Optics concepts
We explain basic optics concepts here for our memory.
In this page we assume light rays always come from left to right, namely left side is object space and right side is image space.
In Gaussian optics, the cardinal points consist of three pairs of points located on the optical axis of a rotationally symmetric, focal, optical system. These are the focal points, the principal points, and the nodal points. For ideal systems, the basic imaging properties such as image size, location, and orientation are completely determined by the locations of the cardinal points. Cardinal points provide a way to analytically simplify a system with many components, allowing the imaging characteristics of the system to be approximately determined with simple calculations.
Light ray # 1 is parallel with the optic axis. It is converged at point F2. Similarly Light ray # 2 passes point F1 and is parallel with the optic axis behind lens. F1 is called front focal point and F2 is called back focal point.
Extending light ray #1 from left side and right side, the intersection is on the rear principal plane. The principle plane crosses optical axis at point H2, which is called back principal point. Similarly H1 is the front principal point that can be determined with light ray #2.
The front and rear nodal points (N1, N2) have the property that a ray aimed at one of them will be refracted by the lens such that it appears to have come from the other, and with the same angle with respect to the optical axis. If the medium on both sides of the optical system is the same (e.g., air), then the front and rear nodal points coincide with the front and rear principal points, respectively. This is the case of image shown above, where air is the common medium on both sizes.
Since we defined the Cardinal points, now we can define the focal lengths. Same as the Cardinal points, there are two focal lengths from two sides as well. The front focal length (f1) is defined as the distance between the front focal point and front principal point. The back focal length (f2) is defined as the distance between back focal point and back principal point.