Understanding the Exposure Triangle

The exposure triangle is how your camera's aperture, shutter speed, and ISO settings affect one another and work together to achieve correct exposure.

But let's not get ahead of ourselves. Before we can understand what an exposure triangle is, we first need to understand each element. After all, there are three sides to every triangle, each of which is equally important.

Aperture

An aperture is an opening through which light travels. More specifically, it's the hole in a camera's lens makes that is responsible for controlling the amount of light that reaches the image sensor (or film) in a specific amount of time. The opening's size is measured in f-stops, which you may have seen listed as f/numbers like F/2.8, F/4, F/5.6, etc.

The lower the f-stop number, the wider the aperture. Each time you increase your aperture by one full stop -- F/4 to F/2.8 or F/5.6 to F/4, for example -- you double the amount of light hitting your sensor.

We also refer to lenses with wider apertures as being "faster" because more light hitting your sensor generally means you need to expose your image for less time. Because of this, wider aperture lenses also provide more performance in low-light situations.

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On the other hand, the higher the f-stop number, the narrower the aperture. Every time you decrease your aperture by one full stop -- F/5.6 to F/8 or F/11 to F/16 -- you halve the amount of light reaching the sensor.

For creative purposes, it's also useful to know that aperture controls an image's depth-of-field. Wider apertures decrease depth-of-field, giving you blurry backgrounds (aka bokeh) that helps you isolate your subject from their surroundings. In contrast, narrower apertures increase depth-of-field, making more elements in your images clearer, ideal for things like landscape photography.

To summarize, the wider your aperture (smaller your f-stop), the more light will hit your sensor, making your focal plane more shallow. And the narrower your aperture (larger your f-stop), the less light will reach the sensor, giving you a deeper focal plane.

Shutter Speed

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While the aperture is the opening letting the light through, shutter speed is the amount of time the sensor (or film) is exposed to light. It is represented from fractions of a second to whole seconds, ala 1/60s, 1/125s, 1/250s, and so on. (You can also do longer exposures (minutes and hours) with cameras that have a Bulb mode.)

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The faster the shutter speed, the less time you are exposing your sensor to the light passing through your aperture. Each time that you double your shutter speed, you halve the amount of light hitting your sensor. Conversely, the slower the shutter speed, the longer you are exposing that sensor. Each time that you halve your shutter speed, you double the amount of light hitting your image sensor.

Creatively, use faster (shorter) shutter speeds to freeze motion and slower (longer) shutters speeds to enhance blur or convey motion.

ISO

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ISO used to refer to the speed of the film. Folks would use lower ISO films (ISO 160, etc.) for shooting outside in the sun, and higher ISO films (ISO 800, etc.) for shooting inside where there's less available light.

In the modern digital world, ISO numbers are a bit more confusing because different cameras have different ISO ranges and capabilities related to the camera sensor and processor. But don't worry, you still think of it like the old film days.

When you have lots of light, you shoot with lower ISO numbers. And when it's darker, you shoot with higher ISO numbers. Unlike aperture and shutter speed, however, you're not actually changing the amount of light that hits your sensor. Instead, you're boosting your sensor's sensitivity.

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Creatively, though, there's a trade-off in image quality and clarity as you raise your ISO settings. Higher ISOs typically yield more grain and noise, which degrade image fidelity. Why? Because modern ISO is, effectively, digital processing. The great news is that most newer cameras are getting better and better at minimizing grain and reducing the decrease of image quality, but you still have to be careful.

The trick is to figure out a way to shoot with the lowest ISO possible. Test your camera in dimmer light at your highest ISO settings and see where the image starts to lose color, clarity, and contrast. Then drop down below that number.

How to Use the Exposure Triangle

So how exactly do all three elements connect to make up the exposure triangle?

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Imagine the exposure triangle as the three legs of a tripod: if you adjust one leg, you have to change the other two to maintain balance (aka proper exposure).

For example, if you're shooting portraits in the sun and you want a creamy-blurry background, start with a SMALL aperture number, which means you need a HIGH shutter speed and a LOW ISO number. Or if you're shooting cars on a freeway at night, you want a LOW shutter speed (to convey movement), a HIGHER aperture number, and a HIGHER ISO number.

The trick is to pick what look you want -- lots of depth or shallow depth, frozen movement or blurry movement -- and build your shot around a specific look.

The trick is knowing how much to adjust the other legs of your tripod as you create your specific look. The great news is that your camera is always metering, so you can always check in with it, or take a sample image, as your setting up. Also, you'll begin to get a feel for this as you become more experienced.

At the heart of this understanding, is our old friends Stops or F-stops.

Remember these from above? Here's a refresher: if you raise your exposure by one full stop, you double the amount of light hitting your sensor. Conversely, if you lower your exposure by one full stop, you halve the amount of light hitting your sensor.

  • For aperture settings, full stops are F/1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, & 32. Each time you increase the number, you decrease the amount of light hitting your sensor.
  • For shutter settings, full stops are 1/4000s, 1/2000s, 1/1000s, 1/500s, 1/250s, 1/125s, 1/60s, 1/30s, 1/15s, 1/8s, 1/4s, 1/2s, 1s, 2s, and so on. Each time you raise the shutter speed, you decrease the amount of light hitting the sensor.
  • For ISO settings, full stops are ISO 100, 200, 400, 800, 1600, 3200, 6400, 12800, 25600 and so on. Each time you raise your ISO, you increase
So if you want to open your aperture by two stops, you need to slow your shutter or bump up the ISO by the same number of stops. If you need to increase your shutter speed by four stops, and your aperture is already at its max aperture, you need to bump your ISO by four stops.

Make sense? If not, here are some visual examples.


Example #1: The Flower

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Let's say you're trying to take a photo of a flower and your camera's meter tells you the ideal exposure, with your aperture set at F/5.6, is with ISO 200 and a shutter speed of 1/250.

Since it is a lone flower, you want to isolate it by decreasing the depth-of-field so you'll get a creamier background and the flower pops out more. To do this, you increase your aperture by one stop to F/4. But by doing so, you're also doubling the amount of light entering your camera, and if you don't compensate, you will overexpose your shot, and it will come out a little washed out.

To counter this, you need to either drop your ISO setting by one on your camera from 200 to 100 or double your shutter speed by one stop from 1/250 to 1/500.

Example #2: Some Motion

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Similarly, if you want to capture motion like a waterfall, use a slower shutter speed. Let's say your camera is set to F/4, ISO 100, and 1/500 of a second.

Because you're capturing motion, set your shutter speed first, dropping it down to something slow like 1/15s, which is five stops down from 1/500. To get an ideal exposure, you'll have to adjust your ISO down and/or aperture by five stops.

However, since your ISO is already at 100, a minimum ISO setting for many cameras, you must adjust your aperture by five full stops, from F/4 to F/22.

Final Thoughts

It's a little tricky to learn. Just remember aperture controls image depth, shutter controls motion, and ISO helps boost sensitivity. Work with your camera's internal meter to figure out a proper exposure and then, if you need to adjust, remember to count the number of stops you change on one leg of the exposure triangle so you can balance it with the other two legs.

The more you implement what you learned today, the more apparent it'll be in your head. The best way to understand how it all works is to get out in the field. Take your camera, switch it to manual mode, and clock in some shooting time. Experience, after all, is the best teacher. Before you know it, the exposure triangle will feel like second nature.