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The 7D isn’t great at low light focusing and even if it says it locks focus, it might not be in focus. You have only a sliver of DOF at the minimum focus distance of 1.5 feet (like a millimeter!), so focus/recompose is a bad plan way up close. At a more reasonable distance like 5 feet, it’s around an inch.
Toss a flash on the top and set it to only do focus assist, but not to flash. That will really help you focus in low light. Use sub-point focus (the focus box that’s filled in with a spot) and instead of recomposing, use the joystick to move the point where you want in focus or as close as you can get.
If you can’t use a flash with focus assist, you’ll have to use one the vertical center focal spots and recompose. Turn the camera and move your head with the back of the camera. Don’t just turn your head with the camera attached.
The 7D built-in flash can be set to assist, too, but it’s kind of blinding.
You can also use live view and zoom in. This works pretty well, esp from a tripod.
Lastly, you could use an led keychain light to focus assist, half shutter, release light, fire shutter.
There are tons of ways to do it. I’ve been using a hotshoe-mounted assist light for dark venues.
Also, “back focus” usually refers to a faulty lens/camera locking focus in back of the subject. It does not mean using the rear AF button to focus.