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No matter how sensitive a light sensor is, none can find an image in absolute darkness. When it's truly dark, there are no photons available to register on it. To allow you to take photographs even when insufficient light is available, most digital cameras incorporate their own photon source—a flash. As the name implies, a flash unit makes a flash of bright light. Although once based on contained explosions sealed inside small light bulbs, today's flash units rely on the excitation of xenon gas to produce a brief but bright pulse of light. The brevity of the flash allows the near-instant illumination to stop motion (the object illuminated can't move far during the brief pulse during which it is illuminated), much like a mechanical stroboscope apparently stops motion. Repeating flashes can thus operate as stroboscopes, too, and consequently some old-time photographers call their xenon-based flash units strobes. Another common term is speedlight, probably created by anxious marketeers wanting consumers to disregard the lengthy period required by their products to recharge between flashes.

When one is judging flash units or the flash incorporated into a camera, several issues are important. These include the guide number, exposure control, red-eye reduction, and the possibility of external connections.

The guide number of a flash unit allows you to determine the aperture setting of your camera for proper exposure using the flash. Guide numbers depend on film speed, so you'll see them listed as something like "Guide number 50 with ASA 100 film." The guide numbers for the built-in flashes of digital cameras are calibrated to the sensitivity of the camera's image sensor, so they don't have to include an ASA number, although some manufacturers note the ASA equivalent. The guide number represents the maximum distance away a subject can be for a proper flash exposure multiplied by the aperture of the camera lens you want to use. For example, a guide number of 80 indicates that, at an f-stop setting of f/8, your subject should be no more than ten feet away.

Most of the time you don't have to bother with guide numbers. All digital camera flashes are automatic. They are designed to provide just the right amount of light to properly illuminate your subject when taking a picture. But the guide number does tell you the maximum distance at which the automatic flash exposure system will work. Some manufacturers simply list this maximum distance and forego guide numbers entirely so you don't have to bother with the math.

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