# F number and light intensity

1. Jun 6, 2010

### Lachlan1

1. The problem statement, all variables and given/known data
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 recieve the same total light energy, what factor must the required time of exposure change for the picture with double the f number.

2. Relevant equations

f number = focal length/diameter

3. 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.??

2. Jun 6, 2010

### tiny-tim

Hi Lachlan1!
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!

3. Jun 6, 2010

### 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 im not sure how to get that?

Last edited: Jun 6, 2010
4. Jun 7, 2010

### Staff: Mentor

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

5. Jun 7, 2010

### tiny-tim

(just got up :zzz: …)
light energy is just another name for the amount of light