What is Aperture in Photography? Key Concepts Explained

Struggling to understand aperture and how to use it effectively in your photography? Here are some key concepts and simple explanations to help clear up any confusion.

One key concept in photography that a lot of beginner photographers struggle with is aperture.  And for good reason – the numbers that indicate aperture on our cameras don’t make a whole lot of sense at first, and it is tempting to just ignore it and stick with Auto Mode. 

Below I help you break down the mental barriers around aperture so that you can start using this powerful tool to unlock your creative potential today.  

What is Aperture in Photography?

Aperture is one of the three camera settings that control the exposure of an image.  Collectively, these settings are referred to as the “exposure triangle” and consist of shutter speed, aperture, and ISO.  

Aperture controls exposure by increasing or decreasing the relative amount of light that enters a camera lens. 

This seems straightforward enough, however, the details of how aperture works to control exposure is often a stumbling point for many photographers.

Let’s break it down.   

Every camera lens has an internal circular diaphragm of overlapping blades with an opening at the center.  If we were to take a lens apart and look at this diaphragm, we would see an opening in the middle called the “aperture”. 

The diameter of the aperture changes as the diaphragm blades are adjusted, and this controls how much light passes through the lens to the camera sensor.  

The lens diaphragm functions a lot like the iris of an eye, so you can think of the aperture just like the pupil of an eye.  Make the aperture wider – more light will enter the lens.  Make the aperture narrower – less light will enter the lens.

Lens Diaphragm and Aperture

There is a small detail that is often missed about this opening.  Because there is a lot of lens glass in front of and behind the diaphragm, the hole that you see when you look at the end of your lens is actually called the “entrance pupil.  On the camera mount side of the lens, it’s called the “exit pupil.  

The important distinction between aperture (the physical opening) and the entrance pupil (the apparent opening) is that the diameter of the entrance pupil is what is used to calculate the f-number, which represents the aperture setting. 

More on that later.

How Does Aperture Work?

To understand how aperture works as part of the exposure triangle, we first need to grasp a key concept.  We’re going to talk a little bit about how light behaves, so bear with me.

Wavelengths of light entering a camera lens through the aperture travel along the distance of the lens barrel to get to the camera sensor.  

That distance is important in understanding aperture and is known as the “lens focal length”.  Most lenses are identified by their focal length (a 35mm lens, 50mm lens, 70-200mm lens, etc.).

How a lens focal length is shown on lens
Lens focal length range as shown on the lens.

Physics tells us that the intensity of light diminishes as it travels over a distance.  This inverse relationship between light intensity and distance is known as the Inverse Square Law.  

Without getting into the mathematical details (shown if you’re interested), the Inverse Square Law basically indicates that the intensity of light diminishes by one quarter every time the distance from the light source is doubled.  Conversely, when the distance from the light source is halved, the light intensity is quadrupled.

Two people sitting by campfire with Milky Way above

However, we don’t need math to understand the Inverse Square Law conceptually.  

Imagine sitting around a campfire with friends talking about your next photography adventure.  You can see everyone’s faces in the firelight. But, if you walk away from the campfire to gaze up at the stars, the intensity of the light from the fire will be reduced the farther you walk away from it.

The same is true in photography. 

Let’s say we have a 50mm lens and a 100mm lens both set to have the same diameter of the aperture (really, the entrance pupil).  The same amount of light would pass through this opening in both lenses, but the light would have to travel twice as far in the 100mm lens to reach the sensor. 

In this example, the intensity of light hitting the sensor in the 100mm lens would be less than that of the 50mm lens.

Now understanding this relationship between light and distance, we realize that simply controlling the diameter of the entrance pupil isn’t the complete picture (pun intended) in regard to exposure.  

When it comes to controlling exposure, we need to also account for the focal length of the lens.  

So, how do we do that? Enter the f-number!

How Aperture Accounts for Focal Length

Aperture settings are represented by an f-number (also referred to as the f-stop).  The f-number is a dimensionless, relative unit that considers both the lens focal length and the diameter of the entrance pupil.  Some photographers call the aperture the “relative aperture” for this reason.

To account for this relationship, the f-number is represented as a ratio (or fraction) of the lens focal length divided by the diameter of the entrance pupil:  

f-number = focal length (f)/diameter of the entrance pupil (mm)

This ratio is displayed as f/number, where “f” represents the focal length, the “/” represents the fraction, and the number represents the resulting quotient of the fraction.

