Basic Training: Controlling Exposure

Kyle Cassidy
February 2008

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"Exposure" is the amount of light that hits the photo-sensitive element in a camera. Exposure controls the gradations of brightness that you see on your screen - most consumer digital video cameras can support 100 gradations between light and dark. (Your eye can distinguish about 10,000 gradations, and film can distinguish about 1000.) In a properly-exposed shot, no level of brightness will go above 100 (absolute white) or below 0 (blackest black). Videographers refer to brights or darks out of this range as "clipping," meaning that any values greater than 100 or less than 0 have the values of 100 and 0, so you lose detail, because there is no differentiation in the various brightness values. Keeping the light hitting the sensor between 0 and 100 is what's called "proper" exposure. If too many values approach 100, your image is over-exposed (too bright); if too many values approach 0, it's under-exposed (too dark). If your camera has a histogram feature, you can see this in real time - you want to see a histogram with no spikes running off the edges.

Many modern cameras have program modes in which the camera sets all the variables necessary to expose a shot. In many circumstances, the cameras are pretty good and, if the lighting scheme is not too difficult, they're very often correct. Many people, in fact, leave their cameras on program, not even realizing that there are other modes. This doesn't always work for difficult lighting situations and times when you want the camera to perform in a particular way.

• Shutter Speed
  Film cameras have a physical shutter that opens and closes, allowing light to land on the film. Modern video cameras have no actual shutter, but rather the photo-sensitive chip is made sensitive and insensitive (like a tape recorder that's turned off, not like a bad boyfriend) by flicking a "switch." The chip can be sensitive and collect light for a long period of time - "slow" shutter speed, maybe 1/8th of a second - or it can be sensitive and collect light for a very brief period of time, maybe 1/500th of a second. Each of these has its advantages and disadvantages. A fast shutter speed looks jittery if you're videotaping rapid sports action, because you have a 1/500th-second image every 30th of a second, so football players may seem to jump about in their race across the field. On the opposite end of the spectrum, slower shutter speeds can make rapid motion look blurry.
• Aperture
  The aperture is an actual ring behind the lens that opens and closes (making a larger or smaller hole) to allow more or less light through the lens. The wider the hole, the more light that gets in. The aperture also is key in determining the depth of field, which is the area that is in focus. Large apertures (letting lots of light through) give shallow depth of field, maybe enough to have a person's head in focus and the area behind blurry. Small apertures (letting small amounts of light in) give deep depth of field, which may keep a person and the background in focus at the same time. There are reasons for choosing a deep or shallow depth of field when setting up a shot, depending on how you want it to look in the final product.
• Element Sensitivity ("Film Speed")
  What is called "film speed" on movie cameras is equivalent to "gain control" in video. The "gain" controls electronically how sensitive the CCD is: the higher the gain, the more sensitive the element and the less light that needs to hit it in order to register. With gain, like film speed, come problems - the higher the gain, the larger the grain.

There is another way to control exposure - blocking the amount of light that gets to the camera. You normally do this with ND or neutral-density filters. These act like sunglasses and cut back on the amount of light getting to the lens, without changing the colors. These can be very useful if you want to have a wide aperture in bright sunlight. There are also "graduated" ND filters, ones which start out dark at the top and fade to clear at the bottom. These can darken a bright sky without affecting the rest of the image (too much). One problem with graduated filters is that they want a straight horizon; anything that protrudes will also darken.

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