I take two pictures of my dog Kaycee.I use the same film and level of illumination for both pictures,but I quadruple the f-number for the second picture.By what factor will the required time of exposure change if the film is to receive the same total light energy?
A)0.06
B)0.25
C)4.0
D)16.0
E)3.0

Question

I take two pictures of my dog Kaycee.I use the same film and level of illumination for both pictures,but I quadruple the f-number for the second picture.By what factor will the required time of exposure change if the film is to receive the same total light energy?
A)0.06
B)0.25
C)4.0
D)16.0
E)​3.0

Quizplus · Accepted Answer

According to the inverse square law, the amount of light received by the film is proportional to the square of the aperture diameter, which is proportional to the f-number. Quadrupling the f-number reduces the amount of light received by a factor of 16 (2² × 2² = 16). Therefore, to maintain the same total light energy on the film, the required time of exposure needs to be increased by a factor of 16. Hence, the answer is 16.0.

I Take Two Pictures of My Dog Kaycee