OK here's the deal. In auto or semi auto modes which includes Av, Tv, TAv, Sv, the camera will meter for ambient and use you flash as fill only. Unless you are outside on a sunny day intending to use you flash as fill to just lift the shadows cast by the sun, this is not what you want.
Here's a post I did in another thread to answer exactly the same question ===================================================
Yes that is typical, the camera is exposing for purely ambient light in auto or semi-auto modes, and does not take flash exposure into account in that mode. Obviously if ambient gave you the exposure you wanted then you wouldn't even be using flash. So, do not shot in auto or semi-auto modes whilst using pTTL on the flash unless you are only doing subtle fill flash. Indoors is not for fill flash.
So, put the camera in manual mode, ISO ~400 to 800, shutter say ~1/100, aperture to taste (usually wide though). You want the meter to be saying you are under exposing by 1 to 3 stops (this indicates how much ambient light will be let in, the more negative the darker the ambient). The flash should be in pTTL mode. Aim the flash preferably at a wall so light will bounce off that and illuminate your subject. Push the shutter. The camera will then instruct the flash to achieve overall correct exposure. Then chimp and if your subject looks to bright or dim after this then use the FEC compensation on the flash to adjust. Typcially I end up dialing in -0.3 or -0.7 FEC. If ambient is too dark or bright then use ISO, aperture or shutter to adjust, this will alter the mix between ambient and flash but not the total overall exposure (only FEC will do this). The more ambient you let in (slower shutter, higher ISO, wider aperture) the less flash power will be used.
Remember, in pTTL mode the camera will use the flash output to achieve overall correct exposure. The more negative the meter says the scene is, the more light will have to be dumped by the flash to achieve correct exposure (hence the greater the ratio of flash to ambient).
All this sounds complex but is actually very easy once you try it and get the hang of it. The results are fantastic so it's worth mastering.
The best site I've found on using TTL mode on the web is this one
http://neilvn.com/tangents/. The think "the strobist" is the best site for off camera manual flash photography if you don't want to use pTTL.
Good luck and have fun experiementing
Last edited by twitch; 07-26-2011 at 03:57 PM.