Don’t Be Confused With Focusing
The Circle of Confusion Explained
The Circle of Confusion or CoC sounds confusing at first. The name sounds like a band. It is also known by other terms like disk of confusion, circle of indistinctness, blur circle, or blur spot. It doesn’t seem to be a term used in photography, yet it is important to understand what it is all about. If you are confused, then this article can help explain the meaning of the term in case you ever encounter it. That is because you will probably not use this term that often, yet it is an important concept in photography used notably for calculating the Depth of Field (DoF).
CoC Defined
The CoC is defined as:
“… an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source.” (source Wikipedia).
Think of the CoC as the maximum size of a spot or circle of the focal point that has an acceptably sharp focus on a subject. This is of course the focal point at which you have focus on the subject you want to shoot. Anything that is bigger or smaller than the CoC is considered out of focus or too blurry to be acceptable. The CoC value is the diameter of that spot or circle measured in mm (millimeters).
The CoC is used to calculate what are called Depth of Field tables. The DoF determines the sharpness of focus based on the aperture of the lens. If you want shallow DoF or small Depth of Field, you open up your aperture to allow more light or you simply come closer to the subject you are photographing. Smaller aperture can focus on the subject while blurring the background or surrounding. A larger aperture puts more focus on the entire surrounding, like for example taking a photo of a landscape (e.g. mountains, skyline, countryside, etc.). The term hyperfocal distance is used to define the greatest DoF or sharpest focus you can create on an image. To calculate the value of the hyperfocal distance, the CoC value is needed.
This link lists common CoC values for digital cameras.
The CoC values were calculated using the formula:
CoC = (CoC for 35mm format) / (Digital camera lens focal length multiplier)
The focal length multiplier is specified by the camera manufacturer calculated as:
Multiplier = (35mm equivalent lens focal length) / (Actual lens focal length)
When you adjust the focus on your lens, be it manual or automatic, the lens is actually trying to converge the beams of light to a focal point which must fall with the focal plane or the surface of the film or sensor. This brings the subject to a sharp focus since you are changing the point behind the lens where the beams of light from different distances come to a convergence i.e. the focal point.
During the adjustment, imagine that as your focal point comes closer to the focal plane, a circle becomes smaller. That is until it comes to the convergence and this is the point where the subject is acceptably sharp. No image is 100% sharp, it is really the eye’s perception of acceptable sharpness that eventually sees this. This is because not all light beams come to a perfect convergence or the lens itself is not perfect.
The CoC Limit or Criterion
There are three factors to consider in CoC.
- Visual acuity of the eye
- Viewing distance to the image
- Enlargement factor used to produce print or image
The acuity, which means sharpness, in vision varies from person to person. It is really not the same because people have differences in how they see things which is due to age, perception and physical attributes. This is the closest comfortable viewing distance for most people and it is set to approximately 25 cm.
Viewing distance refers to how far away the image being viewed is from the focal point where the image is in sharp focus.
The enlargement depends on the final output to print, as this can be used to determine how sharp an image will look.
This is just a basic overview of the CoC Limit. There is more information about that under this Wikipedia article.
Calculating Hyperfocal Distance
Why do we need the CoC value for hyperfocal distance?
When you want to enlarge prints, you want an acceptably large image that was shot in focus. Shooting on film is a much bigger challenge because you don’t have immediate feedback like digital. Therefore it is best to get the camera settings right before a photoshoot. Take for example shooting a landscape photo of a national park scene. The hyperfocal distance value would be needed. The hyperfocal distance is required to focus the lens to maximize DoF for the photo. The calculation would also require the CoC value.
This is the basic formula for hyperfocal distance:
f = Focal Length
N = Aperture diameter f-number
c = Circle of Confusion value
Let’s look at an example. Suppose I had a lens with a focal length of 28 mm at f/16 using a CoC value of 0.03 mm typically used in 35 mm photography.
H = (28mm)²/16(0.03mm) + 28mm = 1661.33 mm
This means that if the focus is at a distance of 1.66 meters, anything that is half that distance to infinity will be in sharp focus. You can go to this website which has a Hyperfocal Distance Table that can be referenced instead of having to calculate it.
Summary
The CoC is basically the diameter of the focal point at which the image is in focus. When the image is in focus it is sharp and acceptable for print or projection. The complexities of CoC were removed here just to give the basics. If there are some points to clarify or correct regarding CoC, please state them at the comments section.
If you are interested to learn more about Depth of Field, I published this article about that here on this link.
