The Magic Numbers of Photography - Calculating Your Circle of Confusion

Despite sounding like group therapy for dementia patients the circle of confusion is a remarkably simple concept to grasp. It is often talked about as an unfocussed spot of light produced from a cone of light.

If you think about how light travels through a lens and the cones that it produces in doing so it soon becomes clear that the circle of confusion produced by objects at different distances from a lens will produce different circles of confusion when the recording medium is at different distances from the lens. Any combination that creates a circle of confusion that is too small for our eyes to resolve appears as a single crisp spot and is deemed to be in focus. Any combination that produces a larger circle of confusion is deemed to be more out of focus.

In this way the circle of confusion is related to the depth of field. Any circle of confusion that produces a finer resolution than our eye is deemed to be in focus.

As the circle of confusion essentially relates to the image that gets recorded on a sensor or frame of film it is calculated in relation to the size of the sensor or frame of film. Commonly the diagonal length of the recording medium is divided by 1000 or 1500. In the case of a Nikon APS-C sensor the value obtained by dividing the diagonal by 1500 is 0.019