Question

A properly exposed photograph is taken at f/16 and 1/120s. What lens opening is required if the shutter speed is 1/60s

Answer 1

To maintain proper exposure when changing the shutter speed from 1/120s to 1/60s, you would need to adjust the aperture from f/16 to f/22.

Step-by-step explanation:

If a properly exposed photograph is taken at f/16 and 1/120s and you want to maintain exposure but change the shutter speed to 1/60s, you need to adjust the aperture to a smaller size to allow less light in due to the longer exposure time. The relationship between shutter speed and aperture can be understood through the concept of 'stops'. A stop can either double or halve the amount of light reaching the sensor. Moving from 1/120s to 1/60s doubles the amount of light because the shutter stays open twice as long. To compensate and maintain the same exposure, you need to decrease the amount of light entering through the aperture. Since f-stops follow a specific sequence where adjacent settings represent a halving or doubling of light, moving from f/16 to the next smaller aperture, which is f/22, will halve the light entering the camera.

Therefore, the required lens opening to maintain the correct exposure when changing the shutter speed from 1/120s to 1/60s is f/22.

Answer 2

f / 8

Step-by-step explanation:

The f-number is the ratio of the lens's focal length to its diameter and is a measure of the intensity of the light reaching the CCD. The smaller the greater the intensity of the light

I = D / f

f-number = f / D

I = 1 / f-number

the energy deposited in the CCD is proportional to the intensity of the light and the exposure time (Δt)

E = I Δt

E = 1 /f-number Δt

in the exercise they indicate that the system is well exposed (the image is sharp and clear) for fo-number = f / 16

Δt = exposure time = 1/120 s

E =
`(1)/(120) \ \ (1)/(fo-number)`

indicate that the exposure time has been changed to Δt = 1/60 s, which should be the f₁-number

E =
`(1)/(60) \ \ (1)/(f_1-number)`

the energy deposited must be the same per location we can equal the expressions

`(1)/(120) \ (1)/(f_o-number) = (1)/(60) \ (1)/(f_1-number)`

f₁-number =
`(60)/(120) \ \ (1)/(fo-number)`

f₁-number = ½ 16

f₁-number = 8

so the system should be set to f / 8