Point principal (main points)
Ballast main points are part of the cardinal points of a studied optics in terms paraxiales; they correspond to the intersection between the major plans and the optical axis. The main image is useful for the calculation of the effective focal length (effective focal length).
In paraxiales, the principal planes (object and image) are the set of points for which the magnification is 1:
The plan main image is the place where the incident rays parallel to the optical axis with the emerging rays corresponding intersect.
Its main purpose is the place where the emerging rays parallel to the optical axis, with the incident rays corresponding intersect.
O' is the image of the point O, and you can train as many pair of points connected by the same relationship between both main object and image in the paraxiales conditions.
The effective focal length (effective focal length), called f', corresponds to the distance P 'F'. If the optical system is located in the air, FP and P 'F' distances are the same, and the nodal points are confused with the main points.
The principal planes of the human eye are located in the anterior Chamber of the eye.