Question

If the focal length of a lens of a camera is $5 f$ and that of another is $2.5\, f$, what is the time of exposure for the second, if for the first one is $\frac{1}{200}$ s? (where $f$ is focal length/unit aperture)

• A. $\frac{1}{200}$
• B. $\frac{1}{800}$
• C. $\frac{1}{6400}$
• D. $\frac{1}{3200}$

Answer 1

Answer: (B)

$\frac{1}{800}$

Answer 2

Ratio of time of expoiusers
$\frac{t_{1}}{t_{2}}=\left(\frac{f_{1}}{f_{2}}\right)^{2}=\left(\frac{5 f}{2.5 f}\right)^{2}$
$\Rightarrow \frac{1 / 200}{t_{2}} =(2)^{2}$
$\Rightarrow t_{2}=\frac{1}{800} s$