The beam from a flash unit is cone-shaped—shoot a relatively close object and most, if not all, of the light cone will cover it. As you move farther away, though, a smaller portion of the light cone hits the subject, as more of the light sprays wide. At a great enough distance, the flash’s light becomes imperceptible to the eye as well as to a sensor or film. And it’s a square relationship: If you move twice as far away from your subject, you get only 1/4 the illumination on the subject; triple your distance, and you get only 1/9 the light. Setting the zoom head of an accessory flash to tele position will create a narrower cone, but you lose wide-angle coverage. Yet just because you’re far from your subject doesn’t mean your flash has to be. You can position an accessory unit close to your subject and fire from a distance via a wireless trigger.