Introduction to Refractive
Error and Prescription
Writing
Walter Huang, OD
Yuanpei University
Department of Optometry
Vision
Optics
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Cornea
Aqueous humor
Lens
Vitreous humor
Retina
Refractive Error
The result of a mismatch between optics
and the growth of the eye
It is due to a combination of genetic and
environmental influences
It is NOT considered an eye disease
Treatment includes spectacles, contact
lenses, and refractive surgery
Types of Refractive Error
Emmetropia
Myopia
Hyperopia
Astigmatism
Presbyopia
Emmetropia
The average emmetrope has a VA of
20/20 or better
Myopia
When parallel rays of light enter the eye
(with accommodation relaxed) and come
to a single point focus in front of the retina
Myopia
Blurry vision at distance
Clear vision at near
Myopia
It is corrected by divergent or minus lenses
Power of corrective lens needed can be
estimated by finding the far point where the
patient can achieve clear vision
Example
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Far point is at 20cm
Focal length of corrective lens needed = 20cm
Power of corrective lens needed = 1/f =1/0.2m
= -5.00D
Unit for Power = Diopter (D)
Hyperopia
When parallel rays of light enter the eye
(with accommodation relaxed) and come
to a single point focus behind the retina
Hyperopia
Blurry vision at distance and near
Intermittent blurring of vision
Hyperopia
It is corrected by convergent or plus
lenses
A young patient with low hyperopia can
accommodate to focus the distant image
on the retina
Since accommodation decreases with age,
a low hyperopic patient tends to wear
corrective lenses for near work at an
earlier age
Astigmatism
When parallel rays of light enter the eye
(with accommodation relaxed) and do not
come to a single point focus on or near the
retina
Astigmatism
It is due to a distortion of the cornea
and/or lens
The refracting power is not uniform in all
meridians
The principal meridians are the meridians
of greatest and least refracting powers
The amount of astigmatism is equal to the
difference in refracting power of the two
principal meridians
Astigmatism
Distorted vision
Letter confusion
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P versus F
A versus R
H versus N
Astigmatism
It is corrected by cylindrical or
spherocylindrical lenses
Presbyopia
Presbyopia = “old man’s eye” (Latin)
Presbyopia
Decrease in the amplitude of
accommodation or loss of accommodative
ability with age
Presbyopia
It is a natural part of the aging process
The onset of presbyopia is at
approximately 40 years of age and over
though it may be earlier in low hyperopes
Presbyopia
Blurry vision at near
Difficult or impossible to accommodate
sufficiently for near work
Presbyopia
It is corrected by convergent or plus
lenses for near work only (near Add)
Types of Lens
Spherical lens
Cylindrical lens
Spherocylindrical lens
Spherical Lens
A plus or minus lens where the refracting
power is equal in all meridians
Diopter Sphere (DS) is the measuring unit
used to differentiate the spherical lens
from lenses with cylindrical component
Power cross and prescription writing for a
spherical lens require the specification of
the spherical power component only
Spherical Lens
Power cross:
Prescription form: +2.50DS
Cylindrical Lens
A flat or plano (pl) axis meridian
perpendicular to a power meridian
Cylindrical Lens
Diopter Cylinder (DC) is the measuring
unit used to differentiate the cylindrical
lens from lenses with spherical component
Power cross and prescription writing for a
cylindrical lens require the specification of
both the cylindrical power and axis
components
Cylindrical Lens
Power cross:
Prescription form: -4.00 x 180
Spherocylindrical Lens
A toric lens consists of two perpendicular
principal meridians
Power cross and prescription writing for a
spherocylindrical lens require the
specification of the spherical power,
cylindrical power, and axis components
Spherocylindrical Lens
Power cross:
Prescription form:
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+3.00 -1.00 x 180
+3.00/-1.00 x 180
Prescription Writing
Example 1
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Power cross:
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Prescription form: +1.00 -0.50 x 120
Prescription Writing
Example 2
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Power cross:
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Prescription form: -3.00 -0.50 x 084
Prescription Writing
Example 3
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Power cross:
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Prescription form: +1.25 -2.50 x 005
Rules for Power Cross
Specify both power and axis
Power is always represented by plus or
minus sign in front and contains two digits
after the decimal point
Power is presented in 0.25D steps
Rules for Power Cross
Axis meridian starts counter-clockwise
from 0 to 180
Rules for Power Cross
The cross orientation is drawn to the
actual meridian
When axis is at the 0 to 180 horizontal,
use 180 instead of 0
Degree notation may or may not be used
for axis
If degree notation is NOT used for axis,
three digits must be used for axis, except
in the case of 0
Rules for Prescription Writing
Always include power, cylinder, and axis,
except for spherical lenses (specified as
DS)
Degree notation is NOT used for axis
Minus versus Plus Cylinder
In Optometry, prescription writing is in
minus cylinder (-cyl) form
Sphere and axis specified is the most plus
principal meridian
Minus versus Plus Cylinder
In Ophthalmology, prescription writing is
usually in plus cylinder (+cyl) form
Sphere and axis specified is the most
minus principal meridian
Conversion between Minus and
Plus Cylinder
Be sure to know how to convert between
minus and plus cylinder form and back
Conversion between Minus and
Plus Cylinder
Example
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Minus cylinder form: +1.00 -3.00 x 180
Plus cylinder form: -2.00 +3.00 x 090