If a camera lens lets in an amount of light $I$ when it is set to $f / 4$, then how much light will it let in when it is set to $f / 8$ ?
A) $I / 4$
B) $4 I$
C) $16 I$
D) $I / 2$
E) $2 I$

Question

Quizplus · Accepted Answer

The amount of light a lens lets in is inversely proportional to the square of the f-number. So, when the f-number is doubled from $f/4$ to $f/8$, the amount of light decreases by a factor of $4$ ($(8/4)^2 = 4$), making the correct answer $I/4$.

If a Camera Lens Lets in an Amount of Light III

If a Camera Lens Lets in an Amount of Light $I$