Reciprocity Law in Photography

Fujifilm X-T2 demonstrating constant exposure and reciprocity law — This animation demonstrates the reciprocal relationship between the aperture, shutter speed, and ISO and its effect on effective exposure (image brightness). The first two frames start at ƒ/5.6, ISO 400, and 1/500 s. Raising the ISO to 800 (and then 1600) requires adjusting the shutter to 1/1000 s (and then 1/2000 s) to maintain constant effective exposure. In the frames where the ISO remains static, a constant exposure is maintained despite adjustments because as the aperture f‑number is increased (reducing light), the shutter speed is slowed (increasing light) proportionately: ƒ/5.6 and 1/2000 s; ƒ/8 and 1/1000 s; ƒ/11 and 1/500 s; ƒ/16 and 1/250 s. The process reverses to the start before repeating anew.

Combining the aperture and shutter speed

At the beginning of this chapter, you were introduced to the formula Exposure = Intensity × Time. This equation expresses a reciprocal relationship between the intensity of light passing through the lens and the duration that light is permitted to fall upon the image sensor. It demonstrates that you can achieve equivalent exposures by varying both the aperture and shutter speed in inverse proportion to one another. Given their reciprocal relationship, we can make the following statements:

If you decrease the light intensity, exposure time must be increased proportionally to produce the same total exposure; and
If you increase the light intensity, exposure time must be decreased proportionally to create the same overall exposure; and so on.

To determine whether the corresponding changes are inversely proportional, refer back to the concept of the photographic stop as a unit of relative change. When considering the total exposure of the image sensor (and even effective exposure, if discussing ISO), a stop is a stop—it represents either doubling or halving the exposure—irrespective of whether you’re changing the aperture or shutter speed. When considering the total amount of light acting upon the image sensor, you may add one stop by opening the aperture from ƒ/8 to ƒ/5.6, or you may add one stop by increasing exposure time from 1/1000 s to 1/500 s, or you can add two stops by doing both. With this in mind, the statements above may be rephrased with the following examples:

If decreasing the light intensity from ƒ/8 to ƒ/11 (-1 EV), exposure time must increase from 1/250 s to 1/125 s (+1 EV); and
If increasing the light intensity from ƒ/5.6 to ƒ/2.8 (+2 EV), exposure time must decrease from 1/250 s to 1/1000 s (-2 EV); and so on.

Reciprocity law, aperture and shutter speed trade-offs — This series of images demonstrates the trade-offs or compromises required when balancing the exposure using reciprocal variations of the aperture and shutter speed. A) The relatively fast shutter speed freezes the out-of-focus car in place, but the large ƒ/2.8 aperture renders the background out of focus. B) The slower shutter speed (1/60 s) is adequate enough to give the car some semblance of speed while keeping it recognizable, and the smaller ƒ/5.6 aperture renders the background clearer. C) The small ƒ/16 aperture casts the background into greater focus (although not perfectly so), but at the cost of requiring a much longer 1/4 s exposure, which depicts the passing car as a distinct blur. You must raise your camera’s ISO speed when you require a small aperture and fast shutter speeds.

Exposure is a zero-sum game

The examples above demonstrate that equivalent exposure is a zero-sum game: to maintain exposure across various combinations of aperture and shutter speed settings, the sum of the adjustments, expressed in stops, must equal zero. Any deviation from zero will result in a corresponding increase or decrease in exposure. This is true whether your changes are in full stops or fractions of a stop, such as half-stops or third-stops.

At this point, you should have a foundational understanding of how combining the aperture settings with shutter speed influences exposure, how ISO changes effective exposure, and the units describing their relationship. However, it’s helpful to recognize that each parameter—aperture, shutter speed, and ISO—imparts a unique aesthetic to your photography. Correct effective exposure is a delicate balance of all three. In the following chapters, you’ll be introduced to the lens focal length and aperture and their effect on depth of field; shutter speed and its impact on the perception of movement; and, lastly, ISO and image noise.

ISO and Effective Exposure

Aperture and its Effects