What is circle of confusion in photography?

circle of confusion explained

In the world of photography there are concepts that you must know and master, so you will capture images all the time with great quality. Managing these aspects will help you to be ready for any situation that may occur when taking a picture and thus express ideas and emotions.

One of these concepts is the depth of field, which refers to the area of the image that is sharp or focused on the image. This means that if in the image all the elements are sharp, the depth field is large. While if there is only part of the scene that is clear, compared to the rest out of focus, you will have a small depth field.

Another term that may cause some confusion as the name implies is the circle of confusion. Do not worry, before clarifying and fully understanding this concept, the approach is necessary.



Focusing consists in matching the projection of the point that we want to capture in image with the camera sensor. In other words, do everything possible to make the object of the scene look sharp.

What is circle of confusion in photography?

By knowing a little more what the approach refers to, the circle of confusion can be defined as the maximum size that a blurred point on the sensor, when being projected on a surface and seen by a viewer at a certain distance, is still perceived as a point and not as a circle.

You have to take the time to understand these technical aspects in photography and see how you can take advantage of them. That is why in this post we will show you more details of the depth of field and the circle of confusion as well as the relationship between them.

Elements of the depth of field

Depth of field can be defined as the distance ahead and behind the focused point that comes out clearly in a photograph. The elements that you must handle to understand this distance are:

Diaphragm opening

The diaphragm is only the part of the lens that limits the ray of light that penetrates the chamber. Its function is similar to the iris of the human eye, that is, it opens and closes to allow the passage of light.

Then the aperture of the diaphragm is what regulates the amount of light that passes through the lens towards the camera sensor. By controlling that opening, you can handle the depth of field of the scene.

Focal length

It is the distance between the optical center of the lens and the focus. This focus is the point where the light rays are concentrated. In one objective the focal length is that between the diaphragm of the lens and the focus.

If you vary the focal length, you can get more or less approach. Depending on the type of objective you have, you will have a fixed or variable distance.



Distance between the camera and the focus point

The smaller the distance between the focus element and the camera, the lower the depth of field.

Useful depth of field

This is more than a technical concept, is a compositional instrument in photography. By implementing this technique properly, you can compose an image according to your needs. Focus attention on a single point of the image; make portraits that make a difference, show natural landscapes to the fullest, there are endless options.

With the control of this technique, you will change the scene according to your taste and needs. If you are looking to focus the viewer’s attention on an object, or just mark someone’s face, you should handle a shallow depth of field.

If on the contrary you are looking for scenes of cosmopolitan cities, where each element is detailed, you must handle great depth of field.

Calculate the depth of field

Today there are simulators or photographic applications that will allow you to obtain the value of depth of field quickly and clearly.

In this case the most used application is DOF calculator, which is an application for Android system that you can have on your smartphone. With the application you can obtain the focused distance depending on the focal point you use and the aperture.

If you have an iPhone you can download DOF Master to get the depth of field you need for your scene.

These online calculators will not give you exact results if you do macro photography. You should investigate what applications are specifically for this type of images.

Depth of field control

If you want to control the depth of field technique, you need to handle the opening. If you use large openings, you can reduce the depth of field and if you want to increase it, reduce the aperture.

The size of the sensor is also related to the depth of field. If the sensor is large, the depth will be less. The dimensions of the sensor directly affect what refers to the maximum confusion circle.

We will show you some recommendations to obtain little and much depth of field:

Little depth

  • Use a lens with medium focal length. With an order of 85 mm to 200 mm, it is common to use it for portraits.
  • Use a luminous target, with an opening of the order of f /2.8 or higher.
  • Look to be as close as possible to the object of the scene.
  • For these scene conditions where you want to make a portrait, a camera with full frame sensor will allow you to have more background blur.

Much depth

  • Use short focal distances. With wide-angle objectives you will have a lot of depth of field.
  • Close the diaphragm a little more, so the depth will be greater. All this if the light conditions allow it.
  • Put yourself at a greater distance from the object of the scene. As you move away the depth will increase.

Circle of confusion and how it is defined

At the moment that the rays converge before or after the focal plane, a point is no longer generated in the sensor but a circle. According to this, the circle of confusion can be defined as the projection size on the sensor that can still be seen clearly.

Another way of referring to this term is like the maximum diameter of a circle of light in the sensor that can be distinguished as a clearly defined point in the final scene.

The circle of confusion is measured in fractions of millimeters and is used to determine what is focused and not in an image. Any area of light with a smaller size of the circle of confusion will be recorded in a clear and defined manner, while the larger areas will appear out of focus.

 

circle of confusion example

 

Elements of the circle of confusion

This term depends on the following aspects:

Visual acuity

This is about the visual ability to specify the distance between points. This will depend on each person observing the scene.

Distance to the captured image

If you are near or far from the scene, the ability to discriminate points will decrease or increase.

Enlargement of the original image

Each time you enlarge the image size, the sharper you will perceive the magnification.

Calculate of the circle of confusion

Several ways are used to determine this value. One of them is to make a print on paper with specific dimensions, to observe it from a pre-established distance. In this way, you can check the area of a circle on the capture surface that is clearly visible.

Another way is to use the applications currently available, such as Photopill, which don’t ask you the camera model parameters, the size at which you are going to print or enlarge; but the distance at which you are going to observe the image.

Relation between the circle of confusion and the depth of field

One of the parameters used to calculate the depth of field in the photographic simulators is the confusion circle. To obtain this value, the parameters mentioned in the camera model and sensor resolution are used.

When calculating the depth of field, you should know the maximum diameter of those circles that will be perceived as sharp points in the image. As well as reducing the aperture of the diaphragm, you generate a greater depth of field.

That is to say that the depth of field will depend on the maximum diameter of the circle of confusion that appears in the image captured by the sensor.

An acceptable circle of confusion is that point that enlarged in a printed photo of 20 x 25 cm observed at a distance of 61 cm; it is not perceived by the human eye.

It is established that the circle of confusion should not be larger than 0.25 mm in diameter.

Conclusions

Capturing images that captivate viewers, not only depends on your camera or the scene element. There are also concepts from the world of photography that you must know and master to achieve those photos with a unique and unrepeatable look.

Remember if you want to adapt the composition of the image according to your needs, you must focus on learning to specify the depth of field. If this depth is too much or too little, it will play a key role in the final aspect of the image, whether it is a portrait or a landscape.

The circle of confusion is one of the parameters used to obtain the depth of field value. Their relationship is based on the dependence of the maximum diameter of those circles that will be observed as sharp points in the image.

 

Author: condruzmf

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