What Is An F-Stop and How Does Aperture Control Depth of Field?


Photography is extremely seductive for a lot of people around the world, it can be practiced at a professional level or as a very strong hobby. One of the first things people want to learn about photography is how to use a camera in manual mode. Here is an introduction to f-stop and aperture

Manual mode empowers anyone to create extremely creative images beyond the “perfect” modes the camera figures out when shooting in Auto or Program. In photography, everything is about capturing light and today we are going to talk about one of the most complex features of exposure – that is, what is an F-Stop and how apertures control depth of field.

If you are a beginner in photography, the words f-stop and aperture may sound new or quite complicated. So before diving into the details of how these work, we will quickly look at what is an f-stop and what aperture means.

What Is Aperture?

Aperture is quite straightforward and it is nothing but the opening in your camera's lens that allows light to pass through the lens on to the camera's sensor. The size of the aperture is controlled by aperture blades and it is this aperture value or size that decides the amount of light entering the lens.

Wider apertures let in more light leading to shallow depth of field, whereas narrow apertures let in less amount of light leading to a larger depth of field and photographers can make use of varying aperture values to control exposure and depth of field in their images.

What Is An F-Stop?

F-Stop (N) is called the focal ratio which is the ratio of the focal length of the lens (f) to the diameter of the aperture (D).

In simple terms, this is nothing but the aperture value that your camera displays, for example, f/1.8, f/5.6. f/11, f/16, etc., and it is also called the F-Number.

f/1.8 is a larger (wider) aperture compared to f/5.6 and f/5.6 is a larger aperture compared to f/11 and so on.

If you are someone who gets confused with the term larger apertures, this should help you. Consider the f-numbers as fractions, for example as 1/1.8, 1/5.6, 1/11, etc. In these fractional numbers, 1/1.8 is larger than 1/5.6 and 1/5.6 is larger than 1/11. Keeping this in mind, think of the aperture values where f/1.8 is larger or wider than f/5.6 and f/5.6 is larger compared to f/11. This is quite simple.

It is called f-stop, because it simply stops or limits the amount of light entering the camera depending on the size of the aperture.

Illustration to show how aperture size changes for different f values.

What Does The Letter “F” Stand For in F-Stop or F-Number?

The letter “f” stands for focal length and if you wonder how focal length is related to f-stops or f-numbers, here it is.

Let us consider, for example, a 50mm f/2 lens. At its widest aperture, f/2 = 50/2 = 25 which will be the diameter of the aperture in mm for a 50mm lens at aperture f/2. The same lens, at f/10 = 50/10 = 5 mm will be the diameter of the aperture for a 50mm lens at aperture f/10.

You can see from the above, that, at f/2, the diameter is larger, letting in more light and at f/10 the diameter is smaller, thereby letting in less light.

Why Aperture is So Difficult to Understand?

Don't feel bad if you are struggling with aperture, it is one of the most difficult features to master when it comes to exposure, and we'll explain why. First of all, the strange numbers. Everything in exposure controls stops of light, and aperture controls how much light will pass through your lens depending on aperture values.

These are Aperture Values for 1 stop of light

f/1.0 f/1.4  f/2.0 f/2.8 f/4.0 f/5.6 f/8.0 f/11 f/16 f/22

f/1.0 has a very large aperture thereby letting in the maximum amount of light. f/1.4 lets in half as much light compared to f/1, and f/2 lets in half as much light as compared to f/1.4.

What the above means is, for each step down (decrease or narrowing down) in aperture, only half the amount of light is let in. Remember, this is quite different compared to ISO and shutter speed.

The table below will clearly explain how the f-stops work for aperture. When it comes to aperture, the actual area of the opening is taken into consideration to calculate the amount of light that enters the camera.

