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When taking a portrait photo, light ratios determine the look and mood of our portraits. A light ratio is the difference between the amount of light coming from one source, in relation to another light source. Usually in portrait photography, we work with two light sources, namely natural light and artificial light.
Natural light comes from the sun, while artificial light can come from any other light source, eg. speedlights, studio strobes, street lamps, chandeliers, and so on.
Note that the term ‘available light’ is commonly used to refer to natural light, but in practice it can also mean any light that is available without the photographer introducing additional lighting, which makes ‘available light’ include any other artificial light like street lamps.
Light ratios can also be applied to landscape photography, as described in my earlier article, Light Ratios for Landscapes.
Light Ratio 1 (High Key)
Background: High / Subject: Low
This is referred to as a high-key shot, and can be achieved using either natural light or artificial light. This type of shot requires the amount of light on the background to be at least 2 stops more than that which is falling on the subject.
To achieve this with natural light, simply place your subject in a shaded area on a bright sunny day. Overcast days are fine too, if you can find a shaded area that is dark enough. The trick is the light ratio between subject and background.
Example of a high-key shot with natural light:
With artificial light, you can do this in a room or studio, using speedlights or studio strobes. If you can ensure that the amount of light on your subject is 2 stops less than the amount of light hitting the background, you will get a nice clean white background.
Example of a high-key shot with artificial light (top half):
Light Ratio 2 (Low Key)
Background: Low / Subject: High
This would be the low-key shot, where the background is typically darker than your subject. The light ratio is the reverse of what a high-key shot would use, which means that the amount of light on the background is less than that which is falling on the subject.
Using natural light, a possible scenario would be a darkened room with a narrow shaft of light coming in from a small window. The more focused the light, the bigger the difference in the light ratio. Focused light coming into a window would happen in the morning or evening, depending on which direction your window is facing. A west-facing window will have the highest concentration of light in the evening, while an east-facing window will have the highest concentration of light in the morning.
Example of a low-key shot with natural light:
With artificial light, a low-key shot is achieved with using the correct balance of ambient light and speedlights (or another type of light if you wish). The trick is to keep the ambient light low, and the speedlight output higher than the ambient.
Example of a low-key shot with artifical light: