UNIT 2: Optics, Refractive Errors, Anomalies of Refraction, and Visual Acuity Flashcards
Refractive Errors
An error that doesn’t permit the eye to bend light properly onto the retina.
Emmetropia
-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
Myopia
-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.
Hyperopