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Track 2 · Light2 min read

LESSON 08

Flash

A burst of light far shorter than the shutter. One frame holds two exposures: the flash, and the light already there.

ONE FRAME · TWO EXPOSURES

A long bar for the time the shutter is open, with one tall, very narrow spike near its start for the flash.

A very short light

An electronic flash reaches full brightness almost at once and is usually over in less than a thousandth of a second, shorter than the shutter speeds used with it. That brevity is why flash freezes motion.

Two exposures in one

The light already in the scene builds up for as long as the shutter stays open; the flash adds its burst once. So the shutter speed mainly sets how bright the background is, while flash power and distance set the subject. Fill flash works this way: expose for the background, then let the flash lift the face.

The sync speed

Most single-lens reflex cameras use a focal-plane shutter with two curtains. At fast speeds the second curtain starts closing before the first has fully opened, so a flash would light only a strip. The fastest speed with the whole frame open is the sync speed, often 1/60 to 1/125 s on film cameras and as fast as 1/500 s on a few modern ones.

Distance matters

Light from a small source falls off with the square of the distance: twice as far away, a quarter of the light. Full power that suits a distant subject can badly overexpose one close by.

Try it today

  1. 01Find a backlit subject, with a window or the sky behind it.
  2. 02Expose for the background with the flash off.
  3. 03Turn the flash on, keep the settings, shoot again.
  4. 04Lower the flash power or flash compensation until the face looks natural.
  5. 05Try a slower shutter and watch only the background brighten.

Figures from Wikipedia, 'Flash (photography)', 'Flash synchronization', 'Focal-plane shutter', 'Fill flash' and 'Inverse-square law', read 2026.10.16. The steps under Try it today are working practice, not a standard.