Law of Reciprocity
- Exposure ∝ Aperture Area × Exposure Time × Scene Luminance
The law of reciprocity describes how light intensity and duration trade off to make an exposure—it defines the relationship between shutter speed and aperture, for a given total exposure. Changes to any of these elements are often measured in units known as "stops"; a stop is equal to a factor of two.
Halving the amount light exposing the film can be achieved either by:
- Closing the aperture by one stop
- Decreasing the shutter time (increasing the shutter speed) by one stop
- Cutting the scene lighting by half
Likewise, doubling the amount of light exposing the film can be achieved by the opposite of one of these operations.
The luminance of the scene, as measured on a reflected light meter, also affects the exposure proportionately. The amount of light required for proper exposure depends on the film speed; which can be varied in stops or fractions of stops. With either of these changes, the aperture or shutter speed can be adjusted by an equal number of stops to get to a suitable exposure.
Light is most easily controlled through the use of the camera's aperture (measure in f-stops), but it can also be regulated by adjusting the shutter speed. Using faster or slower film is not usually something that can be done quickly, at least using roll film. Large format cameras use individual sheets of film and each sheet could be a different speed. Also, if you're using a larger format camera with a polaroid back, you can switch between backs containing different speed polaroids. Digital cameras can easily adjust the film speed they are simulating by adjusting the exposure index, and many digital cameras can do so automatically in response to exposure measurements.
For example, starting with an exposure of 1/60 at f/16, the depth-of-field could be made shallower by opening up the aperture to f/4, an increase in exposure of 4 stops. To compensate, the shutter speed would need to be increased as well by 4 stops, that is, adjust exposure time down to 1/1000. Closing down the aperture limits the resolution due to the diffraction limit.
The reciprocity law specifies the total exposure, but the response of a photographic material to a constant total exposure may not remain constant for very long exposures in very faint light, such as photographing a starry sky, or very short exposures in very bright light, such as photographing the sun. This is known as reciprocity failure of the material (film, paper, or sensor).
Read more about this topic: Science Of Photography
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