The human eye simultaneously registers a wide range of light intensity. Due to their limited latitude image sensors are unable to do this. The difference in the level of light falling on or being reflected by a subject is called contrast. When harsh directional light strikes a subject the overall contrast increases. The highlights continue to reflect high percentages of the increased level of illumination whilst the shadows reflect little extra. Without contrast photographic images can appear dull and flat. It is contrast within the image that gives dimension, shape and form. Awareness and the ability to understand and control contrast is essential to work successfully in the varied and complex situations arising in photography.
Inherent Subject contrast
Different surfaces reflect different amounts of light. A white shirt reflects more light than black jeans. The greater the difference in the amount of light reflected the greater the subject contrast or ‘reflectance range’. The reflectance range can only be measured when the subject is evenly lit. The difference between the lightest and darkest tones can be measured in stops. If the difference between the white shirt and the black jeans is three stops then eight times more light is being reflected by the shirt than by the jeans (a reflectance range of 8:1).
One stop = 2:1, two stops = 4:1, three stops = 8:1, four stops = 16:1
A ‘high-contrast’ image is where the ratio between the lightest and darkest elements is 32:1 or greater.
A ‘low-contrast’ image is where the ratio between the lightest and darkest elements is less than 16:1.