Circle of Confusion
Tiny points of light
A lens cannot resolve a point exactly. Instead it creates a small circle of light called the ‘Circle of Confusion’. Inside the ‘Depth of Field‘ (DoF) any individual point on the subject is focussed onto the sensor as a tiny circle of light. If we see it the point appears to be a sharp representation of the same point on the subject. In reality we see it as a tiny dot or may not be able to see an individual one at all. Put all the dots together, from all the points on the subject inside the DoF, and we have a sharp part of the picture.
Outside the DoF the focus is poor. Instead of focusing almost exactly on the sensor plane the focus falls short (tree above). Alternatively, the focus point is beyond the sensor (rock). In both cases the circle of confusion creates a larger circle on the sensor. These points on the subject(s), the tree and rock, appear out of focus because all the tiny circles (points of light) representing individual points on the subject overlap each other. The resulting image is blurred. This is because all the circles are overlapping in this zone. This is seen in the image as bokeh. The bokeh appears in the areas of the picture outside the DoF.
The ‘Circle of Confusion’ defines the Depth of Field
The term ‘Circle of Confusion’ is used to define the ‘Depth of Field’. Here is how it works…
Inside the DoF we see the circle of confusion as a tiny, sharp point. As our person walks nearer to the edge of the depth of field the circle of confusion will grow gradually. As the person passes the DoF-edge the circle of confusion for the observation-point is large enough for us to see it. The focus is now lost, the point we are observing has become unsharp. It will become more blurred as the subject continues to walk toward the lens. In other words, outside the DoF we see a larger circle of confusion overlapping with other points nearby creating a blurred area of the image.
Circle of confusion – size matters
Size is the critical factor. An ‘acceptably sharp’ circle of confusion is defined as one which is unnoticed on an 8 x 10 inch print viewed from one foot away. In practice this works out to about 1/100th of an inch on that print size. Of course a different standard size would apply to different print sizes. Manufactures use these standards to create DoF markers for lenses.
In the modern digital camera we already have a standard preset for the circle of confusion on the sensor. In practice the individual pixel on the sensor makes a convenient focus-point definition. If the circle of confusion exceeds the size of one pixel it will start to affect the surrounding pixels and the sharpness will be lost.