The speed of a camera lens measures its ability to photograph in dim light. Speed is characterized by $f$ -ratio, also called the $f$ -number, defined as the ratio of focal length $f$ to lens diameter $d$. Thus an $f / 2.8$ lens, for example, has diameter $d=f / 2.8 .$ The actual amount of light a lens admits depends on its area $A$, but the inverse-square law shows that the light intensity at the camera's imaging sensor is proportional to $A / f^{2} .$ Most cameras have an adjustable iris that obscures part of the lens to change the f-ratio in response to available light. Point-and-shoot cameras adjust the f-ratio automatically, but serious photographers use their camera's manual f-ratio adjustment (Fig. 31.37 ). Stopping down is the photographer's term for reducing the lens area using the adjustable iris. A 35 -mm camera lens. The numbers from 22 to 2.8 at the bottom are values for the f-ratio, $f / d .$ Turning the ring with these numbers adjusts the iris that covers the outer part of the lens, thus changing the f-ratio. (FIGURE CAN'T COPY)
Zooming your camera's lens for telephoto shots increases the focal length. With no change in the lens area, this will
a. increase the f-ratio and increase the lens speed.
b. decrease the f-ratio and decrease the lens speed.
c. increase the $\mathrm{f}$ -ratio and decrease the lens speed.
d. not change the $\mathrm{f}$ -ratio or the lens speed.