Just as a flower growing where you don’t want it is a weed, light splashing where you don’t want it is a spill. Light spills cause all sorts of problems: they emphasize the wrong areas of the video image, they cause distracting highlights and reflections, and they give away the fact that the setting is lit artificially.
Preventing or removing light spills is fairly simple when you know some tricks of the trade, so here they are. Fundamentally, there’s only one way to prevent a spill: by blocking the unwanted part of the light before it hits the spill surfaces. But as the Wicked Witch muses in The Wizard of Oz, "The question is, how to do it. These things must be done dehhhh-licately!"
Any masking will create a line where the light stops; so try to match the light edge to a natural line in the image, like an inside wall corner, a ceiling line, or a doorway. If you can’t find a suitable straight-edge, make the light border as soft (diffused) as possible. Edge sharpness varies with the distance between the light and the flag or other mask: the farther apart they are, the sharper the edge. A hard edge works well with a natural set line; but if you have no obvious boundary, a soft edge will fade the light gradually and look more realistic.
The Indispensable Spotlight
At any distance between light and mask, the sharpness of the edge also depends on the size of the light source — the smaller the better. That’s one reason why spotlights are so widely used in lighting. The light is not only small, but focusable as well. By moving the lamp forward or back in the light housing, you can change the beam spread, just like a MagLite flashlight. The narrower the beam, the harder its natural edge and the easier it is to control. For even better focusing, use a spot with a Fresnel lens. Spotlights in location kits rarely have lenses — another argument for creating a custom kit rather than buying a pre-packaged outfit.
Most importantly, spotlights have barn doors: opposing pairs of metal flaps that can swing into the light path to mask the edges. When units have two pairs of doors — one on the sides and the other top and bottom — the second pair consists of two half-width blades instead of flaps. By partially overlapping these blades, you can narrow their front edges so they fit inside the other pair of barn doors.
Or so they claim, because this system is clumsy and doesn’t always work well. So, when masking all four sides of a light, first set the solid flaps to kill the more important spills; then add the split flaps. Remember, barn doors are mounted on rotating rings. By turning the ring you can align the doors with diagonal edges, as well as parallel to walls or ceilings.
When you need a gradual falloff instead of a fairly hard line, set the lamp in the spot to the full flood position. Then, if you still need more diffusion, place a half-screen in the filter ring, with its edge parallel to the line you want to soften.
Sometimes you want the light edge harder rather than softer. Beyond a certain point, barn doors cannot deliver a really knife-crease edge. When this happens, you must swing the barn door out of the way and use one of the freestanding masks discussed below. Sometimes you can compromise by using a gooseneck flag. This is a paddle-shaped metal card attached to a flexible arm about two feet long with a clamp at the bottom end. Clamp the arm to the light or the light stand and bend the gooseneck until the paddle stands up in the path of the light. Since this extended barndoor is as much as a foot or more away from the spotlight, it creates a somewhat harder edge. Gooseneck flags are also very useful when you need extra masking at an odd angle that regular barndoors can’t handle.
Spills from Large Sources
The floodlights supplied with location kits are often small broads: long (about 6-inch) skinny lamps set in open trough reflectors fitted with one pair of barn doors on their long axes. These barn doors work reasonably well because the small reflectors create a fairly hard light. And since we often use broads for lighting walls and other backgrounds, the gradual light falloff created by their barn doors looks natural.
Large light sources like soft boxes and fluorescent pans are quite another story. The same diffusion that gives them their natural wraparound quality, wraps equally around attempts to mask them, making them impossible to control — at least with masks placed close to the light. In studio setups you can exercise a degree of control by setting these soft sources well back from set flats or studio draperies, which act as giant flags; but even then the masking is crude, and the farther back you position the lights, the less illumination you get on the subjects. (Remember the inverse-square law: when a light source is moved back to twice as far away, the light on the subject drops to one-fourth its previous level.)
Umbrellas are somewhat more manageable (which is why I personally prefer them as large light sources). If you fit the most reflective umbrella cloth — typically, silver Mylar fabric — you’ll get a light beam somewhere between that of a broad and a soft box, and the higher output will allow you to place lights farther from the subjects. By spotting really large masks such as big foamcore sheets or reflectors as close to subjects as possible, you can indeed reduce light spill, albeit with extremely wide, soft edges.
Spill Control Tools
Used like this, foamcore sheets are one type of flag, the most common tool for controlling light spills. Classic studio flags are thin plywood paddles, painted black and attached to rods for mounting (some are black velour cloth stretched over thin steel frames). Though flags come in all shapes and sizes, 12-by-24 inches is a typical proportion, big enough to use some distance from a light source, while small enough to take on location.
The most versatile flag material is foamcore: rigid foam sandwiched between sheets of paper. I buy 24-by-24 inch sheets of the inch-thick type, for sturdiness. For versatility, I buy them with one side black and the other white, so that they can double as reflectors when needed.
Whether they’re foamcore sheet or rod-and-paddle, all flags must hold for long periods in exactly the right position to mask off light spills. By far the best tool for doing this is the Century stand, or C-stand for short. Think of a light stand with a universal clamp on top, holding out a long rod with a second universal clamp on its outer end. Grasped in this extended clamp, a flag will hold in place in almost any position at any convenient distance from a light. (For me, two C-stands are the absolute minimum number for even the smallest professional light kit.)
How to Do It
To deploy a flag and C-stand rig, mount the flag on the arm clamp, at more-or-less the proper height and angle for masking the spill. Then carry the unit forward and back between subject and light until you get as much edge sharpness as you want (or until the flag intrudes into the frame). When you’ve established the right distance from the light, fiddle with the stand height, arm position, and clamping angles to fine-tune the spill mask. With practice, you can do all this in about 30 seconds.
Because set surfaces and lighting angles don’t always produce straight-line spills, you’ll often need to make custom-shape flags. For these single use models, keep a supply of half-inch foamcore, which is easier to cut than the thicker stuff.
In making odd-shaped flags, you have several options. Sometimes, you need to gang up multiple flags, each on its own C-stand. Or else, build the irregular shape you need by progressively cutting, fitting, and duct taping foamcore pieces together, checking the effect as you go. For simple shapes, you can sometimes cut a multi-edged flag out of a sheet of foamcore. I rarely do this though, because when my first attempt is not quite right, I have to start over with a fresh, expensive sheet.
Finally, don’t forget that you can use flags outdoors too, with reflectors. Here the trick is to employ a shiny aluminum surface reflector to get as hard-edged a beam as possible (making sure it doesn’t shine into subjects’ eyes). Then place the reflector well back from the subjects. For masking spill, use a big sheet of foamcore or another reflector (angled away from the subjects, of course). If you’re using foldable hoop-and-fabric units for reflector and/or flag, watch the frame very carefully for light ripples. Those light cloth models wave in the slightest breeze.
Contributing Editor Jim Stinson is the author of the book Video Communication and Production.