This document discusses eye optics and refractive errors. It provides 3 case studies: a 14-year-old boy unable to see the blackboard, a 51-year-old man with difficulty reading the newspaper, and a 6-year-old girl who squints when looking farther than 2 feet away. The objectives are to understand eye optics, visual acuity assessment, common refractive errors, and color perception assessment. Key refractive errors like myopia, hyperopia, presbyopia, and astigmatism are explained in terms of etiology, presentation, diagnosis, and treatment.

1. 1
Eye Optics and
Refractive Errors
By: John J. Beneck MSPA, PA-C

2. 2
Case 1
• 14 year old boy comes to primary care
office c/o inability to see the blackboard in
school

3. 3
Case 2
• 51 year old man presents c/o difficulty
reading the news paper: “My arms are too
short!”

4. 4
Case 3
• 6 year old girl presents with mom who
states she squints when looking at anything
more than 2 feet away.

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Objectives
• Understand the optics of the eye
• Understand visual acuity assessment
• Understand common refractive errors
• Understand color perception assessment

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Objectives (Cont.)
• Understand common refractive errors in
terms of:
– Etiology/pathology
– Clinical presentation
– Course and prognosis (when appropriate)
– Diagnosis
– Interventions/treatments

7. 7
Abbreviations
• C/o – complaining of or complains of

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Visual Acuity
• Use Snellen chart
– Positioned 20 feet away
• Each Eye Alone, Then Together
• With Corrective Lenses (If indicated)

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Snellen Charts
http://store6.yimg.com/I/sightmart-eye-care-products_1753_2381891 accessed 9/5/03

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Visual Acuity
• Visual acuity is expressed as two numbers
• The first indicates the distance of the patient
from the chart
• The second indicates the distance at which a
normal eye can read the line of letters
– Ex: 20/50

11. Visual Acuity
• 20/20
– ability to see letters of a given size at 20 feet
• 20/40
– what a normal person can see at 40 feet, this
person must be at 20 feet to see.
• 20/200
– what a normal person can see at 200 feet, this
person must be at 20 feet to see.

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Further Acuity
Assessment/Diagnosis
• Optometric examination
– Cornea
– Anterior chamber
– Posterior chamber
• Retinal examination and imaging

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Image Reception
• Optics/Refraction
– Air anterior Cornea
• 2/3 of the refractive power of the eye
– Posterior Cornea aqueous humor
– Iris / pupil
• Variable aperture
– Aqueous humor anterior lens
– Posterior lens vitreous humor

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Image Reception
• Convex refraction
– Refractive index
– Convergence
– Image reversal
• Perception
• Blind spot

15. Refractive Principles of a Lens
• Convex lens focuses light rays
Figure 49-2; Guyton and Hall

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The Refractive Principles of a Lens
Figure 49-8; Guyton and Hall

17. Refractive Principles of a Lens
• Concave lens diverges light rays.
Figure 49-3; Guyton and Hall

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What’s next?
• Emmetropia (normal vision)
• Myopia (near-sighted)
• Hyperopia (far-sighted)
– Inability of the lens to accommodate adequately
for near vision
• Presbyopia
• Astigmatism

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Myopia (Near-Sighted)
• The patient is able to focus on objects near but not far
away
• Typical complaint is difficulty focusing on road signs or
the black board
• The lens is unable to flatten enough to prevent conversion
of images before reaching the retina
• The image comes into sharp focus in front of the retina
• Frequently squinting is compensatory mechanism

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Errors of Refraction
Figure 49-12; Guyton and Hall
Normal vision
Far sightedness
Near sightedness

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Myopia Correction
• Corrective concave lens use
– Glasses
– Contact lenses
• Surgical
– LASIK (greatest range of correction for myopia)
• Laser-Assisted In Situ Keratomileusis
– Epithelial flap cut and lifted
– Laser applied to deep layers of cornea
– Flap repositioned
• Squinting?

