| opt. |
principal ray (the ray from any oblique object point passing through the centres of the iris and pupils of a lens, the ray midway between the extreme upper and lower rim rays, or the ray through the centre of the aperture); chief ray (the ray from any oblique object point passing through the centres of the iris and pupils of a lens, the ray midway between the extreme upper and lower rim rays, or the ray through the centre of the aperture); central ray (the ray from any oblique object point passing through the centres of the iris and pupils of a lens, the ray midway between the extreme upper and lower rim rays, or the ray through the centre of the aperture); axial beam |
