Aperture of lens of a camera is f and its exposure time is `(1)/(60)` s. If aperture becomes 1.4 f, then what will be the exposure time ?

Options

A. `(1)/(42)` s
B. `(1)/(56)` s
C. `(1)/(72)` s
D. `(1)/(31)` s

Answer

A, C, D

Explanation

Exposure time `t prop f^2`
`therefore ((t_2)/(t_1))=((f_2)/(f_1))^2`
`therefore t_2=t_1xx((f_2)/(f_1))^2`
`=(1)/(60)xx((1.4f)/(f))^2`
`=((1.4)^2)/(60)=0.03266`
`therefore t_2=(1)/(31)` s