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Astigmatic Lenses

In an astigmatic lens, all the meridians do not have the same curvature and
a point image of a point object cannot be formed. The lens effectively gives
a minimum power in one direction, gradually changing to a maximum
power in the other, at right angle to the first. These meridians of minimum
and maximum power are referred to as the principal meridians of the lens.
There are two types of astigmatic lenses, namely cylindrical and toric lenses.

CYLINDRICAL LENSES (FIG. 4.1)

Cylindrical lenses have one plane surface and the other curved surface
forming the part of a cylinder. Thus, in one meridian the lens has no
vergence power which is known as the axis of the cylinder. In the meridian
at right angle to the axis, the cylinder acts as a spherical lens. The total
effect is the formation of a line image of a point object. This is called focal
line. It is parallel to the axis of the cylinder (Fig. 4.2).

Fig. 4.1: Cylindrical Lens