CHAPTER 6 HOW TECHNOLOGY LETS THERE BE LIGHT^89
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.5
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 predeter-
mined 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.6
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 lensback and forth to the point where the
most light is concentrated on the subject.3
When the shutter button finally reaches the end of its travel, some flashguns fire a preflash
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.4