I'm no flash pro but the basic idea is the long exposure is for the
background and the flash only illuminates the foreground subject for a
short portion of that exposure. So set it up for the background then add
flash for the subject.

Strobes? As in flash units? Shutter speed doesn't matter a damn- light
output and aperture are the variables, shutter speed should be sync or
lower. If your background hasn't enough light, you need another light on the
background.

Martin

With flash units, you change exposure by changing the aperture. The
flash is faster than pretty much any shutter speed you can use.

If the problem is that the background is too dark, then put more light
on it. It's a studio. You can do that. :-)

If there's continuous light on the background, changing the exposure
time will effect the background exposure.  if that light is *bright*
enough the difference may even be significant.

But this is not a good/normal way to set up lighting in a studio; it's
better to light the background yourself, and then if you want it
brighter, turn up the background lights .

The slower shutter speed will allow more of the ambient light to be
exposed. I used to shoot at 1/30 in the studio just to pick up a little
warmth of the strobes' modeling lights. For weddings, I'd often do 1/15
or 1/8 on some shots to get more of the ambient light.

a focal plane shutter has a max speed for syncing with flash.  By shooting
at your max shutter sync all the time you leave no room for a margin of err,
especially if you use slave sync.

but if you pay attention to the shutter speed you can make use of ambient
(room lights) and flash coordination.  works great with a large room, say a
church wedding formals, select an f/stop that will allow the background to
build up to about one stop under the flash exposure, now you have a whole
lot of detail of the church, but still have a bride that stands out with
some separation.

Depends on how much ambient light there is with respect to the aperture and
shutter speed settings.

I will shoot in a  studio setting, where, if I metered the ambient light at,
say, ISO 100,  F/8  the shutter speed would be something like 2 seconds.  In
other words,  not much ambient light to speak of.   Which is fine,  since
I'm depending on strobes for exposures.

With such little ambient light,  for all intents and purposes,  shutter
speed is irrelevant as long as it doesn't drag enough for ambient light to
creep in much,  or shutter speed doesn't exceed X sync.  So all the shutter
speeds in between these two extremes will have little effect on the
exposure, due to the fact that the exposure is being controlled by flash.
In this situation,  ISO and Aperture have control over exposure,  not
shutter speed.  Shutter speed is irrelevant.  Similar is true if you are
shooting in a tungsten lit room, such as at a wedding reception,  and using
flash to exposure your subjects.   There might be enough tungsten light to
get into the exposure if you drop the shutter speed down to 1/10th of a
second, and shoot at ISO 400.  But the background will be orange-ish,  and
blurry,  and the subject being photographed might display ghosting.  Some
photographers drag the shutter for this effect, intentionally.   Some call
it "art",  but I call it a duff and don't hand it to my client.

Patrick