Try starting, as suggested, with your lowest ISO. And shutter at 1/250th. Adjust the aperture to get your flash level the way you want it. (if your flash & camera is set for TTL, you may want to either dial in some compensation, or set the flash to manual, and adjust the output down some if need be).
Now, 1/250th is the fastest you can go. You can bring the sky brighter by slowing the shutter speed to 1/200, 1/100, etc. It won't affect the flash exposure, as that happens within the first 1/250th of the exposure, anyway, and is already over. The remaining time the shutter is open allows more ambient light in.
With a lower ISO, stopped down for the flash, you may find you really do need to open the shutter longer to bring up the background. |