In the table below, we have used 50mm focal length for example. So,

  • The focal length (f) = 50mm
  • Aperture variations are considered from f/1 to f/11
  • The diameter of the aperture is calculated by dividing the focal length by aperture value
  • Dividing diameter by 2 gives the radius of the aperture
  • We now need to calculate the area through which the light enters the camera, which is nothing but the area of the aperture opening. Since the aperture is almost circular in shape, we will use the formula for the area of the circle (πr²) to calculate the area of the aperture. This is the area through which light enters the camera for each value of aperture.

You can see from the table below, that for each increase in a stop of aperture, that is from f/1 t0 f/1.4, f/1.4 to f/2, f/2 to f/2.8 and so on, the area of the aperture almost halves, which means it lets in half the amount of light.

These are Aperture Values for 1/3 stop of light

f/1.1 f/1.6 f/2.2 f/3.2 f/4.5 f/6.3 f/9.0 f/13 f/18

These are Aperture Values for 2/3 stop of light

f/1.2 f/1.8 f/2.5 f/3.5 f/5.0 f/7.1 f/10 f/14 f/20

The trick here is to see these numbers as fractions, if you manage to keep this in mind every time you move aperture values, you can be sure that you'll master them in no time.

Imagine that the letter “f” is a number 1, and the numbers below are simple denominators, you'll understand that the larger the denominator, the smaller the aperture hole on your lens.

Photo by Markus Spiske

Keep This in Mind:

  • The smaller the aperture opening, the lesser the amount of light entering the camera.
  • The larger the aperture opening, the larger the amount of light entering the camera.

If you keep the aperture value low (wider), you will have brighter images. Whenever you get images that are looking too bright for you, you could narrow down your aperture value a bit to reduce the amount of light passing through your lens.

If you are getting images that look too dark for your taste, you should open (widen) your aperture value in order to increase the amount of light and you'll avoid dark images. The trick here is to find the sweet spot for every lighting condition that you deal with.

What About Depth of Field?

Aperture doesn't only control the amount of light but also how light and distance behaves inside our camera. Large (wide) apertures create a shallow depth of field (DoF), creating beautifully blurred backgrounds in our images.

That's why portrait photographers love to use lenses that allow them fast apertures like f/1.4 or f/1.8. If you close the aperture down, you'll achieve deep levels of DoF, extremely useful for landscape photography.

Photo by Steve Richey

If you don't understand this so clearly, let me explain it in the easiest way there is, by using our own fingers and eyes.

Think of your eyelids as the (controllable) aperture of your eyes, if you put your finger close to your eyes and you try to focus it, you'll instinctively open your eyelids, and your finger will appear in focus, but the background will blur out. If you focus something in the distance, you'll close your eyelids, but everything will remain in sharp focus.

Keep in mind that we have shared this text for you in order to make aperture understandable alone. That's why we have avoided complicating our minds with shutter speeds and ISO values. Aperture is abstract, but we are aiming here to make it extremely easy for you to understand it.

Photography is all about practice and more practice. It takes different amounts of time to master it, and every person has a different learning rhythm. The best thing that you can do in order to become a better photographer is to take your camera wherever you go, and start thinking in terms of light.

If you are interested in learning more about exposure, particularly f-stop and aperture, you can check out this beautiful photography guide – Understanding Light Book One. Whatever you do in photography, do it with passion, and you will have the time of your life!

Further Resources For F-Stop And Aperture:

  1. Bite Size Tips: How To Get A Blurry Background In Your Portrait Photos
  2. Understanding Aperture Is Important for Any Photographer. Read Why…
  3. This is What You Need to Know About Camera Aperture
  4. What Are F/stops and How Do You Define a “Fast” Lens?
  5. Understanding F/Stops & Stops in Photography Exposure
  6. How to Know What F-Stop to Use

About Author

Federico has a decade of experience in documentary photography, and is a University Professor in photography and research methodology. He's a scientist studying the social uses of photography in contemporary culture who writes about photography and develops documentary projects. Other activities Federico is involved in photography are curation, critique, education, mentoring, outreach and reviews. Get to know him better here.

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