The scrim is typically a piece of white fabric stretched on a frame in various sizes. The bigger the better, though, in order to give you more working room. Popular scrim sizes are 4×4, 6×8, 4×8, 8×8 and even 12×12. The largest size, seen commonly in television and movie productions, is useful for large groups and requires a small army to wrangle on set. After all, the thing is just a giant sail, and without someone dedicated to holding it in place on all but the calmest of days, a silk of any size is going to blow over in the wind.
I recently used a 4×8 silk for a portrait session. Because we were working near high noon, the shadow cast by the silk was very small and my subject had to stand very close to the modifier. With an extra set of hands, though, the scrim’s frame could have been held over the subject’s head instead, which would have provided additional space in which to work, but it also would have required the key light to come from above rather than closer to her eye level. How you position your silk affects the light in much the same way that moving a softbox in the studio affects the light. A big silk close to the subject makes for beautiful, soft light.
Westcott makes a series of diffusers called the Scrim Jim. They’re available in a variety of sizes, and they include not only a frame to support the silk but also a variety of different modifiers that can be stretched on the frame—including netting for cutting the intensity of the light or opaque white or silver reflector material for bouncing light as well.
When it comes to using the scrim to diffuse bright sunlight as a key, bear in mind that you’re essentially casting a shadow—albeit a beautiful and fairly bright one—on your subject, so everything in the frame that’s not in that shadow is going to be very bright and overexposed. If that’s not ideal for what you’ve got in mind, you’ll have to start thinking about the scrim less as a way to create a key light and more as a way to cut the sunlight before you add your own illumination via another key source. In this way you can bring down the harsh sun with the scrim and then use another light—a strobe, for instance—to bring up the brightness of the subject under the scrim in relation to the background. That way the background will no longer be so overexposed.
Alternatively, you can position your subject so that the sun is at her back like a hair light and use the scrim to diffuse it before opening up to expose for the now-shaded subject’s face, or prior to again adding your own light source as a key.
Lastly, instead of thinking of the translucent fabric solely as diffusion material that light travels through, you can use it as a reflector and bounce light off of it. For a subject standing in open shade, for instance, the scrim can act as a big white bounce when placed in bright sunlight and angled to reflect light at the subject. In this way, a silk may be the most versatile light modifier there is. It works in studio and outdoors, it can diffuse the key light, or it can bounce a fill. And, best of all, it doesn’t require any electricity to operate and you’ll be amazed at the beautiful light quality it can produce.