Photodiodes catch light from the subject of the photo-to-be. Those photodiodes churn the ethereal light into thin
currents of electricity. If the photographer is changing the zoom on the lens, the currents may vary as the zoomed
frame takes in a changing amount of light.1
Those electrical signals shoot to a microprocessor that determines from the strength of the signals how
much light is streaming from the subject to the lens. Before determining which exposure to set that corre-
sponds to the light meter signals, the camera consults information coming from several other sources.2
Special programs: Some cameras include routines for altering exposure for
situations most photographers, and most light meters, would be at a loss to handle
entirely on their own, such as fireworks or candlelight.3
Exposure compensation: The processor checks to see if the photog-
rapher has ordered that exposure be set for a little lighter or darker than
the light meters themselves would dictate.4
Bracket exposures: This setting tells the processor whether the pho-
tographer has ordered up additional exposures a stop (or fractions of a
stop) above or below what the photodiode devices say the proper expo-
sure should be.5
ISO setting: On many cameras, the photographer has a choice of
ratings referred to as International Order for Standardization (ISO).
They are the digital camera’s equivalent of ASA ratings on film.
Although it’s convenient to think of ISO settings as determining how
sensitive the image sensor will be to the light that washes over it, in fact
the sensor becomes no more sensitive. Instead, the electrical signals that
the sensor generates are amplified, an important distinction as you’ll
see later. But all in all, the greater the number for an ISO setting, the
less light the shutter and aperture must admit.6
Exposure mode: The exposure mode setting tells the processor whether the photographer has already claimed the right to control
the setting of the shutter or diaphragm, or both.7
(^8) Built-in flash: The processor does a quick check on the flash unit to determine whether the flash is open and ready to fire if need be.
Shutter button lock:The processor must check to see if the photographer is holding the shutter button down halfway. If so, the
processor ignores changing signals from the light meters and autofocus and bases the exposure settings on the light readings at the
time the photographer locked exposure and focus by semipressing the button.
9
Based on all this information, when the photographer presses the button completely to take the picture, the processor sends precise
electrical signals to motors and switches controlling the diaphragm, shutter, and image sensor. The diaphragm closes to the proper
aperture, and the shutter opens and closes for a precise slice of time. The image sensor, its pixels excited by the light that bursts
through the lens, quickly sends the electrical current that the pixels produce in three streams representing red, blue, and green to
be converted into digital values that are multiplied by any factor demanded by an ISO setting and stored in memory. Millions of
loitering electrons, in less than a second, have become a photograph.
10
CHAPTER 5 HOW DIGITAL EXPOSURE SIFTS, MEASURES, AND SLICES LIGHT 73