Anatomy of an f-number
The anatomy of an f-number.
Example of relative aperture at two different focal lengths

Let’s look at an example of relative aperture and f-stops:

A 50mm lens with an entrance pupil diameter of 12.5mm would have a relative aperture of f/4.  A 100mm lens with an entrance pupil diameter of 25mm would also have a relative aperture of f/4.  


Because the f-number is a ratio that is dependent on focal length, the aperture setting is a normalized value and indicates the same amount of light exposing the sensor regardless of the lens.  

In this way, all apertures are created equal when it comes to the amount of light traveling through a given lens.  An aperture of f/4 on one lens will allow the same amount of light to hit the sensor as f/4 on a lens of a different focal length even though the diameters of the entrance pupils are different. 

What is the Maximum Aperture of a Lens?

The maximum aperture of a lens is the widest possible aperture of the lens.  It is given as part of the nomenclature of a lens and indicates how fast the lens is.  More on what a fast lens is later.

For example, the Rokinon 14mm f/2.8 lens has a maximum aperture of f/2.8, whereas the Nikon 50mm f/1.8 has a maximum aperture of f/1.8.  Occasionally, the maximum aperture is written as 1:2.8 or 1:1.8 rather than as f/2.8 or f/1.8 on the lens itself.

Some lenses have variable maximum apertures that change according to the focal length.  For example, the Sigma 50-500mm f/4.5-6.3 has a maximum aperture of f/4.5 at 50mm and a maximum aperture of f/6.3 at 500mm.

How maximum aperture is shown on lens
Maximum aperture label on a variable aperture lens

The reason camera manufacturers produce variable aperture lenses in addition to fixed variable lenses is that they tend to be lighter and more economical. 

The downside of using a variable aperture lens is that the longest focal length will have a smaller maximum aperture. This affects depth of field, how well the lens performs in low light, and how fast the lens is at that focal length.

How Do Teleconverters Affect the Maximum Aperture?

Teleconverters are used to extend the focal length of a lens.  They attach between the camera mount and the lens, and some are compatible with a camera’s autofocus system (it is worth checking on this to be sure).  

Examples of Teleconverters

Teleconverters are a lightweight and economical way to get more reach with your lenses. 

For example, a 2x teleconverter on a 200mm lens would make the effective focal length of that lens 400mm.  When I want to pack lightly, say for a day hike with my camera, I’ll often opt to bring a teleconverter rather than lug around a heavy telephoto lens.

Because focal length affects the relative aperture, teleconverters change the maximum relative aperture of a lens.  The diameter of the opening of the diaphragm stays the same, but the overall focal length of the lens changes. So, the f-number changes by the same factor.

For example, when I use a 1.4x teleconverter with my Tamron 24-70mm f/2.8 lens, my maximum aperture becomes f/4, which is 1.4x smaller than f/2.8. 

Similarly, when I use a 2x teleconverter with my Sigma 50-500mm f/4.5-6.3 lens, the maximum aperture at the effective 100mm focal length (50mm x 2) is decreased by 2x from f/4.5 to f/9.  

What Are the Downsides of Using a Teleconverter?

Because teleconverters reduce the maximum aperture of a lens, less light will hit the sensor when the lens is at its widest aperture.  This affects how well the lens and teleconverter combination will work in low light situations. 

To account for the reduction in light, you would have to either use slower shutter speeds or increase the ISO (or both) to get proper exposure.  These changes aren’t always feasible and could reduce the quality of the image.

Secondly, as I mentioned earlier, some teleconverters don’t work with the camera’s autofocus system, which will essentially turn your lens into a manual focus lens when using the teleconverter.  

Additionally, autofocus lenses find focus at the maximum aperture before the shutter is fully depressed, which is the moment when the chosen aperture is then applied.  Because teleconverters reduce the maximum aperture, you may find that autofocus does not work as well (or at all) if the teleconverter changed the maximum aperture beyond the aperture at which the lens can focus.

What Aperture is the Sharpest?

Is there an aperture at which a given lens will yield the sharpest image?  

Generally, yes.  Several factors influence how to get sharp images (a topic for another article), including aperture.  

However, there isn’t one universal aperture that is considered the sharpest because sharpness depends on the optical design of the lens, so the best aperture will vary depending on the lens.

Lenses tend to operate best somewhere in the middle of their f-stop range, usually a few stops down from the maximum aperture (say around f/5.6 – f/8 on an f/2.8 lens), and that is considered its “sweet spot”.  The sweet spot is the f-stop (or f-stops) at which chromatic aberrations and lens diffraction are reduced the most.  

