Flash and lighting
Flash has a bad reputation because of photos of white faces on black backgrounds, but those images come from poor use, not from the tool. Used well, flash fills shadows, freezes motion and lets you decide precisely how much of the background shows. In the simulator you can control flash power and shutter speed separately.
Two exposures in one photo
When you use flash, you're actually making two exposures at once. The ambient light, which lights the background, depends on shutter speed, aperture and ISO. The flash, which lights the nearby subject, depends on power, distance, aperture and ISO, but not on shutter speed, because the burst lasts barely a thousandth of a second.
This gives you enormous control: shutter speed decides how much background appears; power decides how much light the subject gets.
Shutter speed for the background; power for the subject.
At night, with the same flash power and two shutter speeds.
The inverse square law
Flash light loses intensity very quickly with distance: if you double the distance between the flash and the subject, it receives a quarter of the light, that is, two stops less. That's why a flash lights a person well at 6 feet (2 m) but leaves the background black at 30 feet (10 m). To even out subject and background, move the flash farther from the subject or let in more ambient light by slowing the shutter.
Flash sync speed
Most cameras won't let you use flash faster than 1/200 or 1/250, their flash sync speed. At faster speeds, the shutter's second curtain starts closing before the first has finished opening, and a strip of the sensor doesn't get the burst. Some flashes offer high-speed sync (HSS) to get past that limit, at the cost of power.
Bounce and diffuse
Direct flash is a small source: it produces hard light that's frontal and flat. If you turn the head toward a light-colored wall or ceiling, the light bounces and arrives from a large surface, so it becomes soft and wrapping. It's the simplest technique for making a flash photo not look like one.
Bear in mind that bounced light takes on the color of the surface: a wooden ceiling will tint the image orange. With a built-in flash, which can't be turned, a diffuser or even a piece of paper in front softens the result.
Flash bounced off the ceiling: the simplest way to hide the flash.
Fill flash and dragging the shutter
In daylight, with backlight, a low-power fill flash (between −1 and −2 stops of flash compensation) lights the face without its presence being noticed. At night, dragging the shutter combines flash with a slow shutter speed: the burst freezes the subject and the long exposure lets in the city lights.
If you also set the flash to rear-curtain sync, motion trails will fall behind the subject rather than in front, which is how the eye expects to see them.
Your turn: master the flash
With your camera's flash or an external one.
- Photograph a person against the light during the day, first without flash and then with low-power flash.
- At night, take a flash portrait at 1/200 and another at 1/15 without changing anything else, and compare the background.
- If your flash head turns, repeat the photo bouncing it off a white ceiling.
- Analyze the three versions: hardness of the shadows, color and the relationship between subject and background.