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 subj...
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 subj...