What are aberrations?
Aberrations are image defects that result from limitations in the way lenses can be designed. Better lenses have smaller aberrations, but aberrations can never be completely eliminated, just reduced. The classic aberrations are: • Spherical aberration. Light passing through the edge of the lens is focused at a different distance (closer in simple lenses) than light striking the lens near the center. • Coma. The distance from the axis at which an off-axis object point is rendered varies with the distance from the center of the lens at which the light passes. In other words, magnification varies with the distance from the center of the lens. Off axis points are rendered with tails, reminiscent of comets, hence the name. • Astigmatism. Off-axis points are blurred in their the radial or tangential direction, and focusing can reduce one at the expense of the other, but cannot bring both into focus at the same time. (Optometrists apply the word “astigmatism” to a defect in the human eye that
