Here is a demo from the new edition of my textbook that demonstrates what is happening with direct flash. Light spreads out and decreases in brightness as it leaves the strobe. A subject near the strobe receives more light than someone farther away. In a situation where subjects are uneven distances from the flash, only one person can be properly exposed. The demos were shot outdoors at night so as to avoid reflective surfaces such as light-colored walls or ceilings.


In these examples, exposing for the nearest person (A), who is two feet from the flash, requires stopping the lens aperture down to f/16. Because of the light fall-off, the two people who are farther away do not receive enough light and are underexposed.

Properly exposing the middle person (who is six feet from the flash), requires opening the lens two stops to f/8. This overexposes the first person, while the farthest person still receives too little light to be properly exposed.
In the bottom picture, the third person is positioned 14 feet from the strobe. For this person to receive enough light from the flash and be properly exposed (C), the lens aperture must be opened to f/4. The two people closer to the light source are overexposed in this frame.

Changing the brightness of the light on a subject requires changing the aperture on the camera, as in this example, or adjusting the strobe output with the compensation dial either on the strobe back or on the camera. Whichever method you use, remember: when subjects are at different distances from the strobe, only one person will receive the correct amount of light for proper exposure.