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Track 1 · Exposure2 min read

LESSON 03

ISO

How strongly the camera amplifies the light it caught. A higher number brightens the picture and adds noise.

THE SAME GREY PATCH AT FIVE SETTINGS

The same grey patch at ISO 100, 400, 1600, 6400 and 25600: the higher the setting, the more speckled noise.

What the number means

ISO started as a film speed, a measure of how sensitive a film was to light, and digital cameras use a related scale. Each doubling of the number is one stop: at ISO 400 the camera needs half the light it needed at ISO 200 for the same brightness.

What it changes inside

The sensor does not become more sensitive. The camera turns up the gain on the sensor's signal, usually before it is turned into numbers, so the same small amount of light is read out as a brighter picture.

The price is noise

Higher settings cost quality: coarser grain on film, more noise in a digital file, most visible in shadows and in flat areas such as a sky. Lower settings keep more detail, but they need more light or a longer exposure.

Raise it last

Open the aperture and slow the shutter as far as the picture allows. When that is still not bright enough, raise the ISO. A sharp frame with some noise is worth more than a clean one that is blurred.

Try it today

  1. 01Go to a dim room and set A or Av with the lens wide open.
  2. 02Shoot the same scene at ISO 100, 800 and 6400.
  3. 03Enlarge a dark corner of each frame to 100 percent.
  4. 04Find the highest setting whose noise you would accept.
  5. 05Remember that number for night shoots.

Figures from Wikipedia, 'Film speed' and 'Image noise', read 2026.10.16. The steps under Try it today are working practice, not a standard.