How Does Doubling the F Number Affect Exposure Time?

[Lachlan1]

Homework Statement

using the same film and level of illumination two pictures are taken, one with double the f number of the other. for both films to receive the same total light energy, what factor must the required time of exposure change for the picture with double the f number.

Homework Equations

f number = focal length/diameter

The Attempt at a Solution

double the f number couldbe taken as meaning half the diamter. therefore i can guess the answer should be a factor of two. but the f number formula doesn't relate 'total light energy' to anything does it? i thought it was a rating of light intensity.??

 

[tiny-tim]

Hi Lachlan1!

using the same film and level of illumination two pictures are taken, one with double the f number of the other. for both films to receive the same total light energy, what factor must the required time of exposure change for the picture with double the f number.
…
f number = focal length/diameter

If these are with the same camera (it doesn't say), then the focal length is presumably the same, so the diameter is different.

Bigger lens … more light! ​

 

[Lachlan1]

Hi, thanks for that. But the question is asking what is the difference in the time of exposure. do you know what is being referred to as light energy?
-- well i just found out the answer is four, but I am not sure how to get that?

 

[Borek]

Amount of light is directly proportional to the area through which light comes into the camera.

 

[tiny-tim]

(just got up :zzz: …)

… do you know what is being referred to as light energy?

light energy is just another name for the amount of light