Updated: Feb 23, 2019
Many lights (such as the ETC Source Four) have interchangeable barrels (technically “lens tubes”) that change a fixture's beam angle. Choosing the right one is crucial: too narrow and the stage will have dark spots; too wide and your lights will be dim and difficult to isolate.
A bit of barrel background.
Beam angles are measured in degrees (as in, a complete circle is 360°). The four most common barrel sizes are 19°, 26°, 36°, and 50° — but less common barrels can be as narrow as 5° or as wide as 90°.
You might notice that the narrower beam angles are brighter than the wider ones. You aren't crazy; it's physics. Each fixture is producing the same amount of light, but the wider beams are spreading it out over a larger area. It's like using the same amount of butter on a larger piece of toast: it spreads thinner to cover more surface area.
If you want to dive deeper down the physics rabbit hole, google 'Inverse Square Law'. The short version is this: move a light 2x further away and that light will be 4x less bright.
Which barrel size is right for you?
Choosing the “correct” barrel size depends on two factors:
How far away is the light? (Also known as “throw distance”).
How much of the stage do you want that light to cover?
In the example below, both lights cover the same amount of stage, so they're equally bright. The difference is throw distance and barrel size. The light on the left is further away and narrower; the light on the right is closer and wider. Both solutions are fine. There are often factors outside your control that push you in one direction or the other—things like the height of scenery or the location of lighting positions.
Trying to cover the stage with an even wash of light?
You'll want to choose a barrel size that creates overlapping pools of light. In the video below, five lights zoom from 10° to 50°. You'll see a moment when the beams change from tightly focused specials into an even wash of light.
Tip: Right-click the video and select "Loop" to see it play a few times.
There's a point when the beams overlap on the floor, but don't be tricked into thinking it's a even stage wash that performers could move through; it's possible there could be gaps at head height as shown below.
To cover the full stage with a wash of light, you'll need to make sure the beams overlap above the head height of performers.
It's easier to see what's happening if we turn off the center light: notice how these barrels are wide enough to partially light someone standing in the in-between areas.
Can you mix and match barrels?
Yes—it's common when lights that are hung on the same position are focused to different parts of the stage. Below is a classic example of pipe ends. The lights that have a further distance to travel are narrower than the ones lighting the closer parts of the stage.
How much should pools of light overlap?
Here's a good rule of thumb: choose the barrel that creates a pool of light roughly 50% bigger than the area you're trying to cover. If your lights are spaced 8' apart, choose the barrel that creates a 12' diameter beam. And remember, that means a 12' diameter beam at head height, not on the floor. Unless you're cool with people's heads moving in and out of the light, in which case: do you.
How can you tell how big the beam will be?
You've got a lot of options ranging from high tech to low tech.
Option #1: Model in 3D
The illustrations you've seen so far were made using Capture, which is surprisingly easy to use and free for beginners. Vectorworks has robust 3D options too, but the learning curve is steeper and it's only free for students enrolled in high school or college.
Option #2: Draw in 2D
Simple arcs in 2D CAD software show beams equally well. Try giving them a color and turning down their opacity to see the beams overlap. Depending on the angle of light you're trying to see, you'll probably need at least side view and a front view (and maybe others) to measure beam overlap. This illustration was created in Vectorworks, but a program called Drafty offers a smoother learning curve and affordable monthly subscriptions.
Option #3: Draw by hand
You can always break out a good old fashioned protractor. It's not that common anymore, but it's a good reminder that underneath all this digital technology, we're still just dealing with straight lines and angles. And it may be the fastest way to do some quick calculating on a hand-drawn section from the scenic designer.
Option #4: Math shortcuts
Most manuals (like this one for a Source Four 26°) list a number you can multiply by distance to get the size of the beam*. For example, if your light is 18 feet from the performers' faces, then
18 x .45 = 8.1 feet in diameter at face height.
*A note about the word "beam": we've been using its casual, loosey-goosey meaning. TECHNICALLY, the beam is the narrower, brighter part of the light while the field is the full spread. When beam and field are used together in their official meanings, stick with the field. You may notice that a Source Four 26° has a beam angle of 18° and a field angle of 25° ...there's a bit of nomenclature wiggle room.
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