Stigmatic Image

an optical image in which each of its points corresponds to a point of the object being imaged by the optical system. Strictly speaking, such a correspondence is possible only in ideal optical systems where all aberrations are absent or have been eliminated and where the wave properties of light, particularly the diffraction of light, are ignored.

In real optical systems the stigmatic image is a useful and widely employed approximation; any real system represents a point not as a point but as a spot or three-dimensional figure of finite, though small, dimensions. In the case of paraxial rays, astigmatism is the principal aberration disrupting the approximate stigmatic nature of the image.