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28 Ophthalmic Lenses

The total vergence of a spherical lens depends on the vergence power
of each surface and the thickness of the lens. Since, most of the lenses used
in ophthalmology are thin lenses, thickness factor is ignored (Fig. 3.3). Thus,
the total power of a thin lens is the sum of the two surface powers. Refraction
can be thought of as occurring at the principal plane of the lens. In
Figures 3.4A and B principal plane of the lens is shown as AB. The point at
which the principal plane and the principal axis intersect is called the
principal point or nodal point, of the lens which is denoted by N. Rays of
light passing through the nodal point is undeviated. Light rays parallel to
the principal axis is converged to or diverged from the point F, the principal
focus (Fig. 3.5).

Fig. 3.3: Vergence power of thin lens

Figs 3.4A and B: Cardinal points of thin spherical lenses (A) Convex (B) Concave