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The exposure triangle and aperture

Aperture is one of the three essential elements of the exposure triangle. The other two elements are shutter speed and ISO. Aperture seems to be the element that occupies, and often confuses, most beginner photographers.

So, let’s break down the exposure triangle and aperture.

How to use the exposure triangle and aperture

What is aperture in photography?

According to the dictionary, aperture is an opening, hole or gap. For example, with a picture frame hanging on the wall, the opening in the frame for the image is the aperture.

In photography, aperture refers to the opening inside the lens that lets light in.

If you pick up your camera, or just a lens, and look down the lens as you turn the aperture ring, you will see the opening getting bigger and smaller. That is the aperture mechanism at work.

What does aperture do?

When you adjust the aperture setting on your camera, you adjust the diameter of the opening. This controls the amount of light hitting the sensor in the same way that irises work. The more your iris opens, the more light gets into your eyes and the more you see. It’s the same with the exposure triangle and aperture. The wider the aperture, the more light enters the lens and hits the sensor and the brighter your image will be.

Think of cat’s eyes during the day versus cat’s eyes in the dark. Or any eye for that matter.

aperture as an eye in a lens

Of course, aperture is not the only aspect that affects how bright your image will be. Exposure (the brightness of your image) is also affected by the other two elements of the exposure triangle: shutter speed and ISO. For now though we’re concentrating on the exposure triangle and aperture.

Aperture and depth of field

Aperture affects the depth of field in an image and is one of the ways to create those gorgeous blurry backgrounds that photographers shooting portraits outdoors work to achieve. I say one of the ways, as blurriness (or depth of field) is also affected by the focal length of your lens, as well as the distance of the subject from the background.

For the purpose of this blog post we’re talking about aperture creating blurriness, so let’s talk depth of field (DoF).

model shot wide with blurred background

About those blurry backgrounds…

A narrow, or shallow, depth of field means that a very slim area in front of and behind your subject will be sharp. If you have a number of subjects in shot and you’re shooting at a shallow depth of field, you will need to have all your subjects on the same focal plane for them all to be sharp. In other words, it is best that they’re side by side rather than one behind the other.

The focal plane is an area, a certain the distance from the camera, at which the sharpest focus is achieved. Depth of field is the range of distances either side of that point where the focus is sharp. This is not an equal distance, however. Depth of field extends twice as far behind the point of focus as in front of it. So, there is twice as much space behind your subject that will be in focus as there is in front of your subject.

Setting a deep, or long, depth of field means that there is a larger area in front of and behind your subject that will be sharp. You might have heard this referred to as “front to back sharpness”. It is why a deep depth of field is favoured by landscape photographers. They want lots of detail in the foreground, as well as the background of an image.

Further reading:
Learn about depth of field here: Using depth of field for gorgeous photography composition

Learn how distance affects depth of field here: The easy way to a beautifully blurry background


deep depth of field aperture priority

What are aperture settings?

Aperture is measured in f-stops, also called F numbers. The difference between each F number is called a stop, which is why we also call them f-stops. So, f5.6 is a one stop difference from either f8 or, in the other direction, f4. You can alter aperture by less than a full stop, but for now let’s stick to talking about full stops. They are: f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32.

Now, this is where things get a bit confusing… When we say wide aperture, we’re talking about a small F number, such as f2.8. When we talk about a narrow aperture, we’re talking about a large F number, such as f22.

It can be hard to wrap your head around this concept when you’re first starting out. So, here’s a solution…

The change in aperture happens inside the lens, but you can’t see the aperture getting wider (e.g. f2.8) or narrower (e.g. f22). You’re looking at the f-stop numbers in the display on top of your camera getting smaller (e.g. f2.8) or bigger (e.g. f22).

So it gets confusing to be talking about smaller looking numbers being wider. BUT the numbers aren’t actually getting smaller, because they’re actually fractions – f2.8 is actually 1/2.8, which is small that f22, which is 1/22.

The simple way to understand aperture

Think about aperture in terms of depth of field, because that’s what you’re controlling when you set your aperture (your f-stop, or F number).

  • Small number = shallow
  • Big number = deep

I’ll explain with photos…

wide aperture for shallow depth of field

Here’s a fun little scene I set up to demonstrate how aperture affects depth of field. The two images show the difference between shooting at an aperture of f2.8 (above) to get a shallow depth of field and then changing to an aperture of f22 (below) for a deep depth of field.

deep depth of field for front to back sharpness

Think of aperture this way:

Blurry background– shallow depth of field= small number = f2.8
Front to back sharpness– deep depth of field = large number = f22

All the f-stops in between these two examples are varying degrees of depth of field, from shallow to deep depth of field.

You’ll also hear shallow and deep depth of field being referred to as narrow and wide depth of field respectively.

Our really helpful printable cheat sheet lists all the f-stops between f2.8 and f22, including the half stops and third stops. It’s a great reference guide. If you missed it, here it is again.

Now that you know what aperture you need to achieve the type of image you want, you just need to decide on shutter speed and ISO…and that is the exposure triangle.

shallow depth of field to separate subject from background

One last thing about the exposure triangle and aperture:  Aperture priority mode.

Photographing in aperture priority mode

Shooting in manual mode is the goal so that you can take complete control of your camera and achieve the exact results you want. That said, if you are currently shooting in auto mode and don’t want to dive in at the deep end, the next best thing would be to shoot in aperture priority.

What this means is that when you set your camera to aperture priority, you decide what aperture you want to shoot at and the camera then sets the shutter speed accordingly. You will also have to set the ISO and will have to adjust it up or down to ensure a correct exposure for the aperture and corresponding shutter speed.

Shooting in aperture priority will get you used to checking your settings and thinking about what look you want to achieve. Once you’re comfortable photographing in aperture priority, moving onto manual mode won’t be such a big step.

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The ultimate tutorial on how aperture affects the exposure triangle. We also take a close look at how aperture affects depth of field to create a blurred background. Click through to start taking better photos! #phototips #photography #exposure #aperture