How flash works fore best exposure

Pressing the shutter button does more than cause the shutter to open. First it signals that the photographer is happy with the focus and that the photo is framed properly in the viewfinder. The circuits and motors that control focus instantly freeze. Some cameras take a reading of the ambient light levels. These are the light from any source, sun, candles, fluorescent lights—anything except the flash itself. Many camera/flash combinations are able to concentrate on the light levels in the background, where the flash will not reach. Based on the type of exposure the photographer has chosen by selecting aperture or shutter priorities, or scene settings for sunsets, action shots, or as many as two dozen other possible scenes, the camera decides on the best setting for the main subject and the surroundings, information it shares with the flash.

Some flash units determine the most important part of a scene from reading the distance the camera is focusing on when the camera tells the flash the distance it's focused on. Or a flash may send out infrared beams, measure their return time, and apply some photography common sense. There may be mountains in the distance and an elk 20 feet away. But the flash's microprocessor brain sees only the elk. Because its light could never illuminate faraway mountains, the flash settles on something within its range—the elk—and sets its light intensity accordingly.

A camera such as the Minolta 7D or its heir, the Sony Alpha 1, is designed to mate with an integrated but separate flash. The flash uses a complex but precise and quick way to calculate the required flash using the camera's through the lens (TTL) light metering. The camera commands the flash to fire a series of 14 pre- flashes, one for each of the 14 exposure sensors used in the 7D. The readings from those sensors, combined with the distance to the subject reported by the focus mechanism, the position of the subject in the viewfinder, and the ambient light brightness, allow the camera to determine the exact amount of light needed for a proper exposure—the threshold limit stage—and how much of the charge in the unit's capacitor is needed to produce the correct amount of light.

Other flash units depend on a thyristor, a transistor and switch that's snuggled in the flashgun's circuitry and staring a photodiode eye toward the scene being shot. The thyristor measures the amount of light bouncing back toward the camera. When the predetermined amount of exposure has been reached or the TTL system in the camera gives the word, the thyristor switches off the flow of electricity that has been feeding the flash. The thyristor prevents the escape of whatever charge still remains in the capacitor so the flash takes less time to replenish the capacitor for the next shot.

Using the information it gathers from the camera's focusing and its own sleuthing, many high-end flash units respond by activating small motors that move its Fresnel lens back and forth to the point where the most light is concentrated on the subject.

When the shutter button finally reaches the end of its travel, some flashguns fire a pre-flash while the shutter is still closed. This is the camera's reality check. Its autoexposure sensors determine the real, exact brightness of the flash and adjust exposure settings accordingly. Other cameras forgo the preflash, and figure out the correct amount of light to produce on the fly from readings taken through the lens. On less expensive point and shoot cameras, a light meter mounted on the front of the camera reads the light levels. Some flashes store information about how long their bursts of light should last to achieve proper exposures.

Techniques of using cameras flashes