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Correction of Myopic Vision
Figure 49-13; Guyton and Hall
Myopia corrected with
concave lens

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Depth of Focus
Effect of pupil size on focus in myopic patients
Note the difference in divergence of rays as
they reach the retinal surface

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Hyperopia (Far-Sighted)
• The patient is able to focus on objects far away
but not close up
• Typical complaint is difficulty reading
• The image comes into sharp focus behind the
retina

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Errors of Refraction
Figure 49-12; Guyton and Hall
Normal vision
Far sightedness
Near sightedness

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Hyperopia Correction
• Corrective convex lens use
– Glasses
– Contact lenses
• Surgical
– LASIK
• Laser-Assisted In Situ Keratomileusis
– Epithelial flap cut and lifted
– Laser applied to deep layers of cornea
– Flap repositioned

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Correction of Hyperopic Vision
Figure 49-13; Guyton and Hall
Hyperopia corrected
with convex lens

28. Presbyopia; The Inability to
Accommodate
• Caused by progressive denaturation of the proteins of the
lens.
• Makes the lens less elastic.
• Begins about 40-50 years of age.
• Near point of focus recedes beyond 22 cm (9 inches).

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Astigmatism
• Unequal focusing of light rays due to an oblong
shape of the cornea
• Presents with relatively stable blurry vision
• Patient unable to focus on objects near or far
• Near vision is typically better

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Astigmatism
• “Vertical” focal point different from “Horizontal”
focal point
• Cornea lacks discoid continuity
– More curved in one plane than another
• Unable to correct with a single concavity or
convexity index

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Exaggerated Astigmatic Corneal Shape
Notice the difference in
the degree of curve of the
cornea in 2 planes
Cornea: face-on

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Astigmatism Correction
• Cylindrical optical refractive correction
– Glasses
– Contact lenses
• Surgery
– LASIK
• Laser-assisted in situ keratomileusis

33. Cataracts
• Cataracts
– cloudy or opaque area of the lens
– caused by coagulation of lens proteins
• More to come

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Cataract

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Cataract Correction
• Surgical
– The lens is replaced
– Induces presbyopia
– Frequently dramatically improves far vision

36. Pigment Layer of Retina
• Pigment layer of the retina is very important
• Contains the black pigment melanin
• Prevents light reflection in the globe of the eye
• Without the pigment there is diffuse scattering of
light rather than the normal contrast between dark
and light.
• This is what happens in albinos
– poor visual acuity because of the scattering of
light
– Best corrected vision is 20/100-20/200

37. Color Vision
• Color vision is the result of activation of cones.
• 3 types of cones:
– blue cone
– green cone
– red cone
• The pigment portion of the photosensitive
molecule is the same as in the rods, the protein
portion is different for the pigment molecule in
each of the cones.
• Makes each cone receptive to a particular
wavelength of light

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Each Cone is Receptive to a
Particular Wavelength of Light
Figure 50-7; Guyton & Hall

39. Color Blindness
• lack of a particular type of cone
• genetic disorder passed along on the X
chromosome
• occurs almost exclusively in males
• about 8% of women are color blindness carriers
• most color blindness results from lack of the red
or green cones
– lack of a red cone, protanope.
– lack of a green cone, deuteranope.

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Eyes & Visual Pathways
Ishihara Test for Color Blindness
The individual with normal color vision will see a 5 revealed in the dot
pattern.
An individual with Red/Green (the most common) color blindness will
see a 2 revealed in the dots.
http://www.toledo-bend.com/colorblind/Ishihara.html, 2001

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Color Vision
Colorblind individuals should see the yellow square.
Color normal individuals should see the yellow
square and a "faint" brown circle.

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How about those cases
• Case 1
– 14 year old boy comes to primary care office c/o
inability to see the blackboard in school
• Case 2
– 51 year old man presents c/o difficulty reading the news
paper: “My arms are too short!”
• Case 3
– 6 year old girl presents with mom who states she
squints when looking at anything more than 2 feet
away.

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Now, Do You See Things More
Clearly???