First slide

The optical instruments

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

Moderate

A

0.4 sec
B

0.02 sec
C

0.002 sec
D

0.04 sec

Solution

$Time of exposure \propto (f number)^{2} \Rightarrow \frac{t_{2}}{t_{1}} = {(\frac{5 .6}{2 .8})}^{2} = 4 t_{2} = 4 t_{1} = 4 \times \frac{1}{200} = \frac{1}{50} \sec = 0 .02 \sec$

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