There's no need to cry over spilled light, just follow this simple advice.
Just as a weed is any plant, however pretty, that sprouts where you don't want it, spill is any light that falls where it doesn't belong. Pro videographers and gaffers use great ingenuity in controlling light spill, but you can master the basics fairly quickly.
Nearly all spill control involves blocking or at least softening the edges of the beam emitted by a lighting instrument. In each case, how you do this will depend on the effect you want, the type of light you're modifying, and the gadgets available to do so. But, before we look at the how of spill control, let's start with the why.
Spill Control Uses
You employ spill control to confine light to the area you want lit, instead of just letting it splash all over the set. For example, suppose you need to light a dark hallway behind a room (Figure 1a). If you simply aim a spot into the doorway, the light spills all over the adjacent walls (Figure 1b). However, if you use the four barndoors on the front of the spotlight to mask the light edges (Figure 1c), you can confine all the light to the doorway (Figure 1d).
You also use spill control when washing a background wall with a splash of light (Figure 2). Without masking, the light pattern on the wall would spill onto the adjacent wall and the floor (of course, this is sometimes desirable).
When a background pattern is created by a cookie (a foam core or plywood sheet with holes cut for the light to go through) you must confine the beam within the edges of the sheet, so that it doesn't spill onto the background (Figure 3).
Spill control is also important when lighting subjects. By masking light edges, you can illuminate faces with dramatic slashes of light. By masking the edges of a key light pattern, you can keep some light off oversize ears or multiple chins. For the latter trick, however, you might want to use a spotlight screen instead. Let's take a quick inventory of this and other lighting control accessories.
Some lights integrate spill controllers right on the light, while others are well in front of it. But a more important distinction is between hard- and soft-edge control.
Hard-edge Control Tools
Hard-edge accessories slice the light off (more or less) cleanly. Spotlights are the only instruments that permit tight spill control. The ideal spotlight is equipped with four different kinds of light management:
Most light-weight location units sacrifice the lens, but for studio use, look for spots with all four of these features.
Barndoors, again, are hard-edge controllers, but they are too close to the lamp to make a perfectly clean shadow line. For that you need flags: opaque rectangles clamped onto stands and set in the path of the light. Studio flags are often thin plywood or black cloth stretched on frames, but foam core board is the universal choice for versatility, especially on location shots. Found at any art supply store, you can even find foam core with black on one surface and white on the other. For flagging, turn the black toward the light to minimize light scatter or use the white side as a reflector. Buy thick ones (they are sturdier and still almost weightless) and get 'em big. Remember: the farther from the instrument you position them, the harder the masked light edge -- and the wider the light beam. You can place big boards farther away than smaller ones.
Finally, don't overlook the gooseneck accessories that let you clip a small flag well out in front of the light on a light stand. Not only does this deliver a harder edge than the barndoors, but it can also be twisted to any angle to provide masking that the four barndoors can't cover.
Soft-edge Lighting Control
Sometimes you don't want to chop off the light to mask off a door opening or create a similar abrupt transition. Instead, you want to brighten a certain area unobtrusively by letting the extra light fall off gradually. Soft-edge devices are best for this job.
Starting inside the light, you can move the lamp back toward the rear reflector to soften (and widen) the light path. Be careful, though, a fully flooded light will sometimes have a large, darker area in the center.
It's often better to place partial scrims made of screen wire in round frames behind the barndoors. You can rotate screens that cover half or a third of the circle so that they cut down the light in the desired part of the beam. Two-layered screens increase the effect. Graduated screens, some in four parts (three layers, two layers, one layer, and none) allow the light to fall of very gradually. Don't worry about obvious transition lines: the screens are too close to the light source to show visible lines between the sections.
Diffusion materials (milky gels or spun glass sheets) used on the light itself do not act as flags because they soften the whole light beam. Instead, you can use translucent flags just like the opaque models to soften the edges of the light. Screens (usually frame-mounted cloth screening) reduce the light without changing its character. Like light-mounted screens, they're available in a variety of densities and you can buy this material at garden stores and home centers. Spun glass sheets soften the light quality and they can be clipped to stand arms and then trimmed into custom shapes for complicated masking jobs.
Keeping Light off the Lens
So far, we've focused on controlling light on subjects and backgrounds, but the peskiest spills are often lens flares. Professional-grade camcorders have (or can be fitted with) large matte boxes that help keep light off the lens. Consumer lens hoods, however, may be nearly useless because they're short enough to stay out of very wide angle lens settings.
Where the camera is fairly static, a flag mounted horizontally on a stand above the camera lens will usually work just fine. Generally, use long skinny flags so that the camera can pan back and forth. For moving shots, use ingenious gooseneck-mounted flags fit into the camcorder's hot shoe and adjusted so they are as close to the lens as possible without entering the frame.
Often, it's simpler to clear a lens flare by repositioning a light. Sometimes, simply raising it a foot will do the job or moving it slightly more to the side. If this doesn't work, try manipulating barndoors or standalone flags to mask the lens.
Controlling Spill Outdoors
Lest we forget, the sun is a classic spotlight that throws a hard, tight beam because it's so far away. You can't hang barndoors on it, but the same flags and screens used in the studio will work just fine outdoors.
Hard-faced aluminum reflectors are easy to mask down - and they often need controlling. When used as key lights they can throw a beam like a prison searchlight on backgrounds behind the subject, so flag them on both sides to prevent this. Incidentally, remember to keep shiny reflectors far away from talent because their hard beams can bother or even injure the eyes.
[Sidebar: Spotting Lens Flares]
Usually, we say you should rely on a good production monitor to evaluate image quality, but this doesn't always work with lens flares. Hard reflections will be obvious, but some flares simply degrade the entire image slightly, subtly changing contrast and degrading color saturation. To spot light spilling onto the lens, always look at the lens. If it ain't there, it ain't there.
[Sidebar: Crunchy or Chewy?]
Cookies can throw harder or softer patterns, depending on how far they're placed from their lights. For dramatic Venetian blind slashes, keep the light as far back as possible. For mottled moonlight-through-the-leaves effects, keep the light closer. Obviously, you can play with these positions until you get the right effect.