PHLEARN MagazineUnderstanding Aperture: Controlling Light Based on the Exposure Triangle

Understanding Aperture: Controlling Light Based on the Exposure Triangle

“Photography isolates the world via an aperture and gives the photographer the means to see differently, to achieve a spontaneous vision that is direct and uncompromising.”

– Ellsworth Kelly (American painter, sculptor and printmaker)

Let There Be Light…

There is one magical key to understanding the aperture definition. You may have guessed it from our on-the-nose subtitle: the key to understanding aperture is knowing that it’s all about harnessing light.

Adjusting the amount of light that travels through your lens can be a matter of personal style, but, before you can begin creating that style, you’ve got to learn the basic rules. Plus, you’ve got to understand the fact that aperture is not the only way you can control the light entering your camera.

There are three fundamental elements (referred to as the exposure triangle) that help to control the light coming into your camera: aperture, shutter speed (impacts the amount of time your camera body is exposed to light, measured in seconds), and ISO (simply put, a setting used to control your camera’s sensitivity to light). Tweaking these elements will control the amount of light in your image (much more to come on the triangle later).

The aperture is a hole in the lens of your camera, allowing light into your camera’s body. You can visualize what the aperture does by imagining a futuristic movie where a portal to another universe opens and closes – except, in this case, the universe is the image you create.

Here are a few examples of lenses that allow varying amounts of light to travel through the lens:

Controlling the amount of light entering your camera can greatly change the outcome of a photo. Changing the amount of light reaching your camera’s sensor can create a shallower depth of field (where part of the image is blurry/fuzzy) or a deeper depth of field (where all of your image is sharp and crystal clear).

In the shot on the left, the aperture is wider (f/3.5), so you can see that the building is not quite in focus in the very foreground and the higher up you go; there is a selective focus. In contrast, on the right is that same shot with a smaller aperture (f/22). Here, the bricks of the building are in focus throughout the photo:

Aleksandr Khomyakov [CC BY-SA 4.0 (], from Wikimedia Commons

You may often hear of aperture in conjunction with the word f-stop. The f-stop is the measurement of the diameter of the aperture. Understanding how f-stops work will help you to control the size of the hole that light travels through. We’ve included a brief section below to help you understand f-stop lingo and how to control stops on your camera.

Types of photography where aperture is probably the most useful include: landscape, portraiture and macro.


All lenses have a maximum aperture. f/1.4 is likely the largest maximum you’ll find on a non-specialty lens. Most lenses tend to be produce the sharpest images at f/11.

In this guide, we’ll expose the following aspects of aperture:

  • Moving Parts
    • The Blades
    • The Iris
    • The Aperture Chart
  • The F-Stop
    • Functional Fractions
  • The Exposure Triangle
    • Balancing Three Sides
  • What Is Depth of Field (or DOF)?
    • Shallow DOF
    • Deep DOF
  • Aperture Priority Mode
  • A Short Lesson on Lenses
  • What’s Happening in Automatic Mode? (And Other Useful Modes)

Moving Parts

Before we get into the nitty-gritty, let’s talk a couple of camera parts that help to let the light in.

The Blades

In the photo on the left, you can clearly see the aperture blades (where the lens is letting through a smaller amount of light). On the right, the blades are are not discernible, as the aperture has been adjusted to let more light shine into the lens:

The Iris

The iris is the mechanism that allows the opening of your lens to contract or expand, allowing more or less light into the camera. The graphic below captures that movement.

User:Catsquisher [Public domain], via Wikimedia Commons

The Aperture Chart

To give you an idea of the different sizes of aperture rings, take a look at the aperture chart below:


Now that you understand a bit more about our topic, the next thing you’ll have to understand is how to control the amount of light that travels through your camera.

Larger aperture sizes (represented by smaller f-stop numbers) result in more light traveling through your camera lens; conversely, larger numbers and smaller aperture sizes result in less light reaching the inside of your camera.

Yes, this is where our topic gets a bit counterintuitive. To help you wrap your head around the f-stop, we’ll have to get a little fractional.

Fractions and F-Stops

An f-stop is actually a ratio of your lens’ focal length divided by the diameter of your aperture. In other words, an f-stop number represents a fraction of your the lens aperture opening. For example, an f-stop of f/4 means the aperture opening is 1/4th, or 25 percent of the lens is open. If you are using a 100mm lens, f/4 is an opening of 25mm – or about an inch. In the same lens, f/8 represents an opening of 12.5 percent, or 12.5mm – about half an inch.

You don’t need to take this calculation with you on the road; it just helps to illuminate your understanding of our topic. The fraction is the reason a larger aperture is expressed with a smaller f-stop number. For instance, if you have a pie, ¼ of the pie (f/4) is much larger than ? of the pie (f/8).

There’s one more important thing to know about the f-stop:

  • Increasing the size of the aperture by one full stop doubles the amount of light traveling through your lens
  • Decreasing the size of the aperture by one stop halves the amount of light passing through

The Exposure Triangle

The following graphic gives you a sense of what happens when you manipulate the elements of the triangle. To review, these elements are: aperture, ISO and shutter speed.

