F' is the image in the system of the point at infinity on the axis.
The object being infinite on the axis, L1 gives it an image in its image focus F'1.
Of F'1 , L2 gives a definitive image which is the image focus F' de S.
F' is the conjugate of F'1 in lens L2.
To determine the position of F' let us apply the conjugate formula (31)
For the conjugation being considered :