Be patient, I'm going to present how to use flash backwards— show first then explain.
Change the lighting of the room and keep the window the same!
e-TTL II flash is used to light the whole room, swamping the ambient light, with flash exposure compensation of +1.3, 0, and -1.3 stops. The window scene outside of the room is too far away to be affected by the flash, and its exposure is determined by the camera settings f/7.1 1/200s ISO 800.
Change the brightness of the window and keep the room the same!
e-TTL II flash is used to light the whole room swamping the ambient light, with flash exposure compensation of 0 stops,. The window scene outside of the room is too far away to be affected by the flash, and its exposure is determined by the camera settings 1/200s ISO 800 and varying the brightness with apertures of f/7,1, f/5.6 and f4.5.
OK, what's going on? If I try to take the picture without using a flash, the dynamic range of the picture is too wide to use a single exposure. If I meter off the room, using a spot on the molding just above the sink, I use an exposure of f/7.1 and ISO 3200. I am keeping the exposure time at 1/200 s (determined by the sync time of the flash above, just to be consistent), The room is OK but the window is "blown out", that is nearly solid white. On a different day I might even get with maybe some white haze leaking out. Rats. If I meter on the window instead, I get an exposure of f/7.1 1/200s and ISO 800. Now I can see what's out the window but the room is too dark. Turning on the bathroom lights doesn't help much. Using flash allows me to selectively expose the room and the window separately! It is amazing how fast your eye adjusts as it moves from the window to the walls and makes you think you can see both at the same time.
Trying to get a good exposure with only ambient light. (Left) Getting room exposure OK with f/7.1 1/200s ISO 3200, but window to too bright. (Middle) Getting window exposure OK with f/7.1 1/200s ISO 800 but now room is too dark. (Right) Turning on the bathroom lights doesn't help much. This is a rainy day, but problem probably worse on a sunny day.
Note that flash has two different modes: manual and e-TTL for Canon cameras or similar on other brands. Manual mode is like having a light on a dimmer, you can select the power level from full, 1/2, 1/4, 1/8, etc. Manual mode is pretty straightforward, but tedious since you have to hunt around for the right level.
Note that where you point the flash and its angle also affect the amount of light reaching your subject. Also the light drops off like 1/r2 with distance r between the subject and the flash according to the Inverse-Square Law. The Inverse-Square law is why negligible amounts of light from the flash bounce back from distant objects like that in the scene through the window, Until I write a blog entry about the Inverse-Square law, you should google it if you want to learn more.
Back to flash. The automatic mode (called e-TTL for evaluative-Through The Lens) sets off a test flash before the real flash used in the picture. The camera's firmware figures out from sensor signal response to the test flash how to adjust the strength of the main flash so that the resulting image will be on average 18% gray. Great, no hunting around! It is just like how aperture priority and shutter priority modes work. If you don't want 18% gray, you can adjust the overall brightness using flash exposure compensation (FEV). This is just like the exposure compensation in the priority modes. The perfect opportunity to use exposure compensation is when you are photographing rooms with lots of white walls, which case you want the walls white not light gray, done by increasing the FEV by a stop or so.
So in summary, we can control the lighting on our near subject with just the flash and expose far away backgrounds with the exposure trio of exposure time, aperture and ISO. They are independent when the distance to the background is very far away, since the Inverse-Square law indicates that a negligible amount of flash light gets there.