A defect of an optical component or system in which star images become increasingly fan-shaped away from the optical axis; the shape resembles the coma of a comet, hence the name. In positive coma, light passing through the outer zones of the optical system forms a disk rather than a point image. The size and position of the disk depend on the distance of each zone from the centre of the lens, so the images from different zones overlap to form a fan pointing towards the centre of the field. With negative coma, the fan points away from the field’s centre. The effect is usually combined with other aberrations, such as astigmatism and chromatic aberration, which distorts the fan shape.