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34 Ophthalmic Lenses

Fig. 4.2: Image formation by convex cylindrical lens of point object O

NOTATION FOR CYLINDER LENSES AND ORIENTATION OF AXIS

Fig. 4.3: Axis direction of astigmatic lens

A lens with purely cylinder power would be described as, say – 4.00Dcyl
(diopter cylindrical) in order to differentiate from the spherical one, which
is described as – 4.00Dsph (diopter spherical). As cylindrical surface is not
rotationally symmetrical about the midpoint, a notation is required for their
positioning in front of the eye. This is achieved by specifying the angle
between the axis of symmetry of the cylinder and the horizontal. The
universally used “standard” axis notation uses a protractor that reads anti-
clockwise when looking at the face of the lens wearer. Angles upto 180 are

Fig. 4.4: Standard axis notation