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.
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