will want to use a pretty high ISO, because a little noise is less of a problem than blur from a slow shutter speed.
I've never taken pictures of a play, but these general suggestions may be helpful:
To "set the camera for low-light," shoot in Av (aperture-priority mode) and set the aperture to its lowest value (1.8 for the 50mm lens, and from 4-5.6 for the 70-300mm lens, depending on how far you zoomed in). Then increase the ISO if your shutter speed isn't fast enough.
I would highly recommend practicing shooting in Av mode before going to the play if you haven't used Av mode before. Then at the play, take some of your shots in this mode and some in your normal shooting mode (intermission might be a good time to switch modes). This ensures that you will end up with some photos at the quality level you're used to getting, even if you completely blow every picture you take in Av mode.
Choose the lens you bring based on how far you will be from the stage. If very close, the 50mm will be fine. If far away, the 70-300 (but the 50 is pretty light, so maybe stick it in your pocket for pictures of the cast party).
Remember that shooting with wide apertures (i.e. low Av values) you'll be at the narrowest Depth of Field for each lens, so it may be more difficult to maintain focus or keep the entire scene in focus. Usually this is an (artistic) advantage, but it could cause problems for you (again, shoot half your pictures the way you're used to shooting, and you'll end up with at least half of your pictures at your known, expected quality).
"Brightly lit" is an interestingly relative term. I take a lot of pictures in high school auditoriums/gymnasiums (as a yearbook adviser/photographer), and sometimes the lighting can be amazing and sometimes it can be horrible.
First, ignore the advice to get a tripod. Camera shake is less a concern than motion blur from the actors moving, and a tripod/monopod will make no difference when it comes to motion blur.
The 50mm lens is great because it has a large aperture. Open the aperture to f/2.8 (for a lot of light and a little depth of field) or all the way to f/1.8 (lots of light, less depth of field) and you'll be able to take photos in low light. However, a 50mm lens doesn't magnify much. You'll basically need to be in the front row of the auditorium. If you can walk around in the front row, try this; if you'll be back even 5-6 rows, you probably want the zoom lens. Also, don't expect to zoom in and get a close-up on anyone's face. 50mm will do an ok job of getting a shot of part of the stage, but you won't be close enough to get an actor all by themself.
The 75-300mm lens has the disadvantage of having a relatively small aperture. This means less light and (if the lighting of the stage is off) poor lighting in your photos. However, the zoom is great. You can sit anywhere in the front half the auditorium and get pictures of the play - some close ups, some wide outs. Zoom in and out from your seat. When I take pictures in a high school auditorium, I almost always use a 70-200mm lens so that I can have this range.
But here's the tricky part. You want to learn how to use the manual mode on your camera. Why? The lighting is consistent - the stage lights aren't changing. However, your camera will get confused in auto mode when you focus on the back or the front of the stage, and you're pictures might come out poorly because of that.
You want a shutter speed that's fairly quick (1/60 - 1/125 for the 50mm lens, 1/250mm or higher for the 70-300mm lens). You'll have to choose an aperture that's wide enough to let in enough light. You'll probably also have to turn the ISO of your camera up to amplify the light. I use a Canon t1i, and ISO 1600 works ok if I'm in a low light situation. If the lighting crew is any good, ISO 1600 - 3200 should give you enough light to work with.
I've tried shooting in similar situations (at a boxing match, and at a concert) - in both cases without a tripod. My preferred setting for these types of shot is to place the camera in shutter priority mode, and then set the ISO to auto.
The advantage of this setting is that the camera will be able to select 'odd' ISO settings, that you wouldn't be able to select manually, thus optimizing the exposure in a way you couldn't do in fully manual.
Due to the low light, the camera will automatically set the largest aperture for your lens.
I suggest that you use the 50mm, if you can get your self in a range that is suitable for that focal length.
I would set a shutter speed on the slow side, and then set the camera in burst mode, always taking 3-4 images at a time. You will get a lot of shaky images, but some of them will be decent.
Note when shooting without a tripod, you can wrap the strap around your elbow to stabilize the camera. It is difficult to explain, but I learned the technique from this book http://www.amazon.com/Digital-Photography-Book-Scott-Kelby/dp/032147404X/ref=sr_1_1?ie=UTF8&qid=1316503906&sr=8-1
Note, you can just download the kindle sample to check this out, it is displayed in the beginning of the book.
Edit: For shooting video though, I think that you should use a lens with image stabilizer.