Exposure
Exposure refers to light - the amount of illumination the camera picks up. When the scene is too dark, you lose a lot of detail in dark shadows. Worse, your camcorder’s AGC (Automatic Gain Circuit) tries to amplify the available light. The result, which you can see for yourself by filming dim light, is in "video noise" (colored speckles) and unrealistic colors (black becomes a noisy, milky dark grey).
If the scene is too bright, on the other hand, details can wash out, disappearing in white areas.
Contrast
The contrast ratio is the ratio of the brightest highlights in a scene to the darkest shadows. Professional film-makers often set up huge arrays of extra lights to reduce the contrast ratio, thus evening out the illumination so that the camera can record more detail accurately. (When watching a movie being filmed, you sometimes see huge lights set up, even in daylight: they’re there to fill in the shadow areas, so that the camera can "see," for example, the actors’ eyes.)
Film cameras can photograph details in a scene that has a 10:1 contrast ratio (highlights are ten times brighter than the dark shadow areas). Video, on the other hand, can’t capture details outside a contrast ratio of about 3:1 or 4:1. That’s another reason lighting is much more important when using a camcorder, as noted above.
Hard Light vs Soft Light
Hard light comes from a small light source falling directly on an object. It creates hard egdes between the highlight and shadow areas. For example, when someone's standing in direct sunlight, the shadows on his face are harsh and dark. This high contrast emphasizes wrinkles, skin blemishes, baggy eyes, and other facial features - in other words, hard light is unfattering light.
Soft light, on the other hand, is less direct; it offers softer, much smoother gradations of light from brightest to darkest areas. You get soft light from a large light source, usually reflected or diffused (like the outdoors light on an overcast day, or like the light reflected from the umbrellas used by photographers). The result: soft shadows or no shadows; everything is lit fairly evenly.
Soft light is much more flattering to human subjects, because it de-emphasizes wrinkles and other facial contours. Unfortunately, soft light can also make your subjects appear flat and lifeless. Harder light can reveal contours, shapes, and textures, making objects more interesting and three-dimmensional.
The best video lighting, therefore, comes from direct light sources that are mechanically softened. That’s why many video lights have milky translucent covers.
Key, Fill, and Backlights
In professional film and TV work, the most common lighting arrangement is called the three-point lighting setup. It requires that you set up at least three light sources, as shown in Figure below:
•The key light is the primary source of illumination in a scene. This can be the light on the camera, the sun, the overhead light above a table, or the light from a window, for example.
•The fill light comes from a second light source. It’s designed to fill in the shadows caused by the key light. By doing so, fill light reduces the contrast ratio, allowing the camcorder to pick up more details. If your camcorder has a built-in light, that’s usually a fill light, too. It softens the shadows cast by the key light (such as the room lights.)
•The backlight comes from behind the subject. It helps to separate the subject from the background. Backlight is especially helpful in distinguishing a dark subject (such as a person’s hair) and a dark background, because it casts a glow around the rim of the subject’s outline.
Be careful of course: When the light behind the subject is too bright, camcorders respond by dimming the entire picture.
•In professional film and video, technicians sometimes set up a fourth light: the background light, which is pointed at the background to make it easier to see (especially in very dark scenes).
Color temperature
Believe it or not, even ordinary daylight or room light also has a color. In general, daylight has a bluish cast, flourescent light is greenish, and household bulbs give off a yellowish light.
Film makers call these color casts the color temperature of the light. We don’t usually notice the color casts of these common light sources because our eyes and minds have adjusted to it. DV camcorders usually do an excellent job of compensating to avoid noticeable color casts, thanks to the automatic white balance in the circuitry of every modern model.
If automatic white balance doesn’t work, you can manually white-balance the camera. (I always do this!) White balancing means identifying to the camera some object that’s supposed to look pure white (or colorless), so that it can adjust its circuitry accordingly. To use the manual white-balance feature, focus on something white that’s illuminated by the key light -- for example, a clean T-Shirt or piece of paper. Zoom in until the white areas fills the screen, then press the White Balance button. The camcorder responds by compensating for the dominant color in the light.
The 45/45 Rule
This lighting guideline suggests that the key light be at a 45-degree angle to the camera-subject line and at a 45-degree angle above the ground. (see figure below)
General Guidelines for Lighting
The preceding discussion gives you the theory of lighting design. Here’ the executive summary -- a distillation of that information down into just a few points to remember for the most professional-looking lighting.
•The subject should be brighter than the background. Don’t shoot people with a bright window or doorway behind them, unless you want them to disappear into silhouette.
•If the background is bright, shine additional lights on the subject. If you can’t do that, use your camcorder’s Backlight button or its manual-exposure knob.
•Stand so that the key light --the sun, for example -- is behind you. Don’t shoot a subject with the sun behind her (unless you want a silhouette)
•Avoid a key light that’s directly above your subject. That arrangement causes ugly, heavy shadows under the eyes, nose, and chin. (The cinematographers for the Godfather movies set up lights this way on purpose, so that the mobsters’ eyes would be hidden in shadows. That’s not the effect you want when filming the bride in a wedding ceremony.)
•If you’re aiming for professional quality, create soft fill lights by bouncing light off a big square of white foam-core board or a big piece of cardboard covered by foil or newspaper. This arrangement creates a beautiful soft light -- great for close-ups.
Bouncing lights off a white ceiling makes for pleasantly soft key light, too.
•Tracing paper, tissue paper, and translucent plastic (such as shower-door material)make great diffusers for soft light, too. (Just don’t put the paper in contact with the bulb; this kind of paper, especially tissue paper, ignites easily if it gets too hot.)
•Be alert to the presence of shiny surface like windows, glass, chrome, and highly polished wood in your shots. They can reflect your lights into the camera, making it look as though someone is shine a light directly into the lens.
•If the backlight spills into the lens, you get flares -- those oddly shaped patches of light that move across the frame as the camera pans. The sun behind the subject, or car headlights, often causes flares. If you can avoid this effect, do so.
Cinematographers spend entire careers studying lighting; it’s a fantastically complex science. Here’s what they worry about.