Points that form an object–image pair are said to be conjugate to one another, or to form a conjugate pair. Because the paths of light rays are reversible, each point in a conjugate pair is the image of the other. In the case of a source object infinitely far to the left of an optical system, this “point at infinity” is considered conjugate to the back (second) focal point, F2, to the right of the lens. Similarly, the front (first) focal point, F1, is conjugate to the point infinitely far to the right of the lens. Notice that the front and back focal points are not conjugate to each other. In air, the front and back focal points are located at a distance f = 1/P from the front and back principal planes, respectively.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.