You are taking pictures of the beach at sunset Before the Su

You are taking pictures of the beach at sunset. Before the Sun sets, a shutter speed of 1/100 s at f/11 produces a properly exposed picture. Shortly after the Sun sets, however, your light meter indicates that the scene is only one fifth as bright as before.

(a) If you don\'t change the aperture, what exact shutter speed will produce a picture with the same exposure?


(b) If, instead, you keep the shutter speed at 1/100 s, what exact f-number will produce a picture with the same exposure?

Solution

(a) The exposure time should be increased by 5 times to compensate for the reduced light, which means the shutter speed should be decreased by a factor of 5.

5 (1/100) sec = 1/20 seconds

(b) We know that the A_f should be 5 times larger than A₀.

A₀ = ¼d² = (f / 11)²

A_f = ¼d_f² = (f / f#)²

A_f = 5A₀

(1 / f#)² = 5 (1 / 11)²

f# = 4.919