How to Easily Understand the Exposure Triangle: Aperture, Shutter Speed & ISO

If you find the exposure triangle confusing, you’re not alone! For many photographers, the exposure triangle seems to be a moving target. When you think you understand one side of the triangle, everything changes.

This article will cover how to control exposure in digital photography. I’ll talk through each of the three settings that make up the exposure triangle: aperture, shutter speed, and ISO. You will be able to understand how the settings interact with each other.

Exposure Triangle Basics

Three settings control how bright our image will be: aperture, shutter speed, and ISO. We call this trio the “exposure triangle”.

Light on the sensor is what makes the image. Just like in the story of the three bears, we want the things to be just right. Too much light and our photo will be over-exposed. Too little light and the image is too dark.

The shape of a triangle shows how these settings are balanced and equal. The shape also shows the relationship between three exposure settings on our cameras. Aperture touches ISO and shutter speed. Shutter speed affects aperture and ISO. ISO impacts aperture and shutter speed.

Photographers can control aperture, shutter speed, and ISO independently in manual mode. Or, we can choose one setting and ask the camera to adjust the others to keep exposure balanced. An example of this would be the Aperture Priority mode.

Changing one setting can alter all of the others. It’s the interaction between settings that can be confusing. I prefer to think of the exposure triangle as a Venn diagram. The settings have some independence, but also overlap.

 

Let’s first talk about Aperture.

Understanding Aperture

What Is Aperture?

When I press the shutter button to take a photo, my camera opens up a hole inside the lens. The hole lets light hit the sensor. How much light falls on the sensor depends on how big the opening is. A large hole will let in lots of light. A small hole limits the amount of light that hits the sensor.

We often want our camera’s aperture to work like the pupils in our eyes. In bright light, our pupils become small to limit the amount of light that we see. In the dim light, our pupils get large to let in as much light as possible and increase how much we can see.

In photography, we refer to the size of the opening as “aperture” (from a Latin word meaning “opening”). Aperture is also referred to as f-stop. There is a debate about the origin of the term “f-stop”. If it helps, one theory is that it stands for Finestra, which is Italian for window.

The shutter opens a window into your camera.

Photographers use a number system to refer to the size of the opening. What can be confusing is that larger apertures have smaller numbers. Smaller apertures have larger numbers.

An aperture of f2.8 is higher than an aperture of f16. For more about aperture, see the Expert Photography article “How to Understand Aperture in 5 Simple Steps”.

Control Focus With Depth of Field

Aperture controls how much light hits my camera’s sensor, but it also controls how much of the scene is in focus. This is called depth of field.

A wide depth of field means that more of the scene is in focus. A shallow depth of field means only some of the scene is in focus.

Many variables affect how much of a scene is in focus. In terms of aperture, f2.8 will generally have a shallower depth of field than a smaller aperture like f16. More of the scene will be in focus at f16, but f2.8 might give me a beautifully blurred background (bokeh). See the Expert Photography article “6 Tips for Choosing Shallow vs Deep Depth of Field” for more about the depth of field.

Here are two photos of a sunflower taken within seconds of each other. The only setting I changed was the aperture. I took the first photo with an aperture of f2.8, the second with f11. Notice how the focus changes.

I often shoot in Aperture Priority mode. You may find this option labelled with an A or an AV on your camera. I set the aperture because I want to control how much of the photo is in focus. Sometimes, I want a blurred background and sometimes I want everything sharp. In Aperture Priority mode, my camera chooses the best shutter speed to balance the exposure.

But here’s the problem and the reason I need to understand the exposure triangle.

In a landscape scene, for instance, want my aperture set at f16 to get the widest depth of field. I want everything in front of my camera to be in focus. But the opening in my lens at f16 is reasonably small and won’t let very much light strike my sensor. At dusk or on a stormy day, this might be a problem because the sun isn’t providing very much light. I need to get more light into my sensor.

Luckily, I have two other settings that determine how much light I let into my camera. Let’s talk about shutter speed next, and then I’ll return to my landscape problem and tell you how I solved it.

Selecting Shutter Speed for the Right Exposure

What Is Shutter Speed?

When I press the shutter button on my camera, I expose my camera’s sensor to light. I can open the shutter for a fraction of a second or minutes. For instance, I can leave my shutter open for 1/100th of a second or 1 second or even 100 seconds. The longer the shutter speed, the more light hits my camera’s sensor.

If I want more light, I leave the shutter open longer.

More light is good, right? But if I leave my sensor open too long on a sunny day, my image will be so over-exposed that I’ll ruin the photo.

Aperture and shutter speed work together to determine how much light hits my sensor. I can set my aperture and use shutter speed to balance the light. Or I can set the shutter speed and use aperture to balance the light.

In dim conditions, I want to gather as much light as possible into my camera. I can choose a large aperture (lower-numbered f-stop) and leave my shutter open for a long time.

Let me return to the landscape problem I set up earlier and show you how changing the shutter speed let me keep my small aperture.

I photographed the Smokey Mountains at sunset. My aperture is set at f16 to keep everything in focus. Notice how the exposure changes as I leave my shutter open longer.

Shutter Speed Limitations

There is no real limit for how long I can leave the shutter open. Most cameras have a built-in, 30-second limit in the menu settings. But this time limit can be extended by putting the camera into “bulb” mode. Bulb mode lets me manually control the amount of time my shutter is open using an external remote control to trigger the shutter button.

Opening my shutter for these long exposures means setting up my camera on a tripod.

There is a limit to how slow the shutter speed can be when I’m hand holding the camera. I naturally move a little bit, and this motion will show in the image at slow shutter speed.
Some photographers are very steady and can hand hold at 1/10th of a second. But most people are more comfortable at faster shutter speed.