To manipulate the triangle, you’re going to need to know what a “stop’ is.

When photographers refer to stops, they are talking about doubling or halving the light coming through their lens – you can add or take away stops by changing one of the three elements in the triangle.

This chart will help you to learn more about how stopping up and stopping down affects each part of the triangle. Each part of the triangle will control the amount of light that reaches your sensor, but they affect your image in different ways. Notice how increasing aperture adds background softness. See how slowing down your shutter speed introduces motion blur into your image and that increasing ISO introduces noise?

It’s important to note that double the f/stop is a little bit different than the other sides of the triangle. Since the f-stop is based on an equation, you don’t simply double or halve the numbers. For instance, f/4 does NOT let in double the light compared to f/8. Instead, refer to the aperture chart in the graphic above. The numbers immediately beside f/4 represent the numbers you will need to choose to move up or down one stop. See the examples in the chart below to get a proper sense of this.

How to ‘Balance’ the Triangle

In a given photo, there is one exposure that looks the best – it is the elements you change within the triangle that allow you to express your creativity.

Each time you take away a stop from one side of the triangle, you must compensate by adding a stop from one of the other two sides in order to keep the same exposure.

For example, if you take two stops away from the aperture, you must add two stops to shutter speed and/or ISO.

For another example, if you add four stops to the aperture, you must take four stops away from shutter speed and/or ISO.

When you’re balancing the triangle, you want your light meter (the little graph at the bottom of your viewfinder) to be in the middle (at 0).


Stops of 1.4 of 2.0 will allow more light in, which will allow you to do things like use faster shutter speed to capture motion and produce a shallow depth of field.

A stop of f/11 will reduce the amount of light coming into the camera. However, images are sharper and have a much wider depth of field. This is ideal when you want more of your scene to be in focus.

Stops over f/13 up to f/22 or f/29 are all used frequently for expansive, breathtaking landscape photos. Photographers may also use these apertures for night shots, where any point of light turns into a starburst (due to the laser sharp light of the tiny aperture). A couple of notes here, though: using a tripod on night shoots and high f-stops are not ideal for capturing faces.

What Is Depth of Field?

Depth of Field (DOF) is the range of distances from the camera where the subject of your image will still appear to be in focus.

The ability to control the DOF allows us to add life to our shots. We control that life, by controlling the light with the size of our apertures (you may be sensing a real, light-heavy theme to this guide by now – that’s a good thing!).

Here’s a popular photography analogy used to illustrate this definition in simpler terms (in case our DOF definition muddied the waters a bit).

Imagine a swimming pool with deep and shallow ends.

In the deep end, there’s more water, which means more of the image is in focus (ie. there’s a higher f-stop and smaller aperture).

In keeping with our pool analogy, here’s an example (f/14, 1/250s, ISO 200):

Notice how the entire image is in sharp focus, from the foreground to the water in the background.

On the other hand, in the shallow side of the pool, there’s less water. For us, that means that less of the image is in focus (ie., a lower f-stop, and larger aperture).

Here’s an example at this end of the ‘pool’ (f/2.2, 1/1000s, 1,000):

Notice how the subject is in sharp focus but the foreground and background are soft.


Your lens focal length will affect DOF as well. The longer the focal length, the shallower your DOF will be. A longer focal length allows you to capture a smaller area of your scene, creating a magnifying effect. This effect magnifies the blur in your photo, which makes your DOF appear shallower.

Here are a few ideal focal lengths for various types of photos:

  • 75 to 100mm are often suitable for portraits with blurry backgrounds
  • 14 to 35mm are often used for landscape shots
    • 14mm is quite wide; you will have a lot to work with in your frame
    • 21mm is a happy medium among the ranges; it’s neither too wide or too narrow
  • Macro lenses range from 15mm to 200mm

Shallow DOF

Shallow DOF keeps parts of the image (likely the parts closest to you, the photographer) in focus, and blurs out parts of the image that are a further distance from the camera. The f-value is smaller, and the size of the aperture opening in the lens is wider.

Here are a few examples of images capturing a shallow DOF:

If you’re a beginner, pay close attention to the f-stop values here. However, if you’re more advanced, take note of how the other settings of shutter speed and ISO interact with the f-stop.

f/1.4 is a good, standard, large-size opening to use when you’re shooting shallow DOF, as this photographer did in the shot below:

Details: Canon EOS 60D, f/1.4, 50mm, 1/13s, ISO 100

Here are a couple close-up examples. Note the slightly higher f-stop numbers than previous examples, but still giving shallow depth of field.

Details: Canon EOS 100D, 50mm, f/3.2, 1/125s, ISO 100

Details: Canon EOS 5D Mark II, 70mm, f/6.3, 1/320s, ISO 200


The physical distance of your subject from the background impacts the DOF in the image. So you’ll need to either back-up your background or pull your subject forward if you don’t like what you’re seeing.

