Ever get stressed over which light to hang where?
Depending on the manufacturer, ellipsoidals (lekos, spotlights) are known by their lens system (6x12, 6x9), or by their field angle in degrees (20 degree, 30 degree).
What do those numbers mean? In the term "6 by 12", the first number is the diameter of the lens (6 inches), and the second number is the focal length of the lens system (12 inches). Not the most useful set of numbers when you want to know how wide a pool of light you'll get.
If the light is called a "20 degree", that means the beam of the light (the field angle) is 20 degrees wide. Useful to know if you happen to have a scale drawing of your stage and a protractor handy.
Field angle or beam angle? The field angle is the effective width of the whole beam of light, measured in degrees. The beam angle is the bright center of the beam. If the light has been aligned to a nice flat field, the beam angle will be about 2/3 of the field angle. In the examples on this page, we use the field angle equivalent.
So how do you figure (easily) which light should hang where? First of all, divide your set into overlapping acting areas. Then try and figure how far away from the actor the light needs to be (throw distance) to create a pool of light that is 8-10 feet in diameter at head height (we light faces, not feet).
Use the Multiplying Factor: Multiplying factor x Throw Distance = Diameter of area covered (MF x T = D).
Example: A 6x12 has a MF of .43. If the throw distance is 20', then the beam of light will be about 8.6' wide (.43 x 20 = 8.6). Need to know where to hang the same light to get a 10' wide beam? Throw distance equals diameter divided by MF (T=D/MF). Answer: 23'.
Use The Photometrics Handbook for data on almost every lighting instrument ever made in the US.
PNTA Lighting Cheat Sheet
The coverage of an ellipsoidal from any throw distance may be figured by using this formula:
Multiplying Factor x Throw distance = Diameter of area covered (MF x T = D).
To find diameter of beam: MF x T = D (see above)
To find which light given throw distance and beam desired: MF = Diameter divided by Throw.
To find Throw distance needed given the light you have and the beam size you want: Throw = Diameter divided by MF.
Multiplying Factors by Lens System
name field angle MF
4.5x6.5 47°, wide .88
6x9 34.3°, medium .62
6x12 24°, medium .43
6x16 16.7°, narrow .29
6x22 11°, very narrow .19
Multiplying Factors by Field Angle
field angle MF
50°, wide .93
40°, wide .73
36°, medium .65
30°, medium .54
26°, medium .46
20°, narrow .35
19°, narrow .31
15°, x-narrow .26
10°, x-narrow .17
5°, xx-narrow .09
Goal: A beam of light 8' to 10' in diameter at the actor's head height for each acting area. Overlap acting areas across the stage, with an area at center stage.
Divide the playing space into overlapping circles that are 8 to 10 feet in diameter. They should overlap by about 1/3 to ensure even illumination. Why 8 to 10 feet? Smaller areas make it hard to get an even wash of light. Larger areas spread the light too much, making the stage too dark.
Label the acting areas with letters or numbers, stage left to stage right.
When creating acting areas it is helpful to use an odd number of areas downstage and an even number as you move upstage.