What is Aperture in Photography?
Photo by Wan San Yip on Unsplash

What is Aperture in Photography?

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?
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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.

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. The aper­ture set­ting influ­ences how much of the scene from the fore­ground to the back­ground is 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.

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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:

Blurry Bokeh image
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.

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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).

Image of the Christchurch New Brighton Pier with start burst lights
Christchurch New Brighton Pier with start burst lights

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.
  • 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

Photography Club Enthusiast

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