Depth of Field and the Circle of Confusion

In one of my previous articles on this website, I have delved into the concept of depth of field (DoF) as well as the parameters that govern DoF. This is the part of a photographic image that is considered to be in acceptably sharp focus.

As you can recall, the depth of field in a photographic image depends on several aspects. Chief among those being the aperture value in use and the focal point. The wider the aperture, the shallower the depth of field. For the least confusion, it helps to remember that a wide aperture is a lower f-stop number. 

The narrower the aperture, the deeper the depth of field in a photo will appear. This happens when you have set your lens to an aperture number that is higher. 

There is, however, yet another concept that is closely connected with depth of field and that is the circle of confusion.

Before we understand what the circle of confusion is we need to understand how an average lens projects an image onto the camera’s sensor. Or the film if you are using a camera that takes film. The circle of confusion works the same way regardless of whether or not you are using a digital camera or a film camera. 

It’s all about the optics of the camera lens and how they work together to create acceptable sharpness in a photo. The focal plane where the focus point is in focus also influences the circle of confusion. This is basically the viewing distance between the camera sensor in relation to the focal point of the main subjects. The lens focal length also helps to determine the circle of confusion. Some photographers don’t focus on this aspect when considering how confusion affects images.

Understanding What Is Inside Camera Lenses which Can Help Make You A Better Photographer

Inside every camera lens are a number of lens elements (or groups of lens elements) that work in tandem. These make the image project perfectly onto the image medium at the back of the camera. All of the lens elements are important but for this discussion, we will consider only one group and that is the focus element group.

The circle of confusion is affected by how elements are arranged inside camera lenses. The way camera lenses are made somewhat controls the circle of confusion effect and how this relates to the depth of field and the focal plane.

Light travels in a straight line. That means when light originates from a point source it should appear as a point source when it’s perfectly sharp on the focal plane. This is the task of the focus element group. 

The focus element group refracts the light coming through the lens. It then projects it onto the camera sensor or onto the film (this is what I am referring to as the image medium.) This is always at the back of the camera. 

You can see a small circle icon on most camera bodies that indicates precisely the location of the sensor or film. Knowing where this allows photographers to calculate the measurement from the subject, where their focus point is, and the sensor. This can help a photographer to achieve perfect focus when they have a shallow depth of field.

The image medium is also known as the focusing plane. This is the point where the light merges (or at least theoretically should merge) and is known as the focusing point. This sometimes can depend on the confusion diameter limit in a picture.

How do the Depth of Field and the Circle of Confusion affect a Photographic Image?

When the focusing point is perfectly aligned with the focusing plane the result is the perfect focus. This means that the subject where your camera’s focus point is aligned will look in focus to the human eye in the final image. When it is not aligned perfectly, we get what is known as a blurred image. It is an image where the subject is not perfectly sharp on the image plane. This lack of visual acuity is most often very frustrating for many photographers. It’s always best to have at least some parts of the photographs you take have technical correctness about them. 

There are many for an image to be blurry. However, for this discussion, we shall consider it is because the lens was not unable to focus properly. In photography, this is one of the key things to get right. Photographers, no matter how good at focusing they are, cannot fix an out-of-focus photograph made by any regular kind of camera. 

There are some specialist cameras and technologies that work with contrast and distances, along with multiple lenses. These create images that are able to have the circle of confusion altered to achieve visual acuity during post-processing. But this is outside of this discussion on the circle of confusion because most photography is not made with this type of camera equipment.

If the elements are not wildly out of focus, they may still appear to be reasonably sharp. This happens because our eyes accept them to be sharp. This is where the concept of the circle of confusion comes in.

The Degree of Tolerance with the Circle of Confusion in Photography

The circle of confusion is basically about the degree of tolerance that the human eye has. Your eyes make a clear distinction between a subject that is out of focus and one that is in focus. Before this happens there is what is called the circle of confusion. 

Let’s say that there you are doing some photography of an arrangement of dinner spoons. Say you have them laid out in a straight line. With your standard focal length lens on your camera, you choose your point of view low to the line of spoons. You are using an aperture of f/4. You set your AF point on one of the spoons and press the shutter release button on your camera. 

