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Using the Flash on Your Camera
Fast ISO speeds and fast lenses make it easy to work in all but the most dim lighting conditions, but there are times when turning to electronic flash is a better and more predictable alternative. For example, because the duration of flash is so brief (often measured in thousandths of a second), you can use it to get sharp pictures of moving subjects in very dark surroundings, like dancers in the ballroom of a cruise ship. The drawback of flash is that it produces a comparatively harsh and obtrusive light.
Most point-and-shoot and many DSLR cameras have a built-in flash that is capable of producing good results over a modest distance range, typically from about three to 15 feet with an ISO 100 film. Accessory flash units for DSLRs are considerably more powerful and sophisticated than built-ins, frequently providing a maximum shooting range of 100 feet or more.
Most accessory flash units these days are "dedicated," meaning that they're designed for use with specific brands and models of cameras and use incredibly sophisticated circuitry to communicate between camera and flash. The flash automatically knows, for example, the ISO setting of the camera and sets its output accordingly. In addition, virtually all digital cameras use "TTL" (through-the-lens) light metering, so that the camera measures the reflected flash at the sensor plane and shuts off the unit when enough light has been supplied. Flash exposure in general is very accurate.
A problem peculiar to taking portraits with any type of electronic flash is the phenomenon known affectionately as "red eye," the somewhat satanic red glint in subjects' pupils often seen in photos. The effect occurs as the flash reflects off the rear surface of the retina. Many flash units, both built-in and accessory, have a red-eye-reduction feature that uses a series of brief preflashes to constrict the pupil, thereby eliminating the effect. This is a feature worth paying extra for if you photograph people often.Next: "Fill-In Flash"