64 PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES
How a Diaphragm Controls Light
Picture a shy sunbather standing in his skivvies in front of a window hoping to get a tan.
The size of that window does the same job as your camera’s diaphragm. Assuming the
curtains are open—more about them in the next illustration—the window size controls how
much light gets into the room; in much the same way, a camera’s diaphragm changes the
size of a hole at its center to control how much light gets into your camera. Too little
light, and wonderful details in your photo are buried in the shadows. Too much
light, and everything washes out to a pale scene devoid of color and
shape. The diaphragm has only the most fleeting slice of a second to
collapse from its largest opening to its smallest, and then it must retreat
again to be ready for the next instant to be captured. Here’s how it
does it in one of the most common diaphragm designs.
1 Light that has been focused through the lens of a
camera must pass through a round diaphragm
on its way to being registered as an image on
the camera’s sensor. How much light makes its
way into the camera is controlled by an opening
in the diaphragm. The opening, called the
aperture, serves the same function as the
human eye’s pupil. The diaphragm is the camera
version of the eye’s iris.