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.
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 of these parameters—aperture, shutter speed, and ISO—imparts a unique aesthetic to your photography, and that correct effective exposure is a delicate balance of all three. In the following chapters, you’ll be introduced to the lens focal length, aperture, and their effect on depth of field; about shutter speed and its impact on the perception of movement; and, lastly, ISO and image noise.