Artificial light and flash

Continuous light versus flash

Continuous lights (LED panels, lamps) stay on. What you see is what you get, which makes them easy to learn and essential for video.

A flash fires a burst lasting roughly 1/1,000 s or shorter (approximate). It is far brighter for its size and battery, and the short burst freezes motion.

The trade-off is that you can't see a flash's effect until after the shot. Modeling lights on studio strobes, and simply reviewing test shots, close that gap.

Guide number: how powerful is a flash?

The guide number (GN) describes a flash's power: GN = distance × f-number at a given ISO, usually ISO 100. A flash with GN 40 (meters) lights a subject at 10 m at f/4, or at 5 m at f/8.

Guide numbers are given in meters or feet. GN 40 in meters is about GN 131 in feet, so check which one a spec sheet uses.

Makers often quote the GN at the longest zoom setting of the flash head, which flatters the number. Treat it as a rough guide for comparing units (approximate).

Doubling the distance needs four times the light (the inverse square law), which is why flash falls off so quickly behind your subject.

Direct, bounce and off-camera flash

Direct flash, pointed straight at the subject from the camera, is small and hard: flat faces, shiny skin and a dark shadow on the wall behind. It's useful, but rarely flattering.

Bounce flash points the head at a ceiling or wall. That surface becomes a large light source, so light becomes soft and comes from above or the side. A white surface works best; colored walls tint the light.

Off-camera flash, triggered by a radio or optical signal, lets you place the light anywhere: to the side for shape, behind for a rim of light. It's the biggest step toward studio-like results.

Diffusers and modifiers (domes, softboxes, umbrellas) make the light bigger. Remember that softness depends on size relative to the subject: a big softbox close up is soft, the same box far away gets harder.

Fill flash outdoors

In bright sun, faces get deep shadows under the eyes and nose, and backlit subjects turn dark. A little flash fills those shadows in.

Set the flash about 1 to 2 stops below the ambient exposure, for example −1 to −1.7 FEC on TTL. The aim is to lift shadows, not to make the flash obvious.

Fill flash also adds a small catchlight to the eyes, which makes portraits look more alive.

Sync speed and high-speed sync

Most cameras use a focal-plane shutter with two curtains. At fast speeds the second curtain starts closing before the first has fully opened, so only a moving slit exposes the sensor.

The sync speed is the fastest speed at which the whole sensor is uncovered at once, typically about 1/160–1/250 s (approximate). Go faster with normal flash and you get a dark band across the frame.

High-speed sync (HSS) makes the flash pulse rapidly during the whole exposure so it works at any shutter speed. The cost is much lower effective power and range, so the flash needs to be close.

HSS lets you use wide apertures with flash in bright sun, for example f/1.8 at 1/2,000 s for a blurred background.

First- and second-curtain sync

By default the flash fires just after the first curtain opens (first-curtain or front-curtain sync). With a slow shutter, a moving subject's blur then appears in front of it, as if it were moving backward.

Second-curtain (rear-curtain) sync fires the flash just before the second curtain closes. The blur trails behind the frozen subject, which looks natural for cars, dancers and light trails.

Both only matter when the shutter is slow enough to record ambient light; at 1/200 s you usually won't see a difference.

Mixing flash with ambient light, and gels

Think of each photo as two exposures in one. Shutter speed controls the ambient light only, because the flash burst is shorter than any normal shutter speed. Aperture and ISO affect both. Flash power and distance affect only the flash.

A simple workflow: first expose the background as you like with shutter speed, aperture and ISO, ignoring the subject. Then add flash and adjust its power until the subject looks right. To darken the background, use a faster shutter speed (up to the sync speed).

Flash is roughly daylight-colored, about 5,500 K, while indoor bulbs are warmer. A CTO (orange) gel on the flash matches it to tungsten light so colors agree; set white balance to the room light. Colored gels can also be used creatively, for example on a background.