Exposure triangle: the relationship between shutter speed, aperture and ISO
3.6.2019
Translation: machine translated
Shutter speed, aperture and ISO are mathematically interdependent. You need to know this in order to understand your camera's automatic exposure. In rare cases, you may have to do the maths yourself.
Three factors determine the exposure of a photo: shutter speed (exposure time), aperture and ISO value.
- Shutter speed: The longer you expose, the more light reaches the sensor.
- Aperture: The larger the aperture, the more light reaches the sensor.
- ISO: The higher the ISO value, the more sensitive the sensor is to light.
Because these factors influence each other, they are often represented in a triangle. Hence the name "exposure triangle".
If I reduce the shutter speed, I have to increase the ISO, open the aperture or both. But by how much exactly?
The interdependence
With shutter speed and ISO, doubling the respective value also means doubling the amount of light. 200 ISO brings twice as much light as 100 ISO, and 1/50 second twice as much as 1/100 second.
A photo is therefore equally exposed with:
- 100 ISO and 1/50 second
- 200 ISO and 1/100 sec.
- 400 ISO and 1/200 sec.
This always assumes that the aperture remains constant.
With the aperture, the calculation is a little more complicated. Firstly, the number is the denominator of a fraction. Aperture F4 is actually written F 1/4, or F/4. This is why F4 is the smaller aperture than F2; after all, ¼ is smaller than ½.
Furthermore, the f-stops are not linear. To double the amount of light, the aperture is not doubled, but multiplied by the square root. This results in the following rounded f-stop series:
1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32
Each number in this series means a doubling of the amount of light. Aperture 1 captures twice as much light as aperture 1.4, and aperture 22 twice as much as aperture 32.
Conversion: an example
You rarely need to convert. It can be useful for night shots: You take a test photo with a high ISO and short exposure so that you don't have to expose for several seconds each time to find the right brightness. As soon as you have found the right exposure, convert the exposure. Example:
- At 6400 ISO it takes 1/10 second
- 6400 ISO is 64 * 100 ISO
- therefore: 64 * 0.1 seconds = 6.4 seconds for 100 ISO
Understanding automatic exposure
More important than such calculations is that you know how your camera's automatic exposure works. On the camera, you can set all three factors of the exposure triangle to manual or automatic. With most cameras, you can control the aperture and shutter speed via the camera mode. This determines whether you manually control only the aperture (A), only the shutter speed (S), both (M) or neither (P).
You set the ISO value to automatic or to a specific value regardless of the mode.
The camera thus ensures that the image is automatically correctly exposed, unless you have set all three factors to manual. However, there are limits to what the camera can do. For example, a lens with a speed of F/4 cannot provide an aperture greater than F/4. Therefore, an image can be overexposed or underexposed even if you have set one or more values to automatic.
When light is scarce, you have to make a choice: You can't have a low ISO for the best quality, a fast shutter speed for frozen motion and a closed aperture for lots of depth of field all at the same time. What you set manually is the value that is really important for your image and where you don't want to make any concessions.
David Lee
Senior Editor
David.Lee@digitecgalaxus.chMy interest in IT and writing landed me in tech journalism early on (2000). I want to know how we can use technology without being used. Outside of the office, I’m a keen musician who makes up for lacking talent with excessive enthusiasm.
