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Lighting Ratios for Portrait Photography

An important concept to understand when dealing with studio lighting for portrait photography is contrast. Contrast is the difference in the amount of light that falls on the dark areas of a scene and the amount of light that falls on the highlight areas of a scene. Humans can see a wide range of contrast while film and digital capture devices are much more limited in the light level ranges that they can record. For this reason, we must use caution when lighting a scene, and consider the ratio of the amount of light between light and dark areas or the Lighting Ratio.

Due to the latitude of film and digital sensors, it is the photographer’s goal to find the exposure that strikes the appropriate balance between the highlights and shadows. You must begin by deciding the desired “feel” of the final image. If you wish to obscure shadow detail and draw attention to the subject, high contrast lighting may be most appropriate. On the other hand, you may wish to show detail in both the highlight and shadow areas which would require lower contrast lighting. Once you know the effect you wish to obtain, you can begin to identify the proper lighting ratio for the shoot.

The definition of a proper lighting ratio is very subjective and can vary widely between photographers. There are, however several lighting ratios commonly used in commercial portrait photography that we can use to illustrate the process for determining this ratio in the studio setting. For example, a lighting ratio of 4:1 is common for traditional portraits. A 4:1 ratio indicates that there is four times (or two stops) more light in the highlight areas of the face than in the shadow areas. A 4:1 ratio gives enough light in the shadows that details can be seen, while creating the sense of depth required for realism.

Holding a light meter at the subjects chinTo produce a portrait with a 4:1 lighting ratio, you would begin by determining the exposure values for the main light. Let’s assume for this example that there is a main light and a fill reflector. The main light is off camera right at 45 degrees and the fill reflector is off camera left at a similar angle. Remove the fill reflector and take an exposure with only the main. Use an incident light meter to measure the amount of light that falls on the side of the face nearest the light. This value will give you the working aperture for the shot. In other words, this exposure value is set such that you get proper exposure in the highlight area of the subject’s face. Now, add the reflector back in and take another exposure. This time, use the light meter to record the exposure value for the light falling on the shadow side of the face. The ratio between these two values should be 1:4 or two stops. So, if the highlight side of the face registered f11 and the shadow side registered f5.6, you have achieved the proper ratio. If you do not get 5.6 in the shadow side, simply move the reflector forward or backward to compensate taking a new reading each time you move it.

When the reflector or second light is placed such that fill light can spill into the highlight areas, extra caution must be used to calculate the resulting ratio. Be sure to measure highlights again and recalculate the ratio after adding a second light or reflector.

Below is a chart that will help illustrate some of the common light ratios and what they mean in terms of f stop differences. As you can see, a rule of thumb is that to calculate ratio, using stop difference, take 2 and raise to the power of the stop difference.

Common Lighting Ratios


Typical flash meter used for calculating lighting ratios
Ratio Stops
Difference
Description
1:1 No
Difference
Flat
lighting
2:1 1
Stop
General
color photography
3:1 1
1/2 Stops
General
black & white photography
4:1 2
Stops
Dramatic
lighting, low key
8:1 3
Stops
Very
dramatic, low key

Lighting Ratios



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