Enrico Marega Interactive photography course
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Flash: main light, fill and sync

A flash is an extremely powerful and extremely short burst of light: it lasts about a thousandth of a second, and much less at reduced power. It’s used in two ways: as the main light, when there isn’t enough light, and as fill light, when there is light but it comes from the wrong side and leaves the face in shadow. To use it well you only need three ideas: flash light falls off quickly with distance, flash is controlled differently from ambient light, and every camera has its own sync speed.

Two lights in the same photo

When you shoot with flash, two exposures add up in the same photo. The ambient light (the sky, the street lamps, the windows) depends on shutter speed, aperture and ISO, as always. The flash burst, on the other hand, is so short that the shutter speed doesn’t change it: its light depends on the flash power, the distance from the subject, the aperture and the ISO.

2Simulator That’s why with flash you think like this: with the shutter speed you decide how bright the background is, with the aperture or the power you decide how bright the subject is. In automatic mode (TTL) the camera fires a pre-flash, measures the light that comes back and decides the power; you correct it with flash compensation, which is a separate setting from exposure compensation.

1Simulator

Main light: light falls off with distance

At night the flash is the only light. Move further away with the flash on the camera: every time the distance doubles, only a quarter of the light reaches me. You find the right aperture with the guide number.

2Simulator

Fill light against the sun

The sun is low behind me and my face is in shadow. Without flash you have to choose: expose for the sky (dark face) or for the face (blown-out sky). Expose for the sky, turn on the flash and set the compensation: between −2 and −1 the face brightens without the flash being noticeable.

3Simulator

The sync speed depends on the camera

At sunset, with the flash on my face. Choose the camera and change the shutter speed: with a focal-plane shutter, beyond the sync speed part of the photo gets no flash; with a leaf shutter it doesn’t happen. Below you can see the shutter in slow motion.

The sync speed, explained

3Simulator DSLRs and almost all mirrorless cameras have a focal-plane shutter in front of the sensor: a first curtain opens and a second one follows it and closes it again. At slow speeds, for an instant the sensor is completely uncovered, and that’s when the flash fires. At fast speeds the second curtain starts before the first has reached the end: the sensor is exposed through a moving slit, and the flash, being so short, lights only the slit. The rest of the photo gets no flash.

3Simulator The sync speed is the fastest shutter speed at which the sensor is still completely uncovered: it’s a feature of the camera, not of the flash. It’s usually between 1/160 and 1/250: my Nikon Z6III goes up to 1/200. Cameras with a leaf shutter, made of blades inside the lens, like my Ricoh GR IV, uncover the whole sensor at the same instant and sync the flash at every shutter speed. With the electronic shutter, on the other hand, many cameras limit the flash to slow speeds or don’t fire it at all.

3Simulator To go beyond the sync speed with a focal-plane shutter there’s HSS, high-speed sync: it’s called Auto FP on Nikon, HSS on Canon and Sony, FP (HSS) on Fujifilm. The flash emits a burst of tiny flashes for the whole travel of the slit: it works at any shutter speed, but the usable power drops a lot. With the leaf shutter of the Ricoh GR IV you don’t need it.

TTL or manual, first or second curtain

1Simulator In TTL the camera decides the power and you correct it with flash compensation: handy when the distance changes. In manual you choose the power (1/1, 1/2, 1/4…) and the light stays identical shot after shot: handy when subject and flash stay still. The guide number tells you how powerful a flash is: distance × aperture, at ISO 100. With a guide number of 32, at 4 metres you need f/8.

Usually the flash fires at the start of the exposure, with the first curtain. With a slow shutter speed and a moving subject the second curtain is better (on Sony it’s called rear sync): the flash fires at the end and the motion trail stays behind the subject instead of in front of it. And remember that direct flash from the camera is hard and flat: if you can, bounce it off a ceiling or a light-coloured wall, or soften it with a diffuser.

Key points

  • With flash there are two lights: the shutter speed controls the background (ambient light), the aperture and the power control the subject (flash).
  • Double the distance = a quarter of the light (−2 stops). Guide number = distance × aperture, at ISO 100.
  • Fill flash: expose for the background and set flash compensation between −2 and −1. The face brightens and the light stays natural.
  • The sync speed depends on the shutter: 1/160–1/250 with a focal-plane shutter (1/200 on the Nikon Z6III), every speed with a leaf shutter (Ricoh GR IV). Faster than that, with a focal-plane shutter, you need HSS.

Try it yourself

On a sunny day, place a person with the sun behind them. Expose for the sky and shoot without flash, then with the flash at −2, −1 and 0. Compare: at −2 and −1 the face brightens naturally, at 0 you can tell you used flash. If your camera has a focal-plane shutter, also try a shutter speed faster than the sync speed and see what happens at the bottom of the photo.

Post the result and tag me @enrico.marega, or send it to me by DM: I’d love to see how you tackled it.

Quick check

The subject moves from 2 to 4 metres away from the flash. How much light reaches them?
With flash, you want a darker background without changing the subject. What do you adjust?
Portrait with the sun behind the person. How do you set the fill flash?
With a mirrorless camera with a focal-plane shutter you shoot with flash at 1/1000 and part of the photo stays dark. Why?