Question

A light meter reports that a camera setting of 1125s at f/4.0 will give a correct exposure. But the photographer wishes to use f/16 to increase the depth of field. What should the shutter speed be?

Answer 1

Answer:

18000 seconds.

Explanation:

Focus to diameter ratio

$\frac{f}{d_1}=4.0$

Shutter time = T₁ = 1125 s

Changing the focus to diameter ratio gives us

$\frac{f}{d_2}=16$

Ratio of the areas

$\frac{A_1}{A_2}=\frac{d_1^2}{d_2^2}\\\Rightarrow \frac{A_1}{A_2}=\frac{4^2}{16^2}\\\Rightarrow \frac{A_1}{A_2}=\frac{1}{16}$

Amount of light is the product of time and area

$\T_1A_1=T_2A_2}\\\Rightarrow T_2=T_1\frac{A_2}{A_1}\\\Rightarrow T_2=1125\times 16\\\Rightarrow T_2=18000\ seconds$

Shutter speed will be 18000 seconds.