The Definitive Guide to Aperture and Depth of Field

One of the fundamental concepts of photography is that of aperture (and how it relates to depth of field). Along with the shutter speed and ISO, aperture is part of the exposure triangle that controls the overall exposure of an image. In addition to this, aperture also affects the depth of field in an image. In short, the aperture (which is measured in f-stops) controls two factors:

  1. The amount of light that is able to reach the image sensor, which in turn controls the image’s overall exposure.
  2. The depth of field (DOF) of the image. The depth of field is simply how much of your image is in focus. Don’t worry, there will be more of this later on 🙂

By completing this article, you’ll understand what aperture is, how it affects an image’s depth of field and how to select the correct f-stop settings to achieve the optimal depth of field for any given scenario. 

Camera Lens Basics

Understanding what aperture is in photography requires that we have some base understanding of camera lenses, as well as some of the vernacular associated with them (such as f-stop, field of view, and focal length). With this in mind, let’s take a look at these concepts before we jump into our explanation of aperture.

In concept, camera lenses are actually pretty simple. They are nothing more than pieces of glass that are designed to refract light and direct it toward an image plane at the back of the camera. The shape of this glass affects just how much distance is required to get these crisscrossed beams of light to properly converge on the image plane. Today we measure this distance in millimeters, which we refer to as the lens’s focal length (e.g. 24mm). 

The focal length will not only affect how light converges on the focal plane but also how an image is rendered to the camera. A short focal length allows the photographer to achieve a wide field of view, whereas a longer focal length (like in the case of a telephoto lens) will result in a much smaller field of view. 

More on Focal Length and Field of View

Before we jump into aperture, let’s go ahead and take a closer look at the focal length of a lens. We’ve already determined that the focal length affects the field of view, but let’s take a closer look at what this means.

Our human eyes have a focal length that is approximately equal to 50mm. Compared to this, a lens with a focal length greater than 50mm will magnify or “zoom in” a scene, thus resulting in a smaller field of view. 

On the flip side, a lens with a focal length less than 50mm will produce a “wide-angle” view of a scene. In other words, the lens will have a field of view that is larger than that of the human eye. It’s also worth mentioning that, when shooting in this field of view, objects will often appear smaller and slightly distorted when compared to what we’re used to seeing with our eyes. 

In general, we tend to classify lenses as falling into one of three categories:

  • Wide Angle Lenses: produce a field of view larger than that of standard human vision. These are usually in the range of 8mm to 35mm.
  • Standard Lenses: produce a field of view that is similar to that of human vision. These will usually be in the range of 40mm to 60mm.
  • Zoom Lenses: produce a field of view that is smaller than that of standard human vision. These will usually be over 70mm.

Effective Focal Length

I’d be remiss if I failed to talk about the effective focal length of a lens. Before we can jump into that topic, however, we need to first understand just a little bit about camera sensors.

Rather than jumping headfirst into the science of how the sensor technology works, we’ll instead focus on just enough to understand the effective focal length (note that, if there’s interest, I can certainly do a much more in-depth article focusing on camera sensor technology).

Camera sensors come in different physical sizes, which we usually express in terms of their width and height, in millimeters. We have what we refer to as a standard size, which is 36mm x 24mm, but we’ll often just refer to this as a full-frame, 35mm format sensor. The reason we consider this the standard size goes back to the days of film, but we won’t worry about that here. The important thing to realize is that the sensor size has an effect on the focal length of a lens.

The focal length that is listed on a lens is the focal length when attached to a camera with a 35mm sensor. The focal length is different when it’s attached to a camera with a different sensor size. This new size is what we are calling the effective focal length.

Let’s consider an example. Many Canon cameras, including my Canon 70D, have what’s called an APS-C sensor size. This sensor is smaller than a full-frame, 35mm sensor. As a result, any lens I attach to it will have a longer effective focal length than what’s listed on the lens. In fact, every lens attached will be zoomed in by a given factor, which we call the crop factor. On the APS-C Canon sensors that I just mentioned, the crop factor is 1.6. So, any lens that I attach to my APS-C Canon camera will have an effective focal length that is 1.6 times longer than the focal length listed on the lens. As an example, a 10mm lens will be the equivalent of a 16mm lens.

