Exposure Triangle | Understand How It Works

Have you ever heard about the reciprocity law? If not, you should know that this is at the core of photography, and the way we use our cameras is based on it. And the effects of the reciprocity law are the foundation for the exposure triangle.

But what is the exposure triangle, and why you should care about this? Keep reading and discover how by understanding and mastering the exposure triangle, you will create better images.

hold three sides of a exposed triangle

What Is The Exposure Triangle In Photography And Why Is It Important?

What is the exposure triangle? Think of it as a way to tell you how the effects of changing, say, exposure must be compensated by modifying the shutter speed and/or ISO, in order to keep the same overall exposure for the image.

The triangle is built on the inverse relationship between aperture and shutter speed in determining the response to light of the film or the sensor.

On The Sunny 16 And The Other Rules In Film Photography

Digital cameras have made it easier for the photographer to get a technically correct photograph, but only a few decades ago, in the film era, things were harder. 

If anything, because in-camera light meters were either not present or less accurate and the photographer had to come up with proper settings by eyeballing the amount of available light and considering the light sensitivity of the film loaded in the camera.

People began to develop empirical rules to help to set the camera for “standard” scenes. One such rule is the famous Sunny 16.

The rule says that in bright daylight and with a film sensitivity of ISO 100 by setting the camera to aperture f/16 and shutter speed 1/100, you will get a properly exposed image.

Similarly, the Overcast F8 rule states that on a cloudy day with a film sensitivity of ISO 100, you should set your camera to f/8, 1/100th of a second.

example of the Overcast F8 rule
For this scene, the Overcast F8 rule (top) gives a result similar to that obtained using the camera in Aperture priority mode. Because most of the scene is in the shadow, I used the overcast rule even if there are no clouds in the sky.

There is even a rule for the full Moon, called Looney-11. You guessed: with a film speed of ISO 100, the full Moon is properly exposed using an aperture of f/11 and a shutter speed of 1/100th of a second.

Following the same idea, other common exposure rules were: 

  • Snowy / Sandy F22
  • Slightly Overcast F11
  • Heavy Overcast F5.6
  • Sunset F4

Nowadays, these rules are only used for educational purposes, as they offer the opportunity to introduce the reciprocity law, on which they, and the exposure triangle, are based.

The Reciprocity Law And The Exposure Triangle

It seems those exposure rules deal with different light conditions by changing only the aperture.

But what happens if you need to change your shutter speed or your aperture? What if you still have a few exposures left on a film with different light sensitivity? Can you still use those rules?

The short answer is: “yes, but you need to adapt them.”

How to adapt those rules is part of a longer answer, which involves the reciprocity law, i.e., the inverse relationship that exists between aperture and shutter speed.  

The law says the response of a film to light is the result of the light intensity times the duration of the film exposition to light. 

Ok, but what does this mean in practice?

Say you are photographing at a Formula 1 Grand Prix: even under the bright sun, a shutter speed of 1/100th will not be fast enough to freeze the car movement. 

need a fast shutter speed to capture the Italian GP car
Panning at the 82nd Italian GP in Monza. Image Credit: Alessandro Torri

Because of the reciprocity rule, the sunny 16 rule will still hold true for a shutter speed of, say, 1/400 if you set the aperture to f/8. 

This works because the reduced exposure of the film to light is compensated exactly by the increase in light intensity, due to the use of a wider f/8 aperture. 

In short, the film response is the same if we photograph at f/16, 1/100s, ISO 100 or f/8, 1/400s, ISO 100.

To know how we can combine different apertures, shutter speed, and ISO while keeping the overall exposure the same, we use the exposure triangle.

linking the three elements of the exposure triangle
The exposure triangle, linking together aperture, shutter speed, and ISO. 

Here how it works: for a given ISO, all aperture-pairs linked by the red dashed line you have the same film response, thus the same equivalent total exposures.

In film photography, the reciprocity law is true only when the film responds linearly to changes in its exposure to light. Outside this range, the reciprocity rule does not hold true, and we refer to this as reciprocity failure.

Astrophotographers in the film era were used to deal with the reciprocity failure: the light intensity from the night sky was so low that the film does not respond linearly to the changing of the setting. 

For those light conditions, it was simply not true that to increase the exposure by 1EV, one could simply double the shutter speed. This link shows how reciprocity failure must be compensated for Ilford film rolls.

Unlike film, digital sensors do not suffer from reciprocity failure, and their response is always linear.

