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Homework Help: Optical Instruments-Cameras

  1. Apr 8, 2007 #1
    You are taking pictures of the beach at sunset. Before the Sun sets, a shutter speed of 1/125 s at f/11 produces a properly exposed picture. Shortly after the Sun sets, however, your light meter indicates that the scene is only one third as bright as before.
    (a) If you don't change the aperture, what exact shutter speed will produce a picture with the same exposure?

    (b) If, instead, you keep the shutter speed at 1/125 s, what exact f-number will produce a picture with the same exposure?

    2. f-number = focal length / Diameter of aperture

    3. I'm not sure what the significance of the picture being 1/3 as bright after the sun sets is. But, I do know that when you change the shutter speed, you need to change the area of the aperture to compensate if you want to maintain the same exposure. Other than that, I don't know where to start on this problem. I would appreciate any help!
  2. jcsd
  3. Apr 8, 2007 #2
    hmmm. To maintain same exposure level I would think require having same number of photons strike the film in each case. Assuming "brighness" here is a measure of the number of photons that strikes the film, how much time to capture the same number as before. Remember shutter speed is a usually a small fraction of a second, 1/125 is 80 millseconds open time.

    Same principle applies for next question. How much larger would opening of aperture have to be to collect the same number of photons as in original case.
  4. Apr 8, 2007 #3
    Yes, I understand that part...you want the same amount of light / photons. I thought the answer to (a) might be 1/375, to make the shutter speed 3x faster so that less light is able to get in and the photo is 1/3 as bright. But, I know that's wrong...
  5. Apr 8, 2007 #4
    yea, thats why I inverted the "shutter speed" to msec open time. Triple the open time, then take reciprocal to get shutter speed..
  6. Apr 8, 2007 #5
    OK, inverting the shutter speed makes it much more easy to understand! I get the problem now and got the right answers. Thank you very much for your help!
  7. Apr 8, 2007 #6
    No problem, happy to be of help.
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