The exposure triangle

The three main controls must be coordinated in order to create a balanced exposure: shutter speed determines the amount of time the sensor is exposed to light, aperture is the size of the opening that the light passes through, while ISO is the sensitivity of the sensor to light. While the scales used for each is different – shutter speed is measured in fractions of a second, aperture in f-stops, while ISO is based on film sensitivity ratings – increasing or decreasing any of these settings to the next unit in its scale is achieved by doubling or halving its value, referred to as a ‘stop’. So a 1/250 sec shutter speed can be doubled to 1/125 sec or halved to 1/500 sec. An aperture of f/5.6 lets in twice as much light as f/8 and half as much as f/4. ISO400 is twice as sensitive as ISO200, or half as sensitive as ISO800. Increase any setting by a stop and you will need to decrease one of the other settings by a stop for the same exposure. But these settings also have an important creative effect over the look of the image. A fast shutter speed freezes movement while a slow shutter speed will record anything moving as a blur. A narrow aperture gives greater depth of field than a wide aperture, meaning more or less of the scene is in focus. And the greater the ISO the noisier the image will be.