Tripling the focal length of a telephoto lens,while holding aperture size constant,will change the light intensity admitted to the film by what factor?
A)0.11
B)0.33
C)3.0
D)9.0
E)0.50

Question

Tripling the focal length of a telephoto lens,while holding aperture size constant,will change the light intensity admitted to the film by what factor?
A)0.11
B)0.33
C)3.0
D)9.0
E)​0.50

Quizplus · Accepted Answer

The light intensity admitted to the film is inversely proportional to the square of the focal length. Tripling the focal length (focal length x 3) would decrease the light intensity by a factor of 9 (3^2), so the new light intensity would be 1/9 of the original, which is approximately 0.11.

Tripling the Focal Length of a Telephoto Lens,while Holding Aperture