Refractive Error
Dr. Corina van de Pol
UyThien Phan
Maricela Robles, LPN
VN Hope
2017
What is Refractive Error?
• Refractive errors occur when parts of
the eye don’t work together to produce
a clear image.
Emmetropia
z Emetropia
y The power
and the length
of the eye are
correct
y Forms a clear
image on the
retina
MYOPIA
NEAR SIGHTED
Myopia
z Causes:
y The eye is too
long and/or
y The power of
the eye is too
strong
Symptoms of Myopia
• Poor distance
vision
• Good near
vision
• Eye strain
Correcting Myopia
• Lenses with negative power (concave)
• The lens is thin in the center and thick
at the edge
• Objects look smaller through the lens
• Light rays DIVERGE so they will focus
on the retina
Example Prescription (Myopia)
R
-3.25 Sphere
L
-2.75 Sphere
HYPEROPIA
FAR SIGHTED
Hyperopia
z Causes
y The eye is too
short and/or
y The power of
the eye is too
weak
Symptoms of Hyperopia
• The eyes have to
focus to see clearly
in the distance.
• The eyes have to
focus more to see
near.
Correction of Hyperopia
• Lenses with POSITIVE power (convex)
• The lenses are thick in the center and
thin at the edges
• Objects seen through the lenses are
magnified
• Light rays are CONVERGED so they
focus on the retina
Example prescription (hyperopia)
R
+3.25 Sphere
L
+2.75 Sphere
ASTIGMATISM
Astigmatism
z Causes
y The cornea is
shaped like an
“american”
football
y The eye forms
two “line” images
90 degrees apart
Symptoms of Astigmatism
• Blurry vision at
distance and near
• Distorted vision
• Headache
• Eye pressure
• Tired Eyes
Correction of Astigmatism
• Cylinder lenses for the correction of
astigmatism are specific to the eyes of
the patient
• They have to be tailored to the patient
Correction for Astigmatism
• Example. -1.00 -1.00 x 180
• Sphere and Cylinder and Axis
– The first number is the sphere power (for myopia
or hyperopia)
– The second number is the cylinder power (for
astigmatism)
– The third number is the axis of the cylinder power
(orientation 0 to 180 degrees)
Example Prescription
(astigmatism)
R
+3.25 -0.75 x 180
L
+2.75 -1.50 x 170
Hyperopia with
astigmatism
Example Prescription
(astigmatism)
R
-3.25 -0.75 x 090
L
-2.75 -1.50 x 085
Myopia with
astigmatism
Presbyopia
Picture of someone in bifocals or
reading glasses
What is Presbyopia?
• The ability to focus
up close is reduced
• The lens of the eye
loses its flexibility
• Only affects near
vision
Presbyopia
z Causes
y Loss of
elasticity of the
lens of the eye
What is Presbyopia?
• Onset between 30
and 40 years of age
• Natural part of aging
• Occurs gradually
over 10-12 years
and then stabilizes
Example prescription (bifocals)
R
-3.25 -0.75 x 160
L
-2.75 -1.50 x 170
ADD +2.50
Myopia with
astigmatism in
a bifocal
Example prescription (bifocals)
R
+3.25 Sphere
L
+2.75 Sphere
ADD +2.50
Hyperopia in a
bifocal
Without
Presbyopia
With
Presbyopia
Happy
Patient