The circle of confusion is a technical term used quite often in photography. It’s related to how your lens focuses as well as the aperture value that you use to shoot your images. It also influences depth of field, as well as how some lenses tend to produce softer images than others. If you’ve always been intimidated by these factors, this video by Mark Wallace should clear your confusion around the circle of confusion:
Understanding Light
To understand the circle of confusion we need to understand how light travels from a point source, through the lens’s many elements and then goes on to hit the sensor inside the camera. Light originating from a point source hits the lens’s front element. Now, there are many elements inside the lens but the one that really matters for this discussion is the focus element.
The focus element (blue in the diagram) refracts the light and focuses it toward the focus point. You have good focus when that focus point lines up with the focal plane. The focal plane, in this case, is the sensor at the back of the camera (or film if you’re shooting with a SLR). When you adjust the focus on your lens, the focus element moves the focus point back and forth. Sound confusing? This diagram should help.
Perfect focus is achieved when the point of focus (small circle on the right) and the focal plane (red line) meet.
What is the Circle of Confusion?
When the focus point and the focal plane do not meet, a point source of light (a round light source) appears slightly bigger and no longer appears to be in sharp focus. However, there is a certain range of tolerance in front of and behind a subject before it starts to appear out of focus to the human eye; that range is what is known as circle of confusion. In the below representative diagram the range is highlighted by the purple rectangle.
Circle of Confusion
What Happens When You Change the Aperture?
If you use a smaller aperture (larger f-number) the convergence is changed. The range within which a point light source will appear in focus now increases. This is clear from the diagram below.
The circle of confusion is extended when the aperture is smaller.
Anything in front of and after the focus point (and falling within the circle of confusion) also appears to be in focus.
The Impact on Bokeh
The concept of the circle of confusion has an overbearing impact on bokeh, as you can see in the images below. The first image was shot at f/1.4. Notice how the lights behind the model are out of focus and blurred.
Background lights blurred at f/1.4
As Wallace continues to stop down the aperture, the background lights start to appear more and more in focus. They have almost become point source lights.
When the lens aperture is stopped down, the lights appear smaller and more in focus.
Chromatic Aberration
The circle of confusion affects something else in photography. Wallace explains that there are three primary colors in light: red, green, and blue. Due to the inherent wavelengths at which they travel, these three colors don’t congregate at the same point behind the lens. The result is color fringing, or chromatic aberration.
Chromatic Aberration
Some lenses, mainly the more expensive ones, have special elements inside them that can correct this problem, forcing the different wavelengths of light to meet at the same point. Cheaper lenses don’t have these extra elements; if you’re using a cheap lens you’re likely to find that your images are softer and have color fringing.
Chromatic aberration is corrected in higher quality lenses.
I hope all this information helped you to clear the confusion around circle of confusion. Go ahead and use this knowledge to make better images. Happy clicking!
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Thanks for the clear and simple explanation, just what I was looking for.