Page 156 - Nov- Dec 2024

ASTIGMATISM CORRECTED BY CYLINDERICAL LENS

Spherical lenses have a single power across all In the meridian specified by the "axis" in
meridians of the lens, such as +1.00 D or -2.50 D. the prescription, the power equals the
However, astigmatism causes directional blur. value listed under "sphere." As you move
around the clock face, the power in a given
Here are two examples of blur caused by meridian steadily approaches the sum of
astigmatism: the letters appear smeared in a the "sphere" and "cylinder" values. At the
specific direction, as if smudged with a thumb on meridian 90° from the axis, the power
charcoal. A cylindrical lens with the correct power equals the sum of "sphere" and "cylinder."
and orientation can rectify this blur. The second
example shows more pronounced blurring,
requiring a stronger cylindrical lens. Additionally, WHEN AN EYE DOCTOR
the second example demonstrates smearing in a MEASURES THE EYE DURING
different direction.
REFRACTION, THEY USUALLY
A spherical lens behaves uniformly regardless START BY DETERMINING THE
of orientation, maintaining consistent BEST SPHERICAL CORRECTION. IF
magnification or blurring. A cylindrical lens,
however, possesses refractive power in only one ASTIGMATISM IS PRESENT, THEY
direction, similar to a bar reading magnifier. The COMPENSATE FOR IT BY ADDING
rotational orientation of this power is specified in THE CORRECT AMOUNT OF
prescriptions using axis notation. CYLINDRICAL CORRECTION
SPECIFICATION OF AN AXIS

The axis specified in a prescription denotes,
in degrees, the meridian of cylindrical power.
This axis can range from 1 to 180 degrees. In the
illustration above, the prescription axis is 20° in AUTHOR:
minus notation or 110° in plus notation.
DR VINEET
The total power of a cylindrical lens varies from SHRIVASTAVA
zero in the axis meridian to its maximum value Founder & Director
in the power meridian, 90° away. In the example Divya Jyoti Eye
above, the axis meridian is located at 20°, while Hospital, Jaipur
the power meridian is at 110°. (Rajasthan)

The total power of a lens combining spherical
and cylindrical corrections changes accordingly.

150 | THE INDIAN OPTICIAN | NOV-DEC 2024 OPTOMETRY