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.??