Question

Focal number of the lens of a camera is 5f and that of another is 2.5 f. The time of exposure for the second camera is $\underset{–}{}$ if that for the first is $\frac{1}{200}s$

$(Givenf=\frac{focallength}{aperature})$

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

Answer 1

The correct option is B $\frac{1}{800}s$

$f$ number decreases by 2

$\therefore$ time of exposure should decrease by $\left(2^2\right)$

$\therefore t_{new}=\frac{1}{4}\times \frac{1}{200}=\frac{1}{800}s$