UNIT 2: Optics, Refractive Errors, Anomalies of Refraction, and Visual Acuity

Question 1

Refractive Errors

Answer 1

An error that doesn’t permit the eye to bend light properly onto the retina.

Question 2

Emmetropia

Answer 2

-No refractive error present
-Cornea and lens shaped correctly
-Distance between fovea and lens is correct
-Light from 20ft is focused on the retina
-The eye can accommodate for near objects
-Need no corrective lens

Question 3

Myopia

Answer 3

-Nearsighted.
-Eyeball too long.
-Distance between lens and fovea too great.
-Light comes to focus in the vitreous humor.
-Sometimes the crystalline lens does not need to accommodate for near vision.
-A minus (diverging) lens is used to correct.

Question 4

Hyperopia

Answer 4

-Farsighted
-Eyeball too small.
-Light from distance may or may not focus on the retina.
-Light from closer source focuses behind the retina.
-A plus, converging lens is used correct.

Question 5

Presbyopia

Answer 5

-This is a condition, not a refractive error.
-Crystalline lens loses natural ability to focus.
-Lens loses its elasticity, ability to accommodate.
-Accommodation lessens with age.
-Bifocals, trifocals, progressive and other multifocal are used to correct.

Question 6

Astigmatism

Answer 6

-The most common refractive error of all.
-The cornea is aspherical in the central zone.
-Light has different focal points in different meridians creating a line focus.
-Meridians are usually 90 degrees apart.
-Almost 2/3 of the population has astigmatism.
-Spherocylinder lenses are used to correct.

Question 7

Simple Myopia

Answer 7

-Axial Myopia
-Corneal Myopia
-Lenticular Myopia

(Use minus, Spherical lenses to correct)

Question 8

Myopia and Near Point Vision

Answer 8

-The eye has no need to accommodate and does not converge.
-The myope has a tendency to under accommodate and under converge.

Question 9

4 ways to deal with uncorrected myopia

Answer 9

-Don’t use one eye.
-Force eyes to converge at near.
-Alternate vision.
-Eyes turn out.

(Leans toward Exophoria)

Question 10

Simple Myopia continued..

Answer 10

-In many cases a myope has convergence without accommodation.
-This causes asthenopia and sometimes exophoria.

Question 11

Simple Hyperopia

Answer 11

-Axial Hyperopia
-Corneal Hyperopia

(Use plus, spherical lenses to correct)

Question 12

Hyperopia and young children

Answer 12

-Young hyperopic children can accommodate at near.
-They over-accommodate and over-converge and lean toward Esophoria.
-Great risk of becoming amblyopic if not corrected by age 6 or 7.

Question 13

4 ways to Deal With Uncorrected Hyperopia

Answer 13

-Ignore one image, develop lazy eye.
-Diplopia
-Asthenopia
-Alternate Vision
-Eyes can become cross-eyed

(Leans toward Esophoria)

Question 14

Simple Hyperopia

Answer 14

-Can accommodate distance vision, while trying to stop the eye from converging.
-This can lead to esophoria if not corrected, and sometimes amblyopia if left untreated.

Question 15

Astigmia

Answer 15

-Focal line instead of focal point.
-Simple myopic astigmatism.
-Simple hyperopic astigmatism.
-Compound myopic astigmatism.
-Compound hyperopic astigmatism.
-Mixed astigmatism.

Question 16

Gullstrand’s Model

Answer 16

-Cornea= +43.00 Diopters
-Crystalline Lens = +19.00 Diopters
-Index of refraction:
-Cornea: 1.376
-Lens: 1.416
-Aqueous/Vitreous: 1.336
-Axial Length: 24mm

Question 17

Optical Image

Answer 17

-The actual image formed by the eye’s optical system.
-It may or may not coincide with the retinal image.

Question 18

Retinal Image

Answer 18

-The image is the image actually on the retina.
-It may or may not be clear.

Question 19

Optical Image vs. Retinal Image

Answer 19

-When the image on the retina is clear, the optical image and the retinal image are the same.
-If the image on the retina is not clear (Blur Circles), then the optical image and retinal image are different.

Question 20

Blur Circles

Answer 20

The farther away the optical image is from the retina, the larger the blur circles on the retina.

Question 21

Changes that occur during accommodation

Answer 21

-The pupil constricts
-Pupillary and edge margin of iris and front surface of the lens shift forward.
-The anterior surface of the lens becomes more PLUS (there is also a slight change in the posterior surface of the lens).
-Crystalline lens sinks down slightly due to gravity.

Question 22

Donder’s table of accommodation

Answer 22

Age and Accommodation Ability:
10 - 14D
20 - 10D
30 - 7.0D
40 - 4.5D
50 - 2.5D
60 - 1.0D
70 - 0.0D

Question 23

Snellen E

Answer 23

-20/20 Line
-8.87mm Tall
-Subtend a visual angle of 1 arc minute
-Snellen/English rating system

Question 24

Other Systems for Measuring and noting Visual Acuity

Answer 24

-Decimal Rating System (1.0)
-Snellen/Metric System (6/6)

(The basis for conversion :
20/20 = 1.0 = 6/6)

Question 25

Other symbols used

Answer 25

-Big E (Snellen E)
-Konig Bars
-Landolt Ring
-Tumbling E