The exposure triangle

If you’ve looked up tutorials on photography or anything like that, then you’ve probably heard of the exposure triangle.

The exposure triangle is referring to the relationship between the three primary camera settings that contribute to the exposure when you take a photo.

Shutter speed

The shutter speed determines the duration that the cameras shutter is open for to allow light to hit the image sensor. Most SLR and mirrorless cameras these days will range from 30 seconds to 1/4000th of a second and faster. It seems obvious that the chosen duration the shutter is open for will allow lots or little light to hit the sensor. A very fast shutter speed will let the light touch the sensor for a very small amount of time. This affects a couple of things primarily. Obviously it affects the exposure as less time to capture the light will mean a darker image (or underexposed) unless other settings are adjusted to make it work. The second thing and most important thing the shutter speed controls is motion.

How shutter speed affects motion in a still photo

Imagine you’re at a sports game but you’re inside a pitch dark box so you cannot see anything. All of a sudden a hole in the front of the box opens, it stays open for 5 seconds and you see through that hole a spectacular kick of the ball and then it closes. You got to see 5 seconds of that ball moving through the air.

Next time it opens, it opens for a split second and that’s it. What you’ve seen of the sports game in that split second is closer to a still image frozen in time. Almost no movement just a fraction of a second and it looks like a photo in your mind.

The cameras shutter speed is like that. Imagine you’re at the same game and you set your camera to a 5 second shutter speed and take the photo. You’ll end up with blurry motion all over your photo as the players and the ball move around. The image sensor captured all the motion. Whereas if you set your shutter speed to 1/1000 of a second, you’ll end up with a still frame with no movement of players to be seen. Like a slice of time saved to a digital file.

Aperture

The aperture in in the lens and is a ring of blades that can open and close to restrict or control how much light gets through. You may ask “isn’t the shutter speed controlling how much light gets through?”

The answer is, not really. It’s controlling how long it records that light, not how much is getting through. The aperture controls how much comes through.

Consider a wide diameter hose attached to a tap. Say water flows through it when it’s unrestricted at 50L/m. Then it bursts in the middle and the only way to fix it is to splice in a smaller diameter hose. At this point, even though most of the hose is still 50L/m capable, the smaller section only let’s through 20L/m and thus becomes the most that hose can now output.

With a camera lens, “stopping down” the aperture means to restrict the size of the opening that allows the light to flow through, meaning that less light overall is going to make it onto the image sensor. Conversely, “opening up” the aperture will make the hole bigger and let more light through to the sensor.

If the shutter speed stays the same, a photo taken at f/11 will be much darker than one taken at f/4 because at f/11 the amount of light coming through the lens is reduced compared to f/4.

The smaller the number, the larger the aperture and the more light will come through. The higher the number the smaller the aperture and the less light will come through.

The aperture and shutter speed both do have an effect on the overall exposure of the photo but for different reasons and purposes.

The other effect that the aperture has is on the depth of field. Or it could be known as the depth of focus. Putting all the complexities aside, it’s really about how much of the depth of the image is in focus. The more open the aperture (larger with more light let through) the shallower the depth of focus is. Meaning that at a wide open aperture there might only be a few centimeters of your scene in sharp focus on either side of the focal point. So while more light will make it through to the sensor, you’ll end up with more out of focus areas of the photo.

The more closed down the aperture (smaller with less light coming through) the deeper the depth of focus. So at f/11 you might end up with many meters either side of the focus point being in sharp focus. At something like f/32 just about everything in the scene would be in focus from right in front to the horizon.

There are many other factors and considerations you should learn about when using extremely shallow or deep depths of focus be we can learn about those another time.

A typical use case for changing the aperture would be a portrait vs a landscape. For a portrait, it’s desirable to have the background out of focus and creamy while the subject is sharp. So a wide open aperture suits well here (f/2.8 for example). Whereas a landscape photo it’s more desirable to have as much as possible in focus, so a stopped down aperture is more desirable for that. (such as f/22).

ISO

So that leaves ISO. Our third point of the triangle. ISO is similar to what was known as ASA in film days and basically refers to the sensitivity of the image sensor.

A low ISO number means low sensitivity and is the most desirable because it doesn’t introduce digital noise in the image. The higher the ISO value, the more sensitive the sensor becomes (by increasing the power provided to it) so it is more capable of capturing lower amounts of light (such as from a very stopped down aperture, or from a very quick shutter speed). However, the byproduct of being more sensitive is more digital noise or graininess in the image.

Some cameras handle it better than others, and it’s improving all the time but it should be the last setting to be changed in the exposure triangle to get the correct exposure.

Camera modes

Most DSLR or mirrorless cameras will have a choice of modes you can use.

  • Aperture priority (You set the aperture, camera chooses the shutter speed)
  • Shutter priority (You set the desired shutter speed, camera chooses aperture)
  • Program (chooses settings for you based on internal programming)
  • Manual (full control of all aspects of the triangle)

In all modes the ISO is either manual or has been set to Auto. If the ISO is on auto, the camera will automatically pick the ISO it thinks it needs to create the correct exposure. If set to manual you will need to set it manually.

When to use what mode?

Manual mode gives you the most control to set the aperture and shutter and ISO for creative means (or because it’s cool to say you shoot manual). You would use this in tricky lighting situations or for more creative control.

Program is pretty good at picking the right settings based on a programmed database of scenes. It tries to find the closest in it’s database and mimics settings for that. It’s pretty good and certainly preferable to missing the shot.

Use Aperture priority mode when the depth of focus/depth of field is the most important factor. The camera will choose a shutter speed it thinks is suitable.

Use Shutter priority when capturing or freezing motion is the most important factor such as at sports games or when shooting fast moving subjects.

Conclusion

Understanding how the exposure triangle works and the impact each setting has on the photo and on the other settings around you is an important part of photography and will enable you to make the best choices to always get the shot you want. It will help you to be in total control when taking the photo rather than leaving the camera to decide everything.

I most often use the semi-auto modes of aperture priority and shutter priority depending on what I am shooting and will only rarely use manual for creative effect or in tricky lighting situations. But understanding how they all fit together is crucial to making the choice of which to use and when to make the photo work.

Self taught software developer and photographer.

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