Basic Photography Series: Exposure Triangle Part I

What is Aperture in Photography?

Frithjof Moritzen
Jul 25, 2020 · 8 min read

Photography is a wonderful hobby which teases and exercises your brain. You will never run out of concepts to master, and one of the first concepts you will come across is ‘Aperture’. Every camera needs it to exist.

You want to know if this article for you? If you do not know all the following in the context of ‘Aperture’, then you’ll learn something new:

  • Exposure control
  • F-stops
  • Why is f/16 a smaller hole than f/1.8?
  • Why is f/8 permitting half the light through the lens than f/5.6?
  • What is a DOF button?
  • What is DoF and how does that relate to aperture?
  • What makes up the shape of the ‘bokeh’?
  • Square root of 2
  • Starburst/Sunbeam effect — how many beams are visible when your lens has 7 blades?
  • Diffraction
  • What does the imprint of ‘1:2.8–3.5’ on your lens mean?
  • Vignetting
  • What are dominant features of a ‘fast’ lens?

Aperture is one of the three principal things to control the exposure (the ‘brightness’) of your image, the other two are shutter speed and ISO. As a beginner, it is confusing to see that most settings and concepts have side effects. You set one thing and another changes at the same time. Same with Aperture. But I try to explain it with as few dependent concepts as I find useful. I hope it works for you. So when I tell you that aperture and shutter speed and ISO hang together, then you don’t have to know for now exactly how that works. I explain that later in another article (spoiler alert: it is the ‘exposure triangle’).

So what is Aperture? The Aperture controls how much light passes through the lens of your camera. It is a variable opening, similar to your eye pupil. It can close to a tiny hole, only allowing a bit of light through the lens and on to the sensor. Or it can stay wide open to let the greatest amount of light through the glass. And you have several steps between the two points. Most lenses do not let you control the aperture fluently, only in these certain steps (‘stops’). Not long ago, most lenses had a scale of these steps printed on the aperture ring of a lens. That vanished now, and most often you can only control the aperture by selecting a setting in your camera. This image here shows that ring, it is the first one. You’ll find values like 22, 16, 11, 8, 5.6, 2.8.

Photo by Markus Spiske on Unsplash

When you look at your camera lens, you can see a lot of glass, but it also has a diaphragm shaped metal ‘iris’ inside. You can’t see it, as it is in a wide open position to let the light in for focusing and metering your scene.

Photo by Markus Spiske on Unsplash

But when you press the shutter button, the camera closes the ‘iris’ to the aperture setting you (or your camera) selected. It controls how much light goes through the lens by changing the size of the hole. The numbers we have seen on the aperture ring tell you how big the hole is. A 22 is a tiny hole, a 2.8 is wide open.

A small animation of aperture change

Wait, is there something wrong? The hole gets smaller when the number gets bigger? Yes, exactly! Do you want to know why? No? You are trying to dodge some maths? That’s OK, but the concept is still important to grasp. Haje Jan Kamps wrote a great article about the weirdness of the F-stop scale, I can’t explain that better than he did, so here you go:

Once you recovered from the math side of things, it might relieve you to hear that you only need to remember that a small f/number means a big aperture opening, and a shallow Depth of Field (DoF). A … what? Yes, another side effect. If you have your subject in perfect focus, the aperture setting influences how much of the scene in front of your subject and behind it is also in acceptable focus. That is (in simple terms) the Depth of Field. This time you are dealing with physics/optics. To keep it simple, know that a small aperture (a big f/number, e.g. f/22) gives you a large DoF. And vice versa, a large aperture (a small f/number, e.g. f/1.4) gives you a shallow depth of field. Well, not always, as the DoF depends not only on the aperture but also on the magnification/distance to the subject. It is hard to explain one concept without opening the box of Pandora.

Depth of Field is something which deserves its own article, but I need to explain it here, as the DoF changes together with changing the aperture, they are married for life, no divorce possible. If you want to see examples of shallow depth of field, have a look at my article about beginner questions:

To complete the topic, it is worth mentioning that many cameras have a function called “DOF preview”. That could be a dedicated button, most often on the front of the camera. Or it could be a function you can assign to a programmable button on your camera. What it does is it lets you preview the effect of the DoF of your selected aperture before you take the image. How does that work? If you look through your viewfinder, the lens keeps the aperture as wide open as possible, to have light for metering and focusing. The aperture only closes to the selected aperture at the time of the exposure. That means that the depth of field you see through the viewfinder is on the shallow side. But when you review your image after capture, you find that the DoF could be surprisingly different to what you saw through the viewfinder. The DOF button now lets you close the iris to the selected aperture while looking through the viewfinder. You can see the change in DoF before you take the image. If you selected a small aperture (I repeat myself, but repetition helps with remembering things: a small aperture → big f/number → large DoF), the viewfinder might get darker as less light is available.

