In the studio you use a low-key or high-key background to create contrast between the background and the face. Which key is most effective for the background is mostly influenced by the tone of the clothing, which needs to blend into the background so as not to overpower the face in the photo. Outdoors, by manipulating the balance of ambient-to-flash with EC and FEC or manual shutter/aperture and flash power, we can render the ambient-lit background normal (perfect match), darker and low-key (underexposed) or lighter and high-key (overexposed) while retaining normal exposure on the face of the subject
When flash is used outdoors there are two factors contributing to the exposure which can be controlled independently. Because the duration of a flash is so short the shutter speed of the camera has no effect on the flash* lit foreground of a photo taken outdoors with flash. Changes in shutter speed will however affect the ambient exposure of the background. That fact allows the background to be made lighter or darker with shutter speed while the foreground is keep the same.
