Disc of light in the image where a point on the subject is not perfectly brought into focus. The eye cannot distinguish between a very small circle of confusion and a true point.

The tiny blurred circle of light that a lens forms as an image of a point of light placed at the subject position.

n. The maximum size on the focal plane that a point of light can be before it appears to be out of focus. On 35mm film this can be from 0.02mm to 0.04mm depending on how critical the application is. A handy way of calculating a critical sized circle of confusion is to divide the diagonal of the image size by 2000. This gives about 0.021mm for 35mm film, and 0.042mm for a 6x6 negative. _______________________________________________________