The best way to get predictable results with a speedlight is to take control over all the parameters for the ambient light.
- Set the mode selector to manual.
- Turn AutoISO off.
- Set the aperture to what you need it to be to get the depth of field you want.
- Set the ISO to the lowest setting you can and still get the flash range you need to light the main subject.
- Set a shutter speed that measures ~-1EV below what the meter tells you, this will balance nicely with the light overspill from the flash into the background.
- Let the iTTL automation handle the exposure for your main subject. If your subject is very dark or very light, so the meter will be fooled, use flash exposure compensation to correct for that.
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