# Amount of light

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

1. ### Man-wai ChangGuest

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

2. ### Nervous NickGuest

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

3. ### Jeremiah DeWitt WeinerGuest

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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-- William Cowper, 1731 - 1800

Jeremiah DeWitt Weiner, Oct 15, 2010
4. ### John McWilliamsGuest

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.

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

--
john mcwilliams

John McWilliams, Oct 15, 2010
5. ### David RuetherGuest

"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
6. ### Dick AlvarezGuest

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