The construction of images

Let us consider an optical system, its focus F and F', its principal points H abd H', its principal planes P and P', an object AB of dimension y. So as to build the image B' from B, let us run from B two luminous rays.

Ray 1 (Red) : Parallel to the axis, cuts P in I. The image ray goes through I' (image of I) and F' (Incident parallel to the axis), we have :

Ray 2 (Blue) : Goes through F, cuts P in J. The image ray goes through J' (image of J) and comes out parallel to the axis (Issued of F), we have :

These rays intersect in B', image of B. Paraxial stigmatism causes that any other ray issued from B crossing the optical system goes through B'.

B' is perfectly defined by the position of the object B and the position of the 4 points (H, H', F, F').

The optical system is perfectly defined by the cardinal points (H, H', F, F').

Point A', image of A, is on the projection, of B' on the axis.