The 3 Components Of The Exposure Triangle Explained

Looking at the exposure triangle alone, you may think any combination of aperture, shutter speed, and ISO giving the same overall exposure is equivalent.This is precisely the feeling I had when my father put in my hands his Olympus OM-1for the first time (and the second, and the third… ) saying: try keeping the light meter’s needle in the middle.

And so I did, by randomly adjusting aperture and shutter speed. 

But I was wrong. It turned out different combinations of aperture and shutter speed will introduce different effects in the images.

Shutter Speed

Shutter speed has two main functions in photography:

  1. It sets how long the film or the sensor are exposed to light;
  2. It is responsible for the way motion is portrayed in the photo.

To freeze the motion of a fast-moving subject, like a racing car, you need a fast shutter speed. 

To create a sense of movement in your images, you have to use slow shutter speed compared to the speed moving subject.

slow shutter speed has blurred the water stream
A nocturnal landscape from my region in Italy. The slow shutter speed has blurred the stream of water in the foreground.

Aperture

Aperture is responsible for the intensity of the light arriving on the film or the sensor. It also affects your image in several ways.

The major effect is on the depth of field, i.e., how much of the scene will look in focus, with narrow aperture giving a wider depth of field than wider apertures.

aperture affects depth of field
The scheme illustrates how the aperture affects the depth of field (DOF) for a given combination of sensor size, focal length, and camera-to-subject distance.

Wildlife photographers prefer using wide apertures to blur everything behind their subject. This way, they create a less distracting background, making the animal stand out more. 

Spotted Woodpecker
A Great Spotted Woodpecker photographed in the wild. Image Credit: Ines Jonkers.

In landscape photography, a narrow aperture ensures that enough of the scene will be in focus.

landscape image where everything is in focus
A classic landscape where everything is in focus. Micro four-thirds camera, f/5.6, and focal length of 12mm. Image credit: Alessantro Torri.

Other things that are affected by the aperture are:

  • Image sharpening
  • Vignette
  • Optical aberration such as purple fringe, etc

ISO

ISO is traditionally included in the exposure triangle because in film photography, it was a real parameter for the exposure. With digital cameras, this is not (technically) true anymore.This is because the sensor has only one native ISO, and you cannot change it with a more sensitive one whenever you need.  Instead, when you change the ISO, all you are doing is digitally amplifying the signal coming from the sensor.

The effects of Increasing the ISO are mainly two:

  1. Higher the ISO, higher is the noise in the images
  2. Higher the ISO, the smaller is the dynamic range the sensor can manage, i.e., the maximum difference in brightness between the darkest shadows and the brightest highlights.

The scheme below shows how by increasing the ISO, the usable range of values a pixel can have is reduced.

effect of boosting the ISO on the dynamic range
The effect of boosting the ISO on the dynamic range.

Assuming a pixel can have a max value of 100 (pure white) like in the scheme above, a pixel that at ISO 400 has a value of only 25, at ISO 1600, will be clipped to pure white. Thus, the dynamic range at ISO 1600 is ¼ than that at ISO 400.

For us astrophotographers, this means that by boosting the ISO we risk clipping 

the bright stars and the core of galaxies and nebulae to pure white.

astrophotography image of M42
My first astrophotography image from a fixed tripod using ISO 20 000, 3.2s at 300mm. All stars are white dots, as well as M42’s core.

The Importance Of The Exposure Triangle For Night Photography

The exposure triangle is essential as it tells us which combination of aperture and shutter speed will result in a correct exposure while introducing the desired photographic effect (motion blur, bokeh, etc.).

In night photography and astrophotography, different considerations dictate what shutter speed, iso, and aperture to use. These are:

  1. Use the lowest usable aperture, i.e., one that produces a decent image in terms of sharpening and optical aberrations (we correct vignetting with flats);
  2. Use the longest usable shutter speed, i.e., the one for which you get reasonable round stars;
  3. Use the lowest ISO value for which your sensor is ISO invariant. Particularly if you are tracking the stars during their journey through the night sky;
  4. Get the histogram peaking about ⅓ of the way from the left edge.

Conclusions

This article concludes a series of four writings, the previous one discussing in detail aperture, shutter speed, and ISO.

By now, you should have a pretty solid understanding of the basic principles of photography and how to make them work at your advantage.All you need now is to go out there, get some practice, and don’t be afraid to break the rules and to experiment with them.

About Andrea Minoia

Andrea Minoia works as a researcher in a Belgian university by day and is a keen amateur astrophotographer by night. He is most interested in deep sky photography with low budget equipment and in helping beginners along their journey under the stars.