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

18000 seconds.

Step-by-step explanation:

Focus to diameter ratio

`(f)/(d_1)=4.0`

Shutter time = T₁ = 1125 s

Changing the focus to diameter ratio gives us

`(f)/(d_2)=16`

Ratio of the areas

`(A_1)/(A_2)=(d_1^2)/(d_2^2)\\\Rightarrow (A_1)/(A_2)=(4^2)/(16^2)\\\Rightarrow (A_1)/(A_2)=(1)/(16)`

Amount of light is the product of time and area

`\T_1A_1=T_2A_2}\\\Rightarrow T_2=T_1(A_2)/(A_1)\\\Rightarrow T_2=1125* 16\\\Rightarrow T_2=18000\ seconds`

Shutter speed will be 18000 seconds.