CIRCLE OF LEAST CONFUSION The theoretically perfect lens is capable of sharply reproducing a point or a line, no matter how small or fine. In telescope and microscope objectives, where only rays near the axis of the lens are used, this condition is nearly fulfilled, but in photographic lenses, where approximate sharpness over an extended field is desired, this critical definition at the centre of the field is sacrificed in order to obtain other qualities. The size of the disc to which the image of a theoretical point is spread out by any lens is called the circle of least confusion, and is a measure of the defining or " resolving " power of the lens ; in British practice, rya of an inch is the maximum diameter of such a disc of which a " sharp " picture can be composed, but latterly on the European Continent of an inch is often taken as the standard of sharpness. To
realise what this means assume that an engraving is composed of lines and dots of an inch in width ; with a lens having a disc of confusion of of an inch a full sized copy would have the lines and dots broadened out to more than rya in., but at io in. distance from the eyes the whole picture would appear to be sufficiently sharp. By stopping down the lens the sharpness can be increased until almost any desired degree is attained. The circles of confusion at the true focus are due to spherical aberration (which see), but they are also found in portions of the image which are " out of focus " and are easily recog nised in the backgrounds of portraits. In this case they are due to the plate cutting the cone of rays at some distance from the point of sharp focus.