What is the Exposure Triangle and How Does It Work?

What is the Exposure Triangle and How Does It Work?

When it comes to photography, light is everything. After all, a photograph is nothing more than light captured on a medium (film or digital sensor). Controlling the amount of light that reaches your camera’s sensor is key to getting the right photograph. There are three camera settings related to exposure, the aperture, shutter speed, and ISO.

Together, they form the Exposure Triangle.

When the settings are right, the correct amount of light reaches the sensor and the photograph is considered properly exposed. If too little light reaches the sensor, the resulting image will appear too dark. This is referred to as underexposed. Overexposure occurs when too much light reaches the sensor and the image appears too light.

Modern cameras usually have an automatic mode where the camera evaluates the scene and makes the proper adjustments to the camera settings for the best exposure.

This shot was taken with my Nikon D5100 in automatic mode. The camera chose an aperture of F/5.6, a shutter speed of 1/125th of a second and ISO200 for this shot. As you can see, it’s a well-exposed photograph.

Now let’s examine what happens to the same photo as we switch to manual mode and begin adjusting the same three camera settings.

Exposure Triangle: Aperture

Aperture controls the amount of light passing through the lens. The smaller the number, the wider the aperture. The wider the aperture, the more light the lens allows to reach the sensor. Aperture is measured in F-stops. Each full-stop reduction in aperture (F-number gets larger) means a 50% reduction in light. A full-stop increase in aperture (F-number gets smaller) means double the amount of light comes through. An F/4 aperture allows in double the light of an F/8. An F/16 allows 50% less light through than F/11.

I switched my D5100 to manual mode and dialed in the same settings chosen by the camera in automatic mode. I then took a sequence of photos, narrowing the lens aperture by one full-stop between shots. We went from an aperture of F/5.6 on the first frame to F/8 in the second. You can see the resulting image is darker. The aperture was one stop smaller so 50% less light reached the sensor.

Narrowing down again from F/8 to F11 resulted in even less light with another 50% drop. These resulting 50% drops in light are referred to as stops of light. So from F/5.6 to F/11, we’ve lost a total of two stops of light. Drop to F/16 and now we’ve lost three stops of light.

Quick Tip: Narrowing aperture by one stop reduces light by half. Increasing aperture by one stop doubles the light.

READ MORE: Aperture and Depth of Field

Exposure Triangle: Shutter Speed

Shutter speed is a measurement in seconds of the amount of time the camera shutter opens, exposing the sensor to the light passing through the lens. A shorter duration allows less light. This is known as a faster shutter speed. A slower shutter speed will keep the sensor exposed longer, generating a brighter image. A 1/125th-second shutter will allow more light to reach the sensor than a 1/250th second. A 1/1000th shutter will allow less light than a 1/500th.

I repeated the same process as above, but this time I left the aperture and ISO settings alone and only adjusted the shutter speeds. The 1/250th shutter speed gave us a darker image than the 1/125th, which makes sense. The shutter was open for half as long, meaning half as much light reached the sensor. Just like narrowing the aperture one stop results in a 50% reduction in light, doubling the shutter speed also results in 50% less light. Increasing from 1/500th of a second to 1/1000th of a second likewise causes a 50% reduction in light on the sensor (one stop of light).

If we look at this photo from right to left, we see slowing down the shutter from 1/250th of a second to 1/125th results in twice as much light getting through because the shutter is open twice as long. The resulting image will be brighter.

Quick Tip: Increasing shutter speed one stop reduces light by half. Decreasing shutter speed by one stop doubles the light.

READ MORE: Shutter Speed and Controlling Motion

Exposure Triangle: ISO

The third point on the exposure triangle is ISO. This is a measurement of the ‘sensitivity’ of the camera to light. If you remember buying film for a camera, you’ll remember you had to buy film that has an ISO rating that matched you shooting environment. You used an ISO200 film for outdoor shooting and ISO800 film for indoor photos.

Once again the shutter speed and aperture are dialed in to match the automatic shot. I increased the ISO one full stop between exposures and you can see the results. Each full stop increase in ISO makes the image brighter. Moving from ISO400 to ISO800 results in an image that twice as bright. Moving down from ISO400 to ISO200 means the image is 50% darker.

While ISO is an algorithm the camera uses to alter the brightness of the photo, I found it helpful to think about ISO as changing how ‘sensitive’ the camera is to light. A lower ISO means the camera is less ‘sensitive’ and a higher ISO means the camera is more ‘sensitive’.

While the actual sensitivity of the camera doesn’t change, this thinking still helped me to understand what was happening to my images as I adjust the ISO.

Quick Tip: Increasing ISO makes the camera twice as ‘sensitive’ to light. Lowering ISO makes the camera half as ‘sensitive’.

READ MORE: ISO and Noise

Dialing In the Proper Exposure

We’ve explored the effects of aperture, shutter speed and ISO on the exposure of our photographs. Moving a setting up or down one full stop has a resulting increase or decrease on the amount of light that reaches our sensor, or the sensitivity of the sensor to that light in the case of ISO.

What’s most important to note is we can compensate for the stop of light lost or gained by adjusting another setting in the opposite direction.

Let’s say we need to narrow down our aperture one stop to increase our depth of field. We’ll lose 50% of our light. We can compensate for that drop by slowing the shutter speed by one stop or by increasing the ISO by a one stop. Either one of those changes will allow us to recapture the lost light.

Without text, the image looks nearly identical to the reference image, but it is the same style compilation as the other examples. Four separate frames, each taken with different camera settings. I started with the reference settings of F/5.6, 1/125th, and ISO200; then made a drastic 3-stop change to one of the settings. The change was compensated for by an equal, but opposite, change in another setting. The result is the same proper exposure.

In the second frame, I narrowed the aperture by 3 stops, which as we know from the above example would result in a substantially darker image. However, I compensated for the loss of light by slowing the shutter speed by 3 stops. This allowed me to regain the lost light by keeping the shutter open longer and properly exposing the image.

The third frame shows the result of a 3-stop increase in shutter speed, from 1/125th to 1/1000th, and a 3-stop bump to ISO 1600 to compensate. The faster shutter reduced the light available to the sensor, but the ISO bump increased the sensitivity of the sensor to the available light.

The last frame I stop down to F/16 and increase to ISO 1600. All three frames are equally exposed, despite the drastically different settings.

Quick Tip: Boosting ISO too high will result in noise in your photos. ISO should be the last adjustment you make and only if aperture and shutter change isn’t enough.

Putting it All Together

Know that we understand how to manually dial in the exposure settings, the obvious question is why bother? If our camera can automatically detect the best settings for the situation, why learn the exposure triangle at all?

I would say because the camera might make a mistake and that may be true. But as a beginning photographer, your camera will nail the proper exposure more often than you will. It’s also a good idea to understand what your camera is doing to get the best exposure when it’s in automatic mode.

However, there’s a lot more to the aperture, shutter speed, and ISO than proper exposure. This is the real reason why you need to understand the exposure triangle. You’ll soon want to step out from the automatic mode and start creating more artistic photographs and you’re camera won’t be able to choose the proper settings. Aperture and shutter speed control different artistic elements of your photograph and your camera’s automatic setting has zero artistic ability.

Aperture controls the depth of field and determines which parts of your photograph are in focus. Shutter speed affects how motion is displayed in your photos. Adjusting both can lead to interesting artistic effects such as blurring the background of a portrait, or creating star trails at night.

Quick Tip: ISO is only for low light compensation unless you consider digital noise to be an artistic element.

For more details on each of these exposure triangle settings and their effects on your photographs, check out these articles:

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