Deep DOF

Deep DOF keeps parts of the image that are both close to the camera and far away from the camera in sharp focus. The f-value is higher, and the size of the hole letting light through is smaller.

In the photo below, the photographer has chosen to shoot a portrait photo that doesn’t blur out the background. We are stricken by the beauty of the model, but the fall leaves have also been highlighted. The subsequent photos show even more clarity and detail because the photographers have selected higher f-stops.

Details: Nikon D3100, 70mm, f/8, 1/3200s, ISO 400

Details: Canon EOS Rebel T7i, 42mm, f/14, 1/640s, ISO 800. Photo by Alex Bierwagen on Unsplash

Details: Nikon D750, 200mm, f/16, 1/500s, ISO 400. Photo by David Clode on Unsplash

What Is Aperture Priority Mode?

Changing your f-stop in aperture priority (Av) mode will cause your camera to automatically adjust its shutter speed to properly expose a photograph. Beginners can start with Av mode to get a clear understanding of how aperture affects shutter speed and ISO. Then, more advanced photographers can switch to manual mode to really master aperture and how it works.

When should you adjust your aperture?

  • Think about the kind of photograph you want to capture. If you want a soft, blurry background, set your camera to a wide aperture, like f/2.8. If you want everything in focus, set your camera to a smaller aperture. Remember, that means a higher f-stop number, like f/11.
  • Usually, you should set your aperture first, so that you can get the type of blur you’re trying to achieve. Then set your ISO and shutter speed accordingly.
  • In a darker environment, you will want to keep your aperture wide (or your f-number low) to let more light into your lens.
  • When you are in a very bright environment, a low f-stop number could result in an overexposed image. This is because your shutter speed and ISO cannot compensate for so much light. In a very bright area, use a narrow aperture (a higher f-stop number) to let less light in.
  • When there is noise in your photo, you may want to stop down your ISO and stop up your aperture to compensate.

A Little on Lenses

There are two types of lenses you need to know about:

  1. Fixed aperture – this means the lens has a constant maximum aperture that remains the same over all focal lengths. Prime lenses and many higher-end zoom lenses have a fixed aperture.
  2. Variable aperture – zoom lenses with variable aperture have a maximum aperture that changes depending on the focal length. They have a wider aperture at the wide end of the zoom and a narrow aperture when zoomed out. Variable aperture lenses are significantly less expensive than fixed aperture lenses.

The kit lens (the lens that comes with your camera) may not have a low enough f-stop for your liking. For instance, if you have a kit lens with a maximum aperture of f/3.5, it may be hard to shoot indoors and get a nice blur in your background. Most kit lenses are variable.


When photographers refer to lenses as “fast” or “bright,” they are talking about lenses with very wide maximum apertures, like f/1.4, f/2 or f/2.8.

If you’re thinking of taking the leap to aperture priority mode or plan to go fully manual, it may help you to know…

What’s Happening Behind the Scenes?

Thinking about how our cameras work in automatic mode can help us understand how to go from aperture priority mode to fully manual.

Macro mode utilizes a large aperture to make up for DOF compression of shooting close-up (compression is where your background appears flattened, and your subject looks closer to that background). You can’t set your exact aperture in this mode, but the automatic settings will help you to keep your subject in focus.

Landscape mode utilizes a wide DOF for the smallest aperture (largest f-stop) to ensure the whole scene, or most of it, is in focus. Your shutter speed will be slower (a slow shutter speed helps you to capture the motion of, say, a babbling brook or ocean waves. This is because the shutter stays open for longer, enabling your camera to capture that motion).

Portrait mode uses a large aperture to produce a shallow DOF. It also uses a lower ISO, which will further blur your background, because it absorbs light more slowly, and therefore, captures more detail. A lower ISO also gives your photos a very fine film grain (ie. less noise in your photo). This makes it easier to create clearer enlargements.

Aperture Priority Mode

Put your camera on aperture priority mode (select the ‘Av’ button), and your camera will select the correct shutter speed to properly expose your photo. This is a great launching off point for beginners who wish to get their cameras out of auto and start having some creative control over their photographs.

Manual Mode

Once you have a grasp on how aperture and the rest of the exposure triangle impact your photos, turn your dial to ‘M’ to become the master of your camera’s settings. This is where the real fun and creativity begin.


Often, we find that learning about f-stops is the trickiest part of understanding aperture. You can get more familiar with the f-stop with our aperture tutorial video, where Aaron explains it all while playing with LEGO!

Another way you can digest the information in this guide (or any new information you’re learning about for that matter) is to teach a friend or family member what you’ve learned here. Use your camera to demonstrate what you’ve learned by teaching them the theory and how they would adjust the settings on their camera. Keep this guide and your camera’s manual close by in case you need to refer to them.

If you can teach someone else what you’ve learned, you know for certain you’re ready to apply the concepts.

Seth Kravitz

Seth is the CEO of PHLEARN and an avid writer, photographer, startup investor, and business mentor in Chicago. He joined PHLEARN in 2016 and has been focused on expanding the community to reach millions of new Phans and make learning fun for the next generation of great artists.



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