Chromatic aberrations are often introduced at wider apertures (f/1-f/4), and diffraction is introduced at narrower apertures (f/22 and above).  Exactly which apertures create aberration or diffraction is lens dependent.

You can find your lens’ sweet spot using the following method:

  1. Take a series of identical images at a fixed distance from your camera using a patterned subject, such as a black and white checkerboard or a patterned fabric that has areas of high contrast.  
  2. While keeping the ISO constant, take a series of images each along the f-stop scale and adjust the shutter speed as necessary to properly expose the image.  In this scenario, because the subject is not a moving object, the shutter speed will not affect sharpness. 
  3. Then, compare the sharpness of each image by zooming in on the areas of contrast on a computer and see if you can identify differences between the f-stops.

Alternatively, you could also check out DxOMark’s lens database to see if your lens has been tested by them.  They do extensive testing of lenses, including testing the sharpness at all of the apertures and focal length combination.  

In the screenshot below, y-axis is the f-stop and the x-axis is the focal length of the Nikon 70-300mm f/4.5-5.6G lens.  The green area represents the aperture and focal length combinations that give the sharpest images. You can see that the sharpness falls off (yellow/orange/red areas) as the f-stops and focal length increase.

DxOMark Lens Sharpness Test (x = focal length; y = aperture)

Should you always use the aperture sweet spot?  Not necessarily. Aperture also affects depth of field, and if you are shooting a scene where the depth of field is important, then you would want to choose the aperture that would give you the appropriate depth of field.

How Does Aperture Affect Depth of Field?

Depth of field (DOF) is defined as the region of an image that is acceptably sharp.  This region is the zone between two points that are at two different distances from the camera where all objects between those points are acceptably sharp.  DOF can be difficult to understand at first because an image is a two-dimensional object, and depth of field is a three-dimensional concept.  

DOF is used as a creative technique to either blur the background of an image, such as in portrait photography, or to have most of the scene from foreground to background acceptably sharp, as in landscape photography. 

When used to separate the subject from the foreground or background, the depth of field is said to be “shallow”, “short” or “narrow”

When used to get as much of the scene as possible acceptably sharp, the depth of field is said to be “deep”, “long”, or “wide”.

Image examples of shallow versus deep depth of field
© Brenda Petrella Photography

Several factors affect the depth of field, including aperture.  As light passes through the aperture opening, it is refracted on its way to the camera sensor.  When the aperture is wide open, the light is refracted more, resulting in a shallow depth of field.  When the aperture is stopped down, the light is refracted less, which results in a deep depth of field.

So, lower f-stops (wider apertures) result in a shallower depth of field, and higher f-stops (narrower apertures) result in a deeper depth of field.

Depth of field versus Aperture
© Outdoor Photography School

How Does Aperture Affect Shutter Speed?

As mentioned in the beginning, aperture, shutter speed, and ISO comprise the three camera settings that control exposure. 

If you are shooting in manual mode, then any change to one of these settings will require the change of one or more of the other two settings in order to maintain the same exposure.

Assuming that ISO is kept constant, then wider apertures (the ones with the lowest f-numbers) will let in more light than narrower apertures (the ones with the highest f-numbers).  Therefore, wider apertures allow for using shorter shutter speeds, and narrower apertures require slowing the shutter speed down to acquire the same exposure.  

What is a fast lens vs aperture
© Outdoor Photography School

As such, lenses with a low maximum aperture (typically f/2.8 or lower) are called “fast lenses” because they permit the use of fast shutter speeds and are capable of freezing motion in low light settings.

I hope this helped clear up any confusion you’ve had about aperture!  If you are still confused, feel free to leave a question in the comments below.


This Post Has 6 Comments

  1. Richard B.

    Excellent. I haven’t read anything that in-depth since my Photography 101 back in ’67. A good refresher.

    1. Brenda Petrella

      That’s great to hear! I’m glad the details were helpful. Thanks!

  2. Tim Shoukry

    Absolutely Marvelleous, you managed to semplyfied it and take it down to basics and still very rich in knowlege, I am passing the article to me friends and I hope they will join in .. Thank you very much Brends Great topic and Great presentaion .

    1. Brenda Petrella

      Excellent! Thanks so much for the feedback and for sharing, Tim!

  3. Joe

    Outstanding article! Well presented and easy to understand. Thanks!

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