Chuck Gardner's Photography Class
Part Two A - Lighting
by Chuck Gardner
Lighting Ratios

The distance between the subject and the two lights determines contrast of the lighting. This is expressed in term of the ratio of reflectance between the fill light intensity (denoted as "1") and the key light. With a standard studio set-up the fill light illuminating the entire subject with one unit of light. If a light of similar intensity is used as the key light and is placed at the exact same distance from the subject as the fill light it will also illuminate the subject with one unit of light, but only on the side facing the key light. These highlights receive two units of light, one each from the main and fill lights. The shadow side gets only the one unit of fill light. Thus the lighting ratio for two lights of similar intensity at the same distance from the subject is 2:1, not 1:1 as some might assume.

If a higher lighting ratio is desired (3:1 is typical for portraits) the key light must be moved closer to the subject until it illuminates the subject with twice as much intensity as the fill. That is to say the highlight areas will receive 2 units of key light plus 1 unit of fill (2+1) while the shadow areas are still only illuminated by the 1 unit of fill light. (2+1):1 or 3:1 lighting ratio.

Moving the fill light in twice as close as the fill will not result in an intensity twice as bright because The laws of physics dictate that the intensity of the light changes by the square of the distance. This is known as the inverse-square law. Fortunately the same laws apply to camera apertures, and those odd f/stop numbers provide a simple and easy to remember guide for setting your studio lighting ratios.

Fill
Distance
Lighting
Ratio
Key
Distance
16 ft. 1:2 16 ft.
16 ft. 1:3 11 ft.
16 ft. 1:4 8 ft.
11 ft. 1:2 11 ft.
11 ft. 1:3 8 ft.
11 ft. !:4 5.6 ft.
8 ft. 1:2 8 ft.
8 ft. 1:3 5.6 ft.
8 ft. 1:4 4 ft.


A simple and practical way to set your studio lights is to tie a string to all of the light head with knots at 16,11, 8, 5.6 and 4 feet then use the following "f/stop = distance" table to set their distances for different :

Set the lights by stretching the string to the subject, starting with the fill light, using one of the knots to set the distance. Then select the desired lighting ratio and key light distance from the chart. Remember this only works if the lights have identical output.



Goto Next Section - Facial Angles>

Goto < Previous Page

Goto Class Outline

Goto super.nova.org my home page.