The electrical current travels to one of two
electromagnets that are paired with two
sets of curtains. The curtains are made of
three to five or more thin blades of alu-
minum, titanium, or metal-coated plastic.
The blades making up one of the curtains
are spread out—but overlapping one
another—to cover the image sensor so that
no light reaches it.2
Mounted on the oppo-
site side of the sensor
from the first curtain,
the second curtain’s
blades are collapsed
so the curtain takes up
little room in the
already crowded inner
chamber of the camera.3
Both curtains are spring-loaded, but the springs are
restrained by metal latches that hook into the
spring mechanism so it can’t move. Poised above
each latch are the two electromagnets. The first
burst of electrical current goes to the magnet asso-
ciated with the curtain that is already open and
covering the image sensor.4
Like most windows, the one in our sunbather’s apartment
has curtains, a necessity to let light in when you want it
and to keep light out when you don’t. Our digital camera
has curtains, and they serve the same function as those in
the window. Most of the time they’re closed to keep the
light off the image sensor, whose pixels are hundreds of
times more sensitive to light than the palest skin. Then for a
fleeting moment, a mere fraction of a second, the curtains
open with the sudden swiftness of a conjuror and the
precision of a surgeon. It’s in that moment that the camera
captures one fleeting moment of time.
How a Shutter Slices Time
66 PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES
At the same time electronic signals
race from one of a digital camera’s
microprocessors to tell the diaphragm
what size to make the aperture, other
signals stream to the camera’s shutter
to tell it how long it should open its
curtains to expose the image sensor
hiding behind them to the light stream-
ing through the aperture. There are
various designs for camera shutters,
but the most common type in the
35mm cameras that have evolved into
the digital camera is the Copal
square shutter, named after the
Tokyo company that developed it in the
1960s. That’s what we’ll look at here.1