EX POSURE 101 Explaining some key concepts of this dark art

Reflected light

To calculate an exposure value for a scene, digital cameras use a built-in light meter to measure the light being reflected from the subject. These meters are calibrated to 18% grey, which corresponds with the amount of light reflected by an ‘average’ scene.

18% grey (midtone)

This value corresponds to a midtone colour halfway between white and black. When a camera struggles to provide the correct exposure value for scenes that include extremes of white and black, an 18% grey card can be used to take a light reading from.

Exposure histogram

The histogram shows the range of tones contained in a digital image. The X-axis represents 256 pixels, and the intensity of those pixels is displayed in the height of the columns. For more on histograms, and how best to use them, see Camera College (p58).

The histogram can be displayed in a mirrorless camera’s electronic viewfinder, or on a DSLR ’s rear screen, with the image preview.

A range of 18% grey accessories is available, including reflectors and lens cloths. As an alternative, grass reflects the same value.

Built-in meters are convenient, but incident light (taken at the subject using a hand-held meter) gives more faithful readings.