I've shot some decent scenes with my 550D, in available/low-light exteriors even with the 18-55mm kit lens at f/3.5. Best option is to get a faster lens, f/2.8 and f/1.8 are easily better choices.
Technicolor Picture Style helps a bit to "get" the maximum information in the "washed" blacks. Not ideal, but helps a bit to process in post after.
Also, make sure to use the "Exp. Override" option and set the shutter speed to 1/24 when shooting 23.976fps. Helps a bit. Everything is (of course) a bit more blurry in movements, but less grainy.
With a minimum work in post using Levels:
...and a good Noise Reduction plugin (NeatVideo comes to my mind), you can get the best, out of a somehow limited camera for low-light shooting. Not the best example, but shows pretty much the power of that plugin:
Hope it helps!
And here's two videos I've done with low-light scenes, shot with the 550D and processed with NeatVideo:
Technicolor Picture Style helps a bit to "get" the maximum information in the "washed" blacks. Not ideal, but helps a bit to process in post after.
Also, make sure to use the "Exp. Override" option and set the shutter speed to 1/24 when shooting 23.976fps. Helps a bit. Everything is (of course) a bit more blurry in movements, but less grainy.
With a minimum work in post using Levels:
...and a good Noise Reduction plugin (NeatVideo comes to my mind), you can get the best, out of a somehow limited camera for low-light shooting. Not the best example, but shows pretty much the power of that plugin:
Hope it helps!
And here's two videos I've done with low-light scenes, shot with the 550D and processed with NeatVideo: