Understanding Exposure Values (EV)
Three parameters control the exposure of an image:
- Shutter speed controls how long the sensor is exposed to light
- Aperture controls the opening of the lens (and thus the amount of light that falls on the sensor per second)
- ISO controls how much the sensor amplifies the light
When two of these parameters are set, the third one can be calculated. For every situation, there's a great number of combinations of these parameters that give the same brightness, but not the same image. Understanding how they interact gives you creative control over the image as you get to decide which of the combinations you prefer.
To express differences in amount of light, photographers use exposure values (EV). A difference of 1 EV corresponds to doubling the amount of light, 2 EV is quadrupling the amount of light (2*2), 3 EV multiplies the light by 8 (2*2*2) and so on. On the negative side -1 EV halves the light and -2 EV quarters the light. An EV is also often called an "F-stop" or just a "stop". As in: you need to underexpose by one stop (1EV).
EV and shutter speed
To increase the exposure value of a picture by 1 EV by shutter speed alone, you need to halve the shutter speed (and thus double the exposure time). So for example a picture shot at 1/60s is 1EV brighter than one taken at 1/120s.
EV and aperture
The diameter of the opening of the lens diaphragm is called the aperture and expressed in F-numbers (this is where the term F-stop comes from). The standard list is as follows:
f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22
With the opening getting smaller from left to right (note that these are fractions: f/1.4 is 1/1.4). Between each of these values lies 1 EV. It's easiest to just memorize this list, but for those who wish to understand I'll provide an explanation in the next paragraph. If you're not interested, just skip it.
As explained earlier, one EV is a doubling of the light. However the amount of light is relative to the area of the opening of the aperture while the F-number is relative to the diameter. The area in turn is relative to the square of the diameter. So, to halve the amount of light we need to halve the area of the apperture opening. This gives us a series of 1/2, 1/4, 1/8, 1/16, 1/32 etc. To get back to the diameter (and thus apperture) we need to take the square root of these values. Doing so gives us the list of appertures above.
EV and ISO
The ISO value represents the sensor's sensitivity to light. If you double the ISO, your resulting image will be 1EV brighter. The standard ISO scale is:
100, 200, 400, 800, 1600, 3200, ...
With again 1 EV between them.
How they all interact
Shutter speed, Aperture and ISO all control how bright your final image is, however there are always multiple combinatons that give the same brightness. For example, the following are all equivalent:
- ISO 100, f/4, 1/120s
- ISO 400, f/8, 1/120s
- ISO 200, f/8, 1/60s
Between the first and the second line we quadruppled ISO sensitivity (a 2EV increase) and kept the shutter speed constant. To get the same brightness the apperture was closed down 2EV from f/4 to f/8. Between the second and the third line we halved ISO (-1EV) and kept the apperture standard. To compensate the shutter speed was halved (+1EV).
Note that even though the brightness is the same for all three examples here, the end result is not because each parameter has another effect on top of changing the brightness:
- Shutter speed controls motion blur
- Aperture controls Depth of Field
- ISO controls noise
Depending on what shooting mode you are using, the camera will let you set some of the parameters and will then calculate the others to match the measurement from the light meter. This is explained on the next page.
