This topic has probably been covered before, but I couldn't find a thread pertaining to it.

What is fill flash? What is it used for? I have a Canon EOS Rebel TI, can I use the built in flash to fill flash? I've heard there are adjustments on how to make the fill flash brighter and dimmer, can I do that as well, and how? What are the advantages.

These are just a few questions I have about it. Sorry if they seem kind of amature, but that's kinda what I am .

Thanks in advance...