ambient light with flash- ISOs, shutterspeed?

Hi all!

In an effort to perfect my skills with basic flash photography, I was wondering what the effects of different ISOs (say ISO200 vs ISO800) and what the shutter speed has to do with getting different ambient lighting. I shoot in M with the flash in auto (+ or - FEC), 550EX.

Basically, ambient exposure is determined by ISO, shutter speed, and f-stop.
Flash exposure is determined by ISO, f-stop, and flash power (essentially how long the flash fires for)

So when you take a flash picture, it is essentially TWO exposures, the flash and the ambient. What this generally means is the background is exposed by the shutter speed/ISO/f-stop, and the subject is exposed by the ISO, f-stop, and flash power (the E-TTL determines the flash power needed to properly expose the subject, or you can set it manually)

See post #3
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