Approached stigmatism

Certain very simple optical systems are rigourously stigmatic :

• The parabolic mirror for an object point to infinity on the axis.

• An elliptic mirror for conjugation between geometrical focus.

• A stigmatic plane mirror pour all points in space.

• ...

These systems are not frequent and are not stigmatic for a unique object point. Generally, optical systems are not rigourously stigmatic.

In this case, the image of an object point on a screen or light receptor (film, CCD matrix) is a diffusion spot. If the dimension of the latter is lower than the film grain or the pixel size of the CDD matrix it will then be seen as quasi-ponctual, the optical system will be equivalent to a stigmatic system. It is said that there is an approached stigmatism.

Calculating for optimisation of optical systems consists in making these diffusion spots small enough on every point of the image.