Understanding aperture and f numbers

Lui Brandstrup
Sep 26, 2015 · 3 min read

Aperture is one of the most confusing things about photography. Why would you measure aperture in f numbers (f/5.6), and why would a smaller f number let in more light?

Aperture has a huge effect on the picture you take, so it is not a good thing to be confused about it. Therefore i have chosen to explain aperture in one of my first blogposts.

How does aperture work?

Aperture on your camera looks like this “f/” followed by a number, fx. it can look like this f./5.6 or f/2.8 and etc.

Before explaining why aperture is measured in f numbers, I will explain how aperture works, because then it will be much easier to understand why it is called fx. f/5.6

Aperture determens how much light is coming through the lens. Aperture is a ring inside the lens that vary in size depending on how much light you want to let in. We measure the size of the ring in numbers. The weird thing is that we measure them backwards, so a low number like fx. f/2, lets in more light than the number f/22. What? Why would a smaller number let in more light. Well, it’s actually pretty simple. an aperture of for example f/2 means the width of the aperture hole in the lens, is the focal length divided by 2 (and f/22 is the focal length divided by 22, etc.)

So if you work with a 50mm lens at f/2, the width of the aperture hole in the lens is 25mm, because we take the number 50 and divide it by 2. That’s the size of the aperture hole. Therefore if you take the 50mm lens and change the f number from 2 to 22, the size of the aperture hole is 2.27mm (50/22). A higher aperture number gives you a smaller aperture hole and therefore you let in less light.

So if i’m working with a zoom lens like the 70–200mm, and put it on fx. aperture f/2. Then when i’m zoomed out to 70mm, my aperture hole has the size of 35mm, but when i’m zoomed all the way in to 200mm, my aperture hole has the size of 100mm, but I still get the same amount of light on my image, because both are set to aperture f/2. How does that work?

It’s pretty simple. when we zoom in to 200mm, the lens has a much narrower field of view, therefore it has to gather the same amount of light from a much smaller area — to compensate the narrow view, aperture has to be bigger.

That’s why we use the term f numbers and not the millimeter size of the aperture, because two different aperture sizes can let in the same amount of light depending on how long you zoom in.

why is it measured in f numbers?

Aperture openings are measured as fractions of the focal length of a lens. A fraction in mathematical terms, is a number usually expressed in the form a/b.

So f/2 can also be expressed like 50/25, where 50 is the focal length and 25 is the size of the aperture hole.

Lastly, when you want to double the amount of light that you let in, you multiply by 1.4. The reason why you don’t multiply with 2 is because you take the formula for the area of a circle and to make the area twice as large you have to increase the diameter by 1.4.

Thanks for reading this post! Did you find it helpfull? Please let me know in the comment section, because it can be pretty hard to explain. And if you have any questions, I will gladly answer them. If you liked it, please press “recommend,” since it really helps me :)

I started this blog, 18 to 55, to challenge myself to only shoot with the 18–55mm kit lens for four months. Read about my challenge and why here.

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Now go shoot some pictures!

Lui Brandstrup

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23 year old entrepreneur and father of two daughters. Writes about personal development, entrepreneurship, and financial indepence.