What is Aperture?

In addition to the above, camera lenses also have a diaphragm that controls the amount of light that is allowed through the lens. This diaphragm is, more or less, just a hole in the lens whose diameter we can control. The mechanism is quite similar to how the pupil of your eye works. When the diaphragm is set to have a larger diameter hole, more light is allowed to enter through the lens. Likewise, when the diameter of this hole is smaller, less light is allowed through. Aperture is simply the diameter of the hole that light enters through the lens. 

Aperture is measured with a number called the f-stop number ( example f-stops include f/2.8 and f/11). You will often hear people use the terms aperture and f-stop interchangeably, but this is not technically correct. Aperture is the actual aperture diameter in the lens, whereas the f-stop is the ratio of the lens’s focal length to the aperture diameter. If you really want to be precise, the f-stop is calculated as follows:

f-stop = lens focal length (mm) / aperture diameter (mm)

Luckily, we don’t really need to worry too much about the nitty-gritty math details to understand aperture enough to use it effectively in our photography! The important things to know are:

  1. The aperture is the size of the hole in the lens
  2. We measure the aperture using f-stop numbers (sometimes also referred to as f-stop values)
  3. A larger f-stop value equates to a smaller aperture, which in turn means less light can pass through the lens
  4. In turn, a smaller f-stop value equates to a larger aperture and, hence, more light that is able to pass through the lens
  5. The aperture affects the depth of field in an image (more on this in the next section)

What is Depth of Field?

In the previous section, we determined what aperture is and that it somehow affects depth of field, but this leaves a big question: What is depth of field?

Depth of Field (DOF), also sometimes referred to as Depth of Focus, simply determines how much of your image is in focus. To be more precise, the depth of field is the distance between the closest and farthest objects in an image that are in focus. All objects that fall between these closest and farthest objects are in focus, whereas those outside these objects begin to fall out of focus (sometimes referred to as being ‘soft’). The photographer can adjust the f-stop value to control the depth of field.

Smaller F-stop Number = Larger Aperture = Smaller DOF
Larger F-Stop Number = Smaller Aperture = Larger DOF

Let’s Look at an Obvious Example

To better illustrate the concept of depth of field, let’s take a look at an example that makes things rather obvious. Consider this image of a kiwi.

Notice how only a very small portion of this image is in focus while the rest is out of focus? That’s what we’re talking about when we say depth of field. This image has a shallow depth of field, meaning that there is only a small portion of it in focus.

Now a More Subtle Example

The previous example of depth of field was quite obvious and the differences between the in focus and out of focus areas were dramatic. The differences aren’t always this distinct. Let’s take a moment to consider a more subtle example.

This first example shows what is known as a deep (or large) depth of field. Notice how everything in the image, from the foreground all the way through the background, is in focus. 

Garland Bend, Kentucky

Now take a look at this second example, which shows a shallow depth of field. Notice how the foreground is in focus (sharp), but the background is out of focus (soft).

Shallow Depth of Field – Shot at f/4

The Focal Point Affects Depth of Field

We’ve now seen how the aperture can affect the depth of field in an image, but the focal point can also affect depth of field. 

I typically use single-point autofocus for my landscape photography. This allows me to manually select the point in the image that I want the camera to focus on. This way I have control over where in the image the focal point is. 

Why does this matter? Because, as we move the focal point, we move the entire focal plane. This changes the depth of field of the image. 

Your goal as a photographer is to select the correct combination of focal point location and f-stop value to achieve your desired depth of field within the image. This is a skill that takes a lot of practice to master, but we’ll discuss some tips on how to do this later on in this tutorial. 

The Focal Length Affects the Depth of Field

In addition to the f-stop value and focal point location, the focal length also affects the depth of field in an image. In general, the following rules describe how the focal length affects the depth of field of an image:

  • Longer focal lengths result in a shallower depth of field
  • Shorter focal lengths result in a deeper depth of field

The difference in depth of field for wider and standard lenses is much less noticeable than it is with long lenses. As you get to longer focal lengths (especially those in excess of 200mm), the difference in depth of field becomes more and more noticeable. 

