Stigmatic image of an luminous point in an optical system

Let us consider a point A in a first space called “object space”. From A let us run a set of luminous rays going through the system. If all these rays converge to the same point A' of the image space, we can write :

• A' is the image of A through the system. It is also said that A' is the conjugate of A

• The system is said to be stigmatic for AA' conjugation

We demonstrate that stigmatism implies a constant value for the optical path (AA')

The case of a real image :

The image may be observed on a screen in the image space.

The case of a virtual image :

The image cannot be observed on a screen. It is nevertheless visible by an observer situated in the image space.