Do all the spoons appear in focus? No. While the spoon you focused on appears nice and sharp, the one immediately in front and immediately behind appears to be out of focus. This is partly because of the circle of confusion. It is also because of the other factors that I have already mentioned that have an influence on the amount of blur in a photo.

Now change the aperture to f/8 and make the exact same image again. Did you notice the difference? The spoon in focus is tack sharp. But the spoon immediately in front and behind also appears to be reasonably sharp. If you set your aperture to f/11 and remake the shot all three spoons will appear perfectly sharp.

Other Variables that Influence the Circle of Confusion in a Final Image

Depending on how far away your viewing distance is, the blur spot in the final image may vary. Your lens focal length and the size of the sensors in your cameras also affect the blur circle. This can create some confusion for some photographers. Especially those who are only considering that the depth of field is controlled by the lens aperture setting.

So, a different focal length lens on your camera and the distance you are from your subject can also influence the depth of field. And so also the circle of confusion. The greater distance you are when you focus on your subject, the deeper the depth of field is and this impacts the circle of confusion.

A long focal length at any given aperture will have a different effect on the circle of confusion than using a shorter focal length lens. Can a viewer perceive the effect of this difference in the circle of confusion created by focal length? And the confusion diameter limit? This could take a whole world of discussion.

For each lens, the hyperfocal distance has an influence on the circle of confusion also. When set to hyperfocal distance the vision of what is in an image and its sharpness is measured by the circle of confusion. When a subject is far enough away from the camera the light will converge differently in the lens. In this case, the background will appear to have more sharpness. The circle of confusion and the coc limit often will not work to create the least confusion.

Why Does Light Passing through a Narrower Aperture Create Less of a Blur Spot?

Light passing through a narrower aperture always results in having more focus in your pictures. This is regardless of what focal length lens you use. The distance you are from your subject makes no difference to the circle of confusion when considering the aperture setting. 

Why does this happen with the circle of confusion? This is because with the aperture stopping down by one stop each time, you are basically increasing the depth of field. With it, however, something else is also happening and that is you are involving the coc criterion of the circle of confusion. Even at f/11 only the spoon that you had focused on is truly sharp.

The other two spoons are merely appearing sharp because even though they are slightly out of focus, your eyes are unable to tell the difference. That in essence is the concept of the circle of confusion.

The circle of confusion is a photography phenomenon. With our natural vision, we experience no circle of confusion. This circle of confusion happens because light passed through a series of lens elements from different distances. It’s the same if you are photographing circles or triangles of any shape at all. It’s not about the subject, but the circle of confusion is about how the light reaches the flat plane of your camera’s sensor or film.

Photography in Two Dimensions

We don’t often photograph two-dimensional subjects. When you do it is possible to avoid seeing the effect of the circle of confusion in your images when all the points on the two-dimensional plane are the same distance from all of the points on your sensor or film.

So long as the whole of a subject is the same distance from your camera, the circle of confusion will not appear in an image. When you chose an angle that is not square to your flat subject, then the depth of field and the circle of confusion become obvious in your photography.

Will Understanding the Circle of Confusion Make you a Better Photographer?

Being a technical type of photographer you may always like to hone your technique in every area. Understanding the circle of confusion and how it works will give you some level of satisfaction. But I seriously doubt that it will make many people want to look more intently at the photographs you take.

When photographers concentrate on technical issues such as the circle of confusion it comes at an expense. This can detract from more fundamental aspects of working with cameras and the overall enjoyment of photography. You can be constantly looking at things like the depth of field tables and completely miss the moment to take an incredible photograph. But, I guess if you like lining up rows of spoons to photograph then this will not be much of a problem.

Take time to study the circle of confusion. You’ll come to realize that it is an important aspect of technical photography that very few photographers will make use of. But before you take a deep dive into how the circle of confusion, think about other things. Consider if it will have a practical and profound influence on your pictures. You’ll be better off studying the 365 Days of Photography Course. This will help you in mastering all the practical aspects of photography that are covered in this course.