[SteveB]

When you meter your subject using a flash there is a consistant amount of light hitting the subject (keeping the flash to model distance the same) on each exposure. This is what determines the f stop. Since the flash is so quick the shutterspeed doesn't really matter for the model. You can alter the shutterspeed to capture the ambient light in the background by slowing it down, or just the opposite keep the background darker by raising the shutterspeed.