All right, was that all to know about ‘Aperture’? Of course not! There is more. You could have a look at the blades which form the ‘iris’. Depending on the lens you have stuck to your camera body, the construction of the diaphragm gives the aperture a specific shape. In the image below you can see that seven blades form a hole, but it is not round.

Photo by Wan San Yip on Unsplash

The shape of the aperture plays a role for the shape of the out-of-focus parts of your image. Photographers call this ‘bokeh’, and a lot of them pay an obscene amount of money to get a lens with a nice bokeh. That is highly subjective, and it varies the same as the pronunciation of the word ‘bokeh’. One example:

Christmas tree lights out of focus, with 5 blade aperture bokeh

You can see the distinct shape of the out-of-focus lights. It directly results from the number and shape of the blades of the diaphragm of the lens. The more blades, the less angular the shape.

Anything else to know about ‘Aperture’? Yes.

There is an interesting photographic effect you can achieve when you select a small aperture (large f/number, e.g. f/22), that is the starburst effect (or sunbeam effect).

You can observe this effect here in the lights, as they have distinct star rays. The shape of the starburst also depends on the shape and number of blades of your lens. The effect is caused by ‘diffraction’ (a topic for another day). Fun fact: An even number of blades (in my example above: 6) causes the same amount of beams, an uneven amount of blades causes a double amount of beams. Interesting, right?

Are we done now? We had (you know, repetition is king):

  • part of the ‘exposure triangle’, controls the amount of light passing through the lens
  • high number → small hole → exposure gets darker (if all else stays the same) and we have a large Depth of Field
  • small number → large hole → exposure gets brighter and we have a shallow Depth of Field
  • number and shape of blades determine the form of bokeh shape
  • you can achieve a starburst/sunbeam effect with a small aperture (big f/number)

There are some more things to note:

  • You can find the greatest aperture of your lens printed on your lens body, e.g. “1:2.8”. 2.8 is the largest aperture of that lens. Zoom lenses can have more than one number, e.g. 1:2.8–3.5. That means that the wider end of the zoom range has a maximum aperture of 2.8, the longer end of the zoom range has the largest aperture of 3.5. This kind of notation is a relation, it means for every 1 mm of entrance pupil, there is 2.8mm of focal length. So what is the entrance pupil? There is a great video from Gerald Undone about that, and some good illustration of aperture, worth the watch.
  • From full f-stop to f-stop the amount of light passing through the lens is different by the factor of 2. Example: f/11 would allow double the amount of light through the lens than f/16, and f/8 half the light of f/5.6.
  • Larger f-stops (large f/number) often cause a decrease in image quality because of diffraction. Expensive lenses mitigate that by complex lens designs. Cheaper lenses usually have the best image quality in the middle range around f/8 as a general rule. Your lens might be different, but you can test that yourself.
  • Lenses with a large maximum aperture require big diameters and more glass. These are ‘fast’ lenses. They tend to be expensive, large and heavy. ‘Fast’, because they allow shorter shutter speeds.
  • a small aperture can cause vignetting (the image is darker towards the edge of the frame)

That’s it, I think.

I hope you learned something new, but even a refresher course is sometimes helpful.

Have fun taking pictures — how about trying a sunbeam image with a large f-stop?

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Frithjof Moritzen

Written by

Photographer, Digital Artist, Photography Club Enthusiast https://GlobalPhotoClub.com

The Global Photo Club

You want to join a Photo Club? You are a photo club member and want to improve on your photography? Get ideas about photo projects? New techniques? Learn about Photography history? Refresh the photography basics? Learn the digital darkroom tools? This is the place.

Frithjof Moritzen

Written by

Photographer, Digital Artist, Photography Club Enthusiast https://GlobalPhotoClub.com

The Global Photo Club

You want to join a Photo Club? You are a photo club member and want to improve on your photography? Get ideas about photo projects? New techniques? Learn about Photography history? Refresh the photography basics? Learn the digital darkroom tools? This is the place.

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