Circle of Confusion

Circle of confusion (also referred to as blur circle, disk of confusion and circle of indistinctness) refers to the effect of non-converging, unfocused light rays that have entered a lens. When light waves don’t converge after passing through a lens, they produce a larger optical spot, instead of coming together at a single point.

Being able to calculate the circle of confusion is important in that it allows a photographer to interpret the depth of field necessary to render a focused image in the photograph.

If the photographer is the proper distance from his subject, he can minimize possible light and chromatic aberrations. This better ensures that the different wavelengths of light entering the camera lens will form a concentrated, focused image on the film.

In simple terms, the circle of confusion is an estimated calculation of the depth of field necessary to produce a clean image.

While some photographers can use a specific equation to calculate the circle of confusion and, therefore, the appropriate depth of field, others prefer to experiment will distances to approximate the necessary depth of field.