EXPLAINING SOME KEY CONCEPTS OF THIS DARK ART. 18% GREY (MIDTONE) EXPOSURE HISTOGRAM

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. 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. 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.

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

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.