Question

The exposure time of a camera lens at the $\frac{f}{2.8}$ setting is $\frac{1}{200}$ second. The correct time of exposure at $\frac{f}{5.6}$ is

• A. 0.4 sec
• B. 0.02 sec
• C. 0.002 sec
• D. 0.04 sec

Answer 1

Answer: (B)

0.02 sec

Answer 2

Time of exposure

$\propto (f.number)^{2}$ $\Rightarrow \frac{t_{2}}{t_{1}} = \left( \frac{5.6}{2.8} \right)^{2} = 4$

$t_{2} = 4t_{1} = 4 \times \frac{1}{200} = \frac{1}{50}$sec = 0.02 sec.