How Much Do These Attributes Affect DOF?

So, we’ve now seen that the depth of field in our images is affected by:

  • The f-stop value we choose
  • The focal length we shoot at
  • Where we place the focal point in an image

At this stage, it would now be reasonable to question just how much all of these affect the depth of field. Unfortunately, however, there is no easy answer to this. 

This is one of those areas where experience comes into play. The absolute best way to master these skills is to practice them. And remember, don’t be afraid to experiment! It’s how each and every one of us has learned the craft of photography!

With that being said, let’s take a look at how we can make an educated decision on how to adjust these attributes. 

Making Creative use of Your F-stop Value

Choosing what f-stop value to choose for your aperture when taking an image is largely a personal, creative decision (though it also comes into play when calculating the exposure). 

What we need to consider when making this creative decision is the effect on the depth of field of the image. So, let’s take a look at how small, medium and large f-stop numbers affect our depth of field. 

Small F-stop Values

Smaller f-stops result in a shallower depth of field. This can be fantastic when you want to isolate your subject from the background (this technique is used a lot by portrait photographers). 

Consider this example, shot at f/5.0. Notice that the background is in focus enough that we can still make out some details, but out of focus enough that the flower is isolated from the background.

Spring has Sprung – Shot at f/5.0

Here’s another example, shot at f/4.0, from one of the very rare times that I’ve done street photography.

Street Photography in Cincinnati, Ohio – Shot at f/4.0

Medium F-stop Values

As a landscape photographer, I use medium f-stops (f/8 – f/13) for approximately 95% of all my images. 

F-stops in this range will provide you with a large depth of field, which happens to be exactly what I want in the vast majority of landscape shots. 

Consider the image below. This image was shot at f/13 in The Great Smoky Mountains. Notice how everything from the immediate foreground to the background is in focus. 

Cades Cove Storm” . Cades Cove, Great Smoky Mountains, Tennessee – Shot at f/11

Large F-stop Values

Before I mention the benefits of shooting with f-stop values larger than f/13, I must first mention that there is a major drawback. Smaller apertures (larger f-stops) are more prone to suffer from something called lens distortion. This problem is especially apparent in f-stops upwards of f/22. I’m not going to delve too much deeper into lens distortion here. Just keep in mind that it can degrade image quality. 

With that disclaimer out of the way, there are actually times where we want to shoot with these higher f-stops. One example is when we want to achieve a longer shutter speed. Remember, a larger f-stop is a smaller aperture, which lets in less light. To account for this we would need a longer shutter speed. 

One reason we might want a slower shutter speed is to give moving water a more silky, ethereal appearance. 

Consider the below example. This image is of Bolton Branch falls in the Daniel Boone National Forest. I shot this image at f/16. Not only did this give me the large depth of field I wanted for the shot, but it also allowed me to use a slower shutter speed (1.3 seconds in this case), which is how I got that ethereal-looking water. 

Bolton Branch Falls, Daniel Boone National Forest, Kentucky – Shot at f/16

The desire for a slower shutter speed isn’t the only reason we might want to shoot at a larger f-stop. Shooting into the sun (or other light sources for that matter) with a small aperture results in what we refer to as a “sun star”. This can be a cool effect that can add interest to a photo.

The below example was shot from Chimney Rock in Red River Gorge. I shot this with a f-stop of f/20, which allowed me to get the sun star.

Chimney Rock Sunset, Red River Gorge, Kentucky – Shot at f/20

As I mentioned before, shooting at f-stops over f/20 or so can result in a decent amount of lens distortion. For this reason, I don’t personally recommend shooting at these f-stop values unless it’s your goal to get a sun star, or if it’s the only option to achieve the exposure you want.

Where to go From Here

The key to fully developing your understanding of the relationship between aperture and depth of field is experimentation. I hope you found this guide useful! Feel free to ask any questions you might have in the comments below 🙂

I’ll leave you with a few more example images for you to study.

Gorgeous Sunset” . Shot at f/18 to achieve the sun star.
Almost Heaven Sunset“. Shot at f/11.
This one was shot at f/4.0. Notice how the large aperture gives us a narrow depth of field.

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