I find TTL flash photography at times a guessing game. Sometimes too complicated and overwhelming. Much like photography itself, the flash exposure affects generally four key factors: the power or strength of the flash, the distance it is from the subject, the aperture and the ISO. I came across this article that does a good simple job of explaining these points and takes the edge off your fears about flash photography. Some good pictures along with explanations. Click here to get a good understanding of TTL Flash. Enjoy!