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The angular magnification of an optical system is the ratio of the angle (with respect to the optical axis) of a ray in image space and that of its conjugate in object space. There exists a pair of conjugate points lying on the optical axis where, for all possible rays, the angular magnification is unity. These are the nodal points. The first nodal point is located in object space and the second nodal point is located in image space. This is set out in Figure 1.10, where for a general conjugate pair, the angular magnification, α, is equal to θ₂/θ₁. For the nodal points, θ₂ = θ₁; that is to say, the angular magnification is unity. Where the two focal lengths are identical, or the object and image spaces are within media of the same refractive index, the nodal points are co-located with the principal points.

Figure 1.10 Angular magnification and nodal points.