Background lighting vs subject lighting

One thing to keep in mind when you use a flash (in this case, behind a shoot through umbrella), you can really set two exposures. See the the earlier exercise here presented in a more theoretical way.

The subject lit by the flash is one exposure, and if the background is farther away a second exposure. The two following photos were shot moments apart using the exact same lighting set up and the exact same power levels. In this case, the subject is lit with a shoot through umbrella at 1/8 power, and the background is lit with a flash pointing straight up.

This first photo was shot using the exact same lighting as the second photo, yet the background in the second photo is brighter, while both subjects are equally well lit.




But the second photo has a longer shutter speed, which does not really affect the subjects lit by the flash, but does affect the amount of ambient light that illuminates the background.

In terms of what is correct for the second exposure is entirely a matter of taste.