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36 Ophthalmic Lenses

Fig. 4.7: Convex spherical surface combined with plano-convex cylindrical surface

Since a plano-cylinder lens has no power along its axis meridian, the
power along the axis meridian of the combination must result from spherical
element alone. The power along the other principal meridian of the lens, at
right angles to the axis meridian of the cylinder surface, is the sum of the
sphere and cylinder. Under the rotation test, it exhibits scissor movement
in the same way as a plano-cylindrical lens and under the movement test;
it exhibits movement along each of its principal meridians. The power of a
sphero-cylindrical lens is expressed by stating the power of the spherical
component first, followed by the power of the cylindrical component, and
finally the direction of the cylindrical axis. Thus, the specification:

– 3.00/– 2.00 × 90º
It signifies that spherical component of the lens is – 3.00D, the cylindrical
component is – 2.00D and the axis of the cylindrical surface lies along the
90º meridian. On representing the power of the sphero-cylindrical lens by
means of optical cross, the principal meridians show – 3.00D on vertical
meridian and – 5.00D on the horizontal meridian as shown in Figure 4.8.

Fig. 4.8: Optical cross representation of – 3.00/– 2.00 × 90°

The pencil of light that results from refraction at an astigmatic lens is
depicted in the Figure 4.9. Since the light does not focus as a point, the
interval between two line foci is called the ‘Interval of Sturm’. The best
focusing occurs somewhere inside the interval of Sturm. This point is called
the ‘Circle of least confusion’, and the complete envelop of the light near
the circle of least confusion is called the ‘Sturm’s Conoid’, named after the
mathematician CF Sturm.