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Lighting ratio

Defining the degree of contrast


The lighting ratio on this subject is very high, because of the great difference in light intensity between the illuminated and the shadow sides.
The lighting ratio on this subject is very high, because of the great difference in light intensity between the illuminated and the shadow sides.

Lighting ratio (or simply "light ratio") is a term used to compare the amount of light striking a subject's shadow side with the amount of light falling on the illuminated side. Because it compares light with dark areas of a scene, it is a way of defining the amount or degree of lighting contrast.

When using two (or more) light sources, lighting ratio is a way of stating the difference between their intensities. If one light source is illuminating the shadow side of a subject with one light unit and another source is illuminating the bright side with two light units - i.e. twice as much light, this is a lighting ratio of 2 to 1. (Note: Some photographers express this as 1 to 2, reversing the values for the amount of light striking the bright and shadow sides. The end result is the same. The higher number refers to the brightest light, and the lower number refers to the light in shadow areas.)

The light sources can, for example, be direct and reflected sunlight, or two artificial lights, or a mixture of sources (e.g. flash and sunlight). In studio flash photography, lighting ratio shows the brightness relationship of the main (or "key") light compared with that of the fill light.

You control the lighting ratio by either choosing light sources that have different levels of output or by adjusting the distance from light source to subject.


ADJUST LIGHT SOURCE INTENSITY TO CHANGE THE LIGHTING RATIO

If your light sources are floodlights or flash units with adjustable output, you can use an exposure meter - either a hand-held incident meter or your camera's built-in meter brought close to the areas to be metered - to measure differences in illumination. Then, you can adjust the output of the light sources to provide a satisfactory lighting ratio.

MOVE THE LIGHT SOURCE TO CHANGE THE LIGHTING RATIO

If the light sources are of the same intensity and their output cannot be adjusted, you can adjust the lighting ratio by moving a light (or a reflector) closer to or further from the subject. Let's suppose two identical-output lights are the same distance from the subject. They will each be striking the subject with the same amount of light. Move one light further away from the subject so that its light-to-subject distance is doubled. If it is, for example, five feet from the subject, place it ten feet away. The intensity of its illumination on the subject falls off to one-quarter of what it was. (Light diminishes over distance in accordance with the inverse square law, which states that doubling the light-to-subject distance reduces the light falling on the subject to one-fourth.)

WHAT IS A SATISFACTORY LIGHTING RATIO?

There is no one lighting ratio that will provide the best results for every photograph. Some pictures benefit from high contrast and therefore need high lighting ratios. Low lighting ratios result in low contrast. For most portraits, low light ratios such as 3 to 1 (expressed as 3:1), are used. Many portrait photographers commonly work with lighting ratios between 2:1 and 4:1. For high contrast images, where there is a sharp difference in brightness between lit and shadow areas, a ratio of 5:1 or 8:1 may be used. But, keep in mind that ratios of 8:1 or higher are ratios that are generally too high for film to record detail in shadow areas. Shadows will be quite dark. If strong, directional shadows are what you want, perhaps to create a mood, then go for the very high lighting ratios.

When there is low contrast (a slight difference between the amount of light on the shadow side and the amount on the illuminated side), the lighting ratio is low.
When there is low contrast (a slight difference between the amount of light on the shadow side and the amount on the illuminated side), the lighting ratio is low.

Measuring light contrast. Lighting ratio is determined by measuring the difference in meter readings of the illuminated side and the shadow side of your subject. The difference in meter readings can be expressed as the lighting ratio.
Measuring light contrast. Lighting ratio is determined by measuring the difference in meter readings of the illuminated side and the shadow side of your subject. The difference in meter readings can be expressed as the lighting ratio.

HOW DO YOU DETERMINE SPECIFIC LIGHTING RATIOS?

Since lighting ratio is based on measuring the light's intensity falling on the subject in two areas - the illuminated side and the shadow side - a comparison of the different light intensities provides the lighting ratio. If there is no difference in light brightness between two areas, the ratio is 1:1. If there is a difference of one ƒ-stop between the bright area and the shadow area, the lighting ratio is 2:1.

The formula to calculate the lighting ratio, using "n" as the difference in ƒ/stops between the two areas, is 2n:1. Here is an example: using a difference of 3 ƒ-stops between the lit and shadow sides, the lighting ratio is 23:1. Since 23 is another way of writing 2 X 2 X 2, or 8, the lighting ratio is 8:1.

Don't feel like calculating the lighting ratio? The table on the left provides the lighting ratios for various differences in ƒ-stops between illuminated and shadow areas.