Subject luminance range
Subject luminance is the amount of light a subject either reflects or transmits, this is the intensity part of the exposure equation (H = Et).
The luminance range is the difference between the maximum and minimum amounts of light reflected or transmitted by a subject. This is measured as a ratio e.g. 100:1. The luminance range of a subject must be placed on the characteristic curve (Fig 3.10) so that optimum recording of the tonal values within the range can occur.
To move the luminance range on the curve either the camera or printer controls are used to adjust either the length of time or the amount of light that can pass through the lens.
If the lowest value subject luminance is given insufficient exposure (underexposure) it will be recorded at a density that will either fail to record at all or place it on the lower part of the toe or where compression of tones will occur. Accordingly, if the highest value is given too great an exposure (overexposure) the values will be recorded in the shoulder region where again compression of tones will occur. A change in contrast is also apparent.