Amount of light

Discussion in 'Digital Photography' started by Man-wai Chang, Oct 15, 2010.

  1. Is there a simple physics equation that relates ISO, white balance,
    aperture and exposure time?

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    Man-wai Chang, Oct 15, 2010
    #1
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  2. Man-wai Chang

    Nervous Nick Guest

    On Oct 15, 10:32 am, Man-wai Chang <> wrote:
    > Is there a simple physics equation that relates ISO, white balance,
    > aperture and exposure time?



    42.

    HTH.

    --
    YOP...
    Nervous Nick, Oct 15, 2010
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  3. Paul Furman <> wrote:
    > f/8 > f/5.6 > f/4
    > (not sure I can explain the math on that one, just memorize them)


    Each whole stop number is approximately 1.4 times the previous one
    (usually rounded a bit for simplicity). Start from 1, and you get f/1.4,
    f/2, f/2.8, f/4, and so forth.

    OK, so why 1.4? That's (roughly) the square root of 2. (Actually,
    1.41421356...it's irrational, it goes on forever, but 1.4 is close
    enough for government work.) OK, so why is _that_ important? Because
    to double the area of a circle, like the circle that is the aperture of
    a lens, you have to increase the radius of the circle by - you guessed
    it - multiplying by 1.4.

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    Jeremiah DeWitt Weiner, Oct 15, 2010
    #3
  4. On 10/15/10 PDT 10:12 AM, Nervous Nick wrote:
    > On Oct 15, 10:32 am, Man-wai Chang<> wrote:
    >> Is there a simple physics equation that relates ISO, white balance,
    >> aperture and exposure time?

    >
    >
    > 42.
    >
    > HTH.


    No!! He asked for the equation, not The Answer.....

    OP: There's a relationship among ISO, Ap and Tv, but not WB.

    --
    john mcwilliams
    John McWilliams, Oct 15, 2010
    #4
  5. "bobwilliams" <> wrote in message
    news:...
    > Man-wai Chang wrote:


    >> Is there a simple physics equation that relates ISO, white balance, aperture and exposure time?


    As others have pointed out, not WB, but f-stop, shutter speed, and
    "sensor speed" (as rated in ASA/ISO) do relate together and with the
    amount of light available. On a hand-held meter with an analogue dial
    and meter pointer and scale, the relationship among the various things
    could be easily seen. Most obvious is that doubling of the light, the time
    the shutter is open, and the sensor sensitivity each doubles the exposure,
    but reducing the f-stop number by 1.4X also doubles the exposure, since
    the aperture area for the light to pass through is doubled in this way.

    > There is a "Rule of Thumb" that relates these variables
    > It is called the "Sunny 16 Rule"
    > In bright sunlight, The correct exposure for an average picture is f16 at a speed of 1 / ISO number ......or some equivalent
    > exposure.
    > EX: At ISO 100, the speed would be 1/100 sec and the f stop at f16
    > OR
    > 1/200 at f11......or 1/400 at f8........
    > You get the picture
    > Bob Williams


    I have never found this to be accurate (maybe our local "bright sunlight"
    is less bright than yours...;-). Ours (on a BRIGHT, CLEAR day) is
    closer to f-11+1/2...
    --DR
    David Ruether, Oct 15, 2010
    #5
  6. Man-wai Chang

    Dick Alvarez Guest

    Man-wai Chang wrote <<Is there a simple physics
    equation that relates ISO, white balance, aperture and
    exposure time?>>.

    White balance does not enter directly into it, although
    color seriously complicates the exposure situation.

    Exposure is highly subjective, and it also depends on
    the light transmission of the lens (beyond just the
    f-number). It also depends on the illumination of the
    object being photographed, and on its reflectance. I do
    not know whether the picture format (.jpg, .RAW, etc.)
    affects the ideal exposure. You should experiment with
    your particular cameras and lenses. But here is some
    information on exposure.

    Wikipedia has some good information on photographic
    exposure, ISO and ASA and DIN ratings, etc., but Wikipedia
    admits that it could use some clarification.

    Wikipedia speaks of "ISO arithmetic scale (ASA scale)".
    so I suppose that they are at least very nearly the same.

    In the 1980s, my company's staff photographer obtained
    the following equation from one of his colleagues. Our
    staff photographer seemed to be serious and reliable, but
    I did not know his colleague. The equation:

    (incident light intensity on the object being
    photographed)/(1 foot-candle) *
    (film speed)/(1 ASA unit) *
    (exposure time)/(1 second) *
    (object reflectance)

    = (f-number)^2 * 4.6875

    Note that reflectance and f-number are dimensionless.

    In the number "4.6875", it seems very excessive to use
    5 significant figures.

    Example:

    incident light intensity: 125 foot-candles
    sensitivity: ASA 100
    exposure time: 1/30 second
    f-number: 4.0
    object reflectance: 0.18

    Linearity tends to fail for film ASA speed below
    about 2.5. Film reciprocity tends to fail for long
    exposures with low light intensity striking the film. I
    don't know about either of these effects with digital
    cameras.

    If anybody has better information, please post it here!
    Dick Alvarez, Oct 17, 2010
    #6
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