When hand holding, the general rule of thumb is to use the focal length of your lens to set the minimum shutter speed. For instance, if I’m using a 50mm lens, I should set my shutter speed faster than 1/50th of a second.

In reality, minimum shutter speed varies from person to person. With advances in-camera stabilization, I can reliably use a slower shutter speed when I’m hand holding my camera.

How Shutter Speed Relates to Motion Blur

Shutter speed determines how much light hits my sensor, but it can also freeze or blur motion.

If I’m trying to photograph wildlife, I want a faster shutter speed to freeze the motion. If I’m trying to blur moving water, I want a slower shutter speed.

I sometimes shoot in Shutter Priority mode, which lets me choose whether I want to freeze motion or capture motion blur. This option may be labelled with an S or a TV on your camera. My camera will choose the best aperture to balance the exposure.

I cover the optimal shutter speeds for wildlife and landscape photography in my articles “12 Tips for Improving Your Wildlife Photography Composition” and “Best Camera Settings for Landscape Photography”. To explore more creative uses of shutter speed, see the article “4 Steps to Understanding Shutter Speed and it’s creative Uses”.

Let me add to the landscape problem I talked about above. If I’m taking a photograph of a landscape at dusk AND the scene includes moving wildlife, I can’t just lengthen my shutter speed to get more light. I want my aperture to be small to get everything in focus, AND I want my shutter speed to be fast enough to capture the animal.

I’ve got one more setting I can change to help me get a bit more light.

Isolating ISO to Increase Light Sensitivity

What Is ISO?

ISO adjusts the sensitivity of my sensor.

Photographers refer to ISO with a system of numbers. The lowest default on most digital cameras is ISO 50 or 100. My current camera has a maximum ISO of 102,400.

If I choose a higher-numbered ISO, my sensor becomes more sensitive to light.

With film cameras, the sensitivity of the film changed rather than the camera sensor. Photographers loaded film with an ISO of 100 into their cameras. If they wanted more light sensitivity, they changed the film in their cameras to ISO of 200 or 400.

Digital photography has made ISO much more flexible and usable. With a turn of a dial or a quick trip to my camera’s menu, I can change the ISO from picture to picture.

If I’m in dim conditions, I can increase the ISO to make my sensor more sensitive to light. If I’m in bright conditions, I can lower my ISO.

In this way, ISO interacts with aperture and shutter speed to balance the exposure. If I need more light, I can increase ISO to make my sensor more sensitive to the light coming into the camera.

Let’s return to my landscape problem. If I’m photographing wildlife at dusk or dawn, I want my shutter speed to be fast enough to freeze motion (faster than 1/500th of a second). But fast shutter speeds don’t allow much light to hit my sensor and at dusk, I won’t have a lot of natural light.

To let in more light, I can increase the aperture, but this will also narrow my depth of field. I may not be able to get as much of the animal in focus as I’d like. Increasing the ISO gives me another option.

When to Use Auto ISO

I often use auto ISO in changeable lighting conditions. In auto ISO, my camera will choose the best ISO to create a balanced exposure, given the aperture and shutter speed settings.

Auto ISO is similar to Aperture Priority and Shutter Priority modes, but this setting works alongside the aperture and shutter speed settings rather than becoming the priority. If my camera senses that there isn’t enough light at the given aperture and shutter speed, it will increase the ISO.

My camera allows me to set a maximum ISO. I often choose ISO 5000. I do this because there is a limit to how high of an ISO I want my camera to choose.

Setting the ISO to Avoid Grain

There is a trade-off that limits how high I set my ISO.

As ISO increases, so does grain in my image. If I set my ISO too high, I get a very bright picture that has lots of visible dots. The dots are called “grain”. I want a clear image rather than one with lots of grain.

Fortunately, with each generation of digital camera, ISO performance increases. This means I can set my ISO higher with little or no perceptible grain. I can also use post-processing tools like Lightroom to take out some of the grain introduced by high ISOs.

Most modern digital cameras can handle ISO 800 to 1600 with little or no perceptible grain.

If I have plenty of light or if my camera is on a tripod, I set my ISO at 100 because it gives me the clearest image. But if I need more light, I increase my ISO to make my sensor more sensitive to the light that is coming into the camera.

If you’re interested in more information about ISO, go to the Expert Photography article “What is ISO and How to Use it in 4 Simple Steps“.

Summary

Three camera settings determine a well-exposed image: aperture, shutter speed, and ISO. I’ve described how each setting works in isolation and provided a few examples of how the settings interact to get a balanced exposure.

• Aperture controls how large the opening will be in your lens.
• Shutter speed controls how long the aperture will be open.
• ISO controls my camera sensor’s sensitivity to light.

In dim conditions, I want to let as much light in as possible. This means setting a large aperture (small number), a slow shutter speed, and a higher ISO. Example settings might be f2.8, 30 seconds, ISO 5000.

In bright conditions, I want to limit how much light gets into my camera. This means setting a small aperture (large number), a fast shutter speed, and a lower ISO. Example settings might be f22, 1/2000th of a second, ISO 100.

But other factors affect my choices. Aperture also controls depth of field (focus). Shutter speed affects whether the motion is frozen or blurred. High ISOs introduce grain into the image.

The example settings given above are extreme. Each scene needs an optimal combination of settings. Because I have three ways to control exposure, I don’t have to rely on only one setting to get a well-exposed image.

It is the combination of settings and how they interact that makes the exposure triangle.

And make sure to check out this cool video on the exposure triangle before you go!