Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Camera aperture/shutter speed

  1. Aug 17, 2006 #1
    camera settings:
    shutter speed: 15, 30, 60, 125, 250. 500, 1000
    aperture f/: 2.8, 4, 5.6, 8, 11, 16.

    a) which aperture setting likes 8 times more like to enter than f/8?
    = f/2.8

    b) which aperture allows 4 times less light to enter than f/5.6?
    = f/11

    c) which shutter speed permits twice as much light to enter camera as 250?
    = shutter speed 125

    d) which shutter speed permits 1/4th as much light to enter camera as 125?
    = 500

    e) camera is loaded with ASA 1000 film and a perfect exposure is taken at 500 and f/8. state two other combinations of settings equivalent to this.
    1 = shutter speed 250 with f/5.6
    2. = shutter speed 125 with f/4


    g) the film is changed to ASA 120. state one combination of camera setting sthat could be used to take a perfect exposure, assuming there is no change in the available light from the situation in part (e).
    = shutter speed 500 and f/2.8

    can someone tell me if im on the right track? im especially iffy on (e). (i have my answers in italics)

    merci

    ~Amy
     
  2. jcsd
  3. Aug 18, 2006 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Each successive f/stop halves the aperture area so it halves the amount of light. (f/stop is inversely proportional to the diameter of the aperature. The area, of course is proportional to the square of the diameter. So f/4 is twice the diameter of f/8 so it allows 4 times the amount of light than f/8).

    The exposure time is the reciprocal of the shutter speed. So 125 is 4 times the exposure of 500 (1/125 sec. vs. 1/500 sec.).

    So your answers to a-d are right. But you have e backward. f/8 and 500 is the same as doubling the exposure time but halving the aperature area; or halving the exposure time and doubling the aperature etc.

    For part g, the ASA number tells you the exposure time (at a certain f/stop for a certain amount of light). So ASA 120 requires 1000/120 more light than ASA 1000 to give a proper exposure. You should be able to work it out from that.

    AM
     
  4. Aug 20, 2006 #3
    thanks Andrew!

    for e) i got shutter speed 1000 with f/5.6., and shutter speed 124 with f/16.

    and for g) i have shutter speed 60 with f/16.

    im pretty sure my answers are correct now :smile:

    ~Amy
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook