CHAPTER 5 HOW DIGITAL EXPOSURE SIFTS, MEASURES, AND SLICES LIGHT 65
Mounted between two of the lens’s elements is the diaphragm, a circular
mechanism made up of tissue-thin metal leaves that overlap each other
like the petals of a flower to form the aperture. The leaves are
tapered and curve in toward the center of the aperture. Their
design creates a rounder circle when the aperture is
bigger and when a diaphragm is made of more
leaves. When it closes down, the edges of the
leaves are more apparent and show up as multi-
sided figures in the bokeh, or unfocused parts,
of a photo. Better lenses have a dozen or
more leaves; cheaper ones have as few
as five.2
Ordinarily, the aperture is open fully
so that as much light as possible
reaches the viewfinder and the photogra-
pher can see the scene easily. Some cam-
eras have a preview buttonthe
photographer pushes to close the aperture to
whatever it’s set for. The preview dims the
image, but it gives the photographer an approxi-
mation of the depth-of-field.3
ring turns, to make the aperture
wide or narrow. The size of the
opening is crucial not only to the correct
exposure of a photograph, but also—as you glimpsed in the previous chapter—to
the image’s sharpness.When the photographer presses the shutter but-
ton, one of the camera’s processors reads the
exposure settings and sends specific electrical
signals to a mechanism such as a tiny servo
motormounted inside the lens on one of two
rings running around either side of the
diaphragm. Using a method called pulse
coded modulation, the servo times the length
of electrical pulses and instantly rotates
a shaft a precise amount, turning
theactuating ringto an
equally exact angle.4
Just as muscles
tug the iris so that it
contracts or expands,
changing the size
of the pupil, the
tugs and prods
of the servos control
the size of the
aperture.
Each of the
leaves of the
diaphragm
typically has
one peg that
fits into the
actuating ring and
another peg, on the
opposite side of the
leaf, attached to a
ring that does not
move. When the
actuating ring turns,
it makes each leaf pivot
around the immobile pins.
All the leaves pivot in unison,
depending on the direction the5
F-stops
The various sizes the aperture is set to are called f-stops, usually expressed as f/ followed by a num-
ber somewhere between 1.2 and 32. The number represents the focal length of the lens divided by
the diameter of the aperture’s opening. If the aperture on a 50mm lens is set to be 25mm wide, the f-
stop is 50/25, or f/2. The ratio lets the same amount of light fall on each square millimeter of the
film or image sensor, although the measured aperture from lens to lens can be different. A 150mm
aperture on a 300mm lens exposes the picture with the same brightness as the 25mm opening on
the 50mm lens (300/150 = f/2). This seemingly easy system loses some of its simplicity when you
realize that bigger apertures have smaller numbers and vice versa. Each smaller f-stop lets in twice
as much light as the next larger f-stop and, of course, a setting of f/8 lets in half as much light as
f/4. If you want to see f-stops in action, visit http://www.photonhead.com, where you can experiment with
its SimCam without having to go to the trouble of actually photographing anything.