The exposure value enables the choosing of an appropriate combination of aperture and shutter speed in order to achieve a correct exposure at a given sensitivity. However, what exactly is this numeric value? Aperture and shutter speed determine how much light reaches the imaging sensor during the exposure. The amount of light can remain consistent with different settings. The aperture/shutter speed combinations that lead to the same amount of light are defined by the so-called exposure value.
To shed more light on this topic, let’s have a look at the components that play an important role for the correct exposure: aperture, shutter speed and sensitivity. The amount of light reaching the sensor is controlled by the aperture and the shutter speed. The opening radius of the aperture regulates the amount of light travelling through the lens system. The smaller the aperture value, the larger the opening and the more light that reaches the sensor, and vice versa. The decrease/increase of the aperture number by one full f-stop leads to a doubling/halving of the amount of light.
The shutter speed determines the duration of the light incidence, meaning the amount of time the shutter stays open to capture light. In combination with the aperture, the amount of light that reaches the sensor during exposure is determined.
Furthermore, the sensitivity of the imaging device influences the exposure. When using a digital camera, the increase in sensitivity is achieved by signal amplification. When i.e. the sensitivity is raised from ISO 100 to ISO 200, the signal will be doubled. That will lead to double the amount of light in the resulting image.
Amount of light in relation to camera settings
Constant ISO and shutter speed:
Aperture number + 1 f-stop (closing) ⇒ amount of light / 2
Aperture number - 1 f-stop (opening) ⇒ amount of light * 2
Constant ISO and aperture:
Shutter speed * 2 (slower) ⇒ amount of light * 2
Shutter speed / 2 (faster) ⇒ amount of light / 2
Constant aperture and shutter speed:
ISO * 2 ⇒ amount of light * 2
ISO / 2 ⇒ amount of light / 2
Amount of light and exposure value
The exposure value describes the different combinations of aperture and shutter speed, which lead to the same amount of light reaching the imaging sensor. The exposure value is defined in such a way that at ISO 100, aperture 1.0 and a shutter speed of 1s, it is “zero”.
for ISO 100 k = 1.0 t = 1s ⇒ EV = 0
EV = log2 (k2/t)
It applies: the higher the exposure value, the more light is available.
Exposure Value + 1 step ⇒ amount of light *2
Exposure Value – 1 step ⇒ amount of light / 2
More exposure values and their aperture/shutter speed combinations can be derived based on the defined exposure value of "zero". Arranged in a table, required settings in relation to a particular sensitivity can be read easily.
The exposure value (EV) can be determined by using a light meter. Select “EV” on the light meter and set it to the desired sensitivity for measuring the exposure value. The result will be shown on the display. Let’s take a look at an example. We would like to take a correctly exposed image at ISO 100. After metering the light, we get the result of EV 14 for 100 ISO. Now we look at the exposure table for ISO 100 and under 14, we can find all aperture/shutter speed combinations that will lead to the same exposure. Assuming we would like to set the aperture to 5.6, we need to look along the row in the exposure chart until we reach 14 and then go up the column to find the matching shutter speed – in this case 1/500s. When selecting an aperture of 4, the matching shutter speed would be 1/1000s.
Deriving the parameters
We do not always have an exposure chart or a light meter handy. In this case, you can use the "Sunny f/16 Rule". Depending on the weather conditions, all necessary settings and further combinations of settings can be derived from this simple rule. The amount of light that is needed for a correctly exposed image has to be taken into consideration. Keeping the relationship between aperture, shutter speed and sensitivity in mind, finding the right combination is easy.
Examples for a constant amount of light at ISO 200
Aperture 5.6, 1/125s = amount of light X for correct exposure
a) Aperture - 1 f-stop: aperture 4, 1/125s = amount of light X *2
b) Aperture - 1 f-stop & shutter speed / 2 = amount of light X ⇒ f/4 and 1/250s at ISO 200
c) Aperture - 1 f-stop & ISO / 2 = amount of light X ⇒ f/4 and 1/125s at ISO 100
The smaller f-number and thus larger aperture of f/4 allows twice as much light to reach the sensor (a) compared to f/5.6. In order to reduce the amount of light to avoid a slight overexposure, we can shorten the duration of the incident light, by either halving the shutter speed (b) or the sensitivity (c).
d) Aperture +1 f-stop: aperture 8, 1/125s = amount of light X /2
e) Aperture +1 f-stop & shutter speed * 2 = amount of light X ⇒ f/8 and 1/60s at ISO 200
f) Aperture +1 f-stop & ISO * 2 = amount of light X ⇒ f/8 and 1/125s at ISO 400
When closing the aperture by one f-stop, the opening becomes smaller and less light can pass (d). In order to maintain the amount of light, we can double the shutter speed (e) or the sensitivity (f). The signal amplification (gain) corresponds to double the amount of light, which therefore compensates the loss of light due to a smaller opening of the aperture.