Question

A camera takes a properly exposed photo with a 4.0 mm diameter aperture and a shutter speed of 1/250 s. If the photographer focuses on the same point, but the diameter to 8.00 mm, what needs to happen to the shutter speed, if it is still properly exposed?

a. Nothing

b. Increases by 4

c. Decreases by 1/4

d. Increases by 2

e. Decreases by 1/2

Answer 1

Proper exposure implies that a certain number of photons should be incident on the film. If the aperture diameter is increased by a factor of 2, then the area of the aperture will increase by a factor of 4. The incident intensity of light is a constant, so if we do not change the shutter speed, then the number of photons that will be incident on the film will increase by a factor of 4, and the film will be overexposed. To avoid this, we need to reduce the shutter speed by a factor of 4, so that the same number of photons reach the film. Thus, the correct option is c, decreases by 1/4.