CHAPTER 3 HOW LENSES WORK^39
Rather than bring an object into focus by walking toward or away from it, the pho-
tographer turns a focussing ringthat runs around the barrel of the lens. (Virtually
all digital cameras also provide autofocus, which adjusts the lens without the aid
of the photographer. That’s the subject of the next two illustrations.)15
Attached to the ring is a part of the lens’s barrel with a spiral groove cut into it. As the groove turns with
the focussing ring, it moves a pin from another part of the barrel. The groove-pin arrangement translates
the focus ring’s circular motion into linear motion.16
That motion, in turn, moves the lens elements toward and away from one another. The different
distances among them change the optical effect they have on each other and shift the paths that
light beams take as they careen through the lenses. When the light rays reach the digital image
sensor, the optical changes now bring to focus a part of the scene that’s closer or farther away.17