The flash duration is about 1/10000 s. So no matter the shutter speed, all the light emitted by the flash will reach the sensor. Ambient light is diminished by the shutter speed, of course, and this gives you the possibility to diminish ambient light. On a sunny day at noon in direct sunlight there is less control of course because the light is too strong. But in shade or at the golden hour you can extinct ambient light completely.
This is the background correctly exposed at about 5:40 PM 1/160, f:5, ISO 160
untitled shoot-004.jpg by
Radu Coman, on Flickr
This was taken eight minutes later, 1/125, f:7.1, ISO 160. I’ve decided to close more the aperture to increase the DOF so shutter speed is longer at 1/125 but still background is underexposed.
untitled shoot-008.jpg by
Radu Coman, on Flickr
As you can see the background is much darker and the subject is correctly exposed.
At 1/250 the background would have been black and still the subject correctly exposed.
I’ve used a flash with a ¼ CTO gel at ¼ power through a white umbrella about 1 meter from the subject.
The shots are on the lousy side of the bad category (my first attempts actually) but I hope these illustrate the concept.
_________________
Radu
Canon PowerShot S100
Canon 50D , SIGMA 10-20 f3.5 ,Canon EF 24-105 L IS USM, Canon EF 100/2.8 macro Canon EF 50/1.4 ,Canon EF 85 f1.8,Canon EF-S17-85 4-5.6 Old Tamron 28-300 inherited from my Canon Rebel G film camera
Canon580EXII
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