I have a Canon 450D/Rebel XSi. How do I adjust the amount of flash from the pop-up for fill-in flash?
Flash exposure compensation (Flash exp comp) is in the second menu page (second red page), second option down. You can adjust exposure compensation up or down by up to two full stops. You can also add flash exposure compensation to your "My Menu", the green menu on the end, for quick access as well. Once set to anything other than 0, your exposure compensation will work in any of the non-automatic exposure modes, such as P, Tv, Av, M, and even A-DEP.
You can adjust the flash output using the flash exposure compensation.
Just go in the menu, page number 2, option #2 should be "Flash exp comp". Slide the marker right to give positive compensation a boost the flash output, or slide to the left for neg comp and reduce the output.
Please note: I think you need to be in creative mode to access the flash compensation feature.