When the current hits the electromagnet, the
magnet pulls the spring latch away from the
spring. The spring moves arms attached to
the curtain covering the sensor and pulls the
blades together, exposing the sensor to the
light from the camera’s lens.5
Meanwhile, an electronic counter is adding up the
pulses from an oscillator. The oscillator passes an
electrical current through a material such as quartz or
lithium niobate, both crystals. The current causes the
crystals to vibrate at a constant predictable frequency.6
The time interval between the withdrawal of the
first curtain and the deployment of the second
one creates a slot through which light can reach
the sensor. Only for the longest shutter times,
such as a second or more, is the second curtain
delayed until the first curtain has completely
revealed the sensor. The faster the shutter speed,
the narrower the slot. For a shutter speed of
1/1000 of a second, it actually takes the cur-
tains several thousandths of a second to complete
their jobs, but each section of the sensor, as the
slot passes over it, is exposed for only a single
thousandth of a second.8
CHAPTER 5 HOW DIGITAL EXPOSURE SIFTS, MEASURES, AND SLICES LIGHT 67
When the pulses add up to a number that has
been determined to be the shutter speed for this
photograph, the processor sends a second burst
of electricity to the electromagnet associated
with the second curtain, which is still
collapsed on the second side of the
sensor. Its latch released by the
electromagnet, the spring
spreads the curtain’s
blades across
the surface of
the image
sensor.7