The ratio of the intensity of key and fill lights to fill light alone.
The brightness of the main light (key light) compared with the brightness of the fill light(s). A ratio of about 3:1 is normal for photography. This is a hand-held incident LIGHT METER, different from a camera's built-in reflected light meter.
The ratio between the key and fill lights.
The ratio of the brightness of light falling on the subject from the main (key) light and other (fill) lights. A ratio of about 3:1 is normal for colour photography, greater ratios may be used for effect in black-and-white work.
Lighting ratio in photography refers to the comparison of key light (the main source of light from which shadows fall) to the fill light (the light that fills in the shadow areas). The higher the lighting ratio, the higher the contrast of the image, the lower the ratio, the lower the contrast.