The Circle of Confusion is one of the less explained, but very important concepts in photography. If you want to take photo professionally you need to learn how to deliver sharp images.
Guess what, understanding the circle of confusion will take you closer to this goal.
Among the most known factors that affect the sharpness of an image, (aperture, focal length and distance), there’s another one called circle of confusion. It’s a technical concept, but I’ll try to keep it as short and simple as possible. I was so bored the first time I read about it and I don’t want to do this to you. Let’s delve into it.
Circle of confusion explained
To understand how the circle of confusion works it’s important to understand how camera lenses work and how they are able to focus light.
Again, I’m going to simplify this concepts for this article. If you’re an expert or know how these things work already, bear with me and don’t hesitate to write in the comments section below if something I say could be explained better.
How Camera Lenses Work
In a convex lens, parallel rays of light are bent inward thanks to a phenomenon called refraction. See image below.
Light rays that travel through the lens are refracted and converge to a single point. That point is called focal point.
Producing a Sharp Image
Let’s try to understand how your camera focuses light.
The process I’ve explained above is not enough by itself to produce a sharp image. One single lens is not enough.
As one single convex lens by itself is not able to produce an in focus image, camera lenses have multiple lens elements inside to help with this process. Without going too much into detail this is the reason why certain lenses are more expensive than others. They might have more lens elements inside to produce better quality images, avoiding imperfections such as chromatic aberration.
Back to sharpness. For a subject to be perceived as in focus by our eyes, points of this object have to be projected by the lens onto the sensor (also known as focal plane) in a way that the focal point is aligned with the focal plane. See image below.
The image is overly simplified. The focal plane never moves, in reality the focus element (not represented) of a lens moves back and forth until light is refracted so that it aligns with the focal plane. The image will be unsharp when the point is off the focal plane and vice versa it will be in focus when it’s aligned.
If all this make sense to you we are very close to define the Circle of Confusion. There is a certain area in which despite the point being outside the focal plane, it’s still perceived as sharp by our eyes. That is the Circle of Confusion and the image below explains it much better.
Circle of Confusion and Depth of Field
We know that to get the sharpest possible images we need to be bale to get the depth of field right. The depth of field depends from they Hyperfocal Distance which is the focusing distance that gives you the greatest depth of field.
So why is the circle of confusion related to depth of field?
It’s because the measurement of the circle of confusion is used by camera manufacturers to determine the Hyperfocal distance of a lens.
If you want to know more about depth of field I invite you to read my article The Ultimate Beginner’s Guide to Landscape Photography
What is in or out the circle of confusion is what we define in or out of focus. I hope that my simplification of this complex concept helped you understand it a bit more.