Shutter Time vs Opening

Discussion in 'Beyond the Basics' started by gdriverx, Jun 25, 2010.

  1. gdriverx

    gdriverx TPF Noob!

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    does anyone know of a graph or chart that compares the shutter times and exposure with lumens as the output. I.e. I wanna see how much difference a 1.8 opening will do compared to a 3.5, how much i would be able to lower the shutter time and still get the same lumens(brightness). Thinking of getting a 1.8 50mm nikkor and I wanna know exactly how much I can lower my shutter speeds.
     
  2. maris

    maris TPF Noob!

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    The brightness difference at the film plane between apertures f3.5 and f1.8 is, according to my calculator, 3.78x. Were you stop down from 1.8 to 3.5 you would have to extend your shutter-open time by the same factor so that the amount (amount = brightness x time) of light arriving at the film plane does not change.

    For a constant exposure outcome your shutter speeds and apertures are linked by a reciprocal rule: half the aperture needs double the time, double the aperture needs half the time, and so on. Apertures are doubled or halved every time the focal ratio (the f number) changes by a factor of 1.4142.

    Typical (approximate) focal ratio steps that represent halving the aperture each time go like this: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, and so on.
     
  3. gdriverx

    gdriverx TPF Noob!

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    so essentially for every time you decrease the aperture number by 1.4 (opening it up) you can cut the shutter time in half? so 1.7/1.4 ~ 1 meaning you could cut the shutter time down to half and get the same lumens?
     
  4. Garbz

    Garbz No longer a newbie, moving up!

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    NO not everytime you decrease by, every time you divide by 1.414 (squareroot of 2).

    E.g one stop brighter than f/22:
    22 / 1.414 = 15.56 = f/16 (rounded)

    one stop brighter than f/2.8:
    2.8 / 1.414 = 1.98 = f/2 (rounded)

    two stops darker than f/4:
    4 * (1.414)^2 = 7.997 = f/8 (rounded)

    Where a stop is a perceived halving or doubling in exposure. I.e. double the shutter speed or half the shutter speed, or half the ISO, or double the ISO.
     
  5. djacobox372

    djacobox372 No longer a newbie, moving up!

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    Each f-stop is a 2x increase/decrease in shutter speed. The f-stop scale is as follows:

    1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64

    At 1.4 you could use a shutter speed slightly more then 5 times faster then at 3.5 (1/500th, vs. 1/90th)

    Here's a nifty exposure calculator:Photographer's exposure calculator
     

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