Lesson 7: ISO: Sensitivity to Light

ISO is the third corner of the exposure triangle, and the simplest to understand: it is how sensitive the sensor is to light. Low ISO numbers mean low sensitivity and the cleanest image; high ISO numbers mean high sensitivity, letting you shoot in dim light — at the price of noise, the grainy speckling that degrades fine detail and colour.

The ISO scale runs in full stops that double each time: 100, 200, 400, 800, 1600, 3200, and higher on many cameras. Each step doubles the sensitivity, which is exactly one stop of light. The name comes from film days, when ISO described a film's sensitivity; a roll of ISO 400 film was twice as sensitive as ISO 200. Digital cameras kept the same numbers so photographers could keep thinking in stops.

When to Raise the ISO

Think of ISO as the setting you raise only when aperture and shutter speed cannot give you the exposure you need. Outdoors in daylight, ISO 100 is the natural home: the image is at its cleanest and most detailed. Indoors or at dusk, ISO 400 to 800 keeps shutter speeds fast enough to hand-hold. At a concert or a night street scene, ISO 1600 to 3200 — or higher on a modern camera — may be the only way to get a usable shutter speed at all.

How much noise is acceptable depends on the camera and the purpose. A modern full-frame camera can produce clean images at ISO 3200 that a phone would struggle to match at ISO 800, because its larger sensor gathers more light per photosite. And noise matters less in a small print or a social-media post than in a large print, where it is easy to see. Some photographers even add grain deliberately in black-and-white work, where it reads as texture rather than a flaw.

ISO and the Triangle

ISO is the only corner of the triangle that does not change how light reaches the sensor — it changes how the sensor's signal is amplified. That is why it is best used last: set the aperture for the depth of field you want, set the shutter speed for the motion you want, and then raise the ISO only as far as the light demands. Many cameras offer auto ISO, which does exactly this for you within limits you set — a genuinely useful feature while you learn, provided you cap the maximum ISO so the camera does not run away into unusably noisy territory.

A good exercise is to photograph the same dim scene at ISO 100, 800 and 3200 with the same aperture, adjusting the shutter speed to match. Zoom into the shadows of each frame and you will see the noise grow. Knowing what your camera's noise looks like — and at which ISO it becomes unacceptable to you — is one of the most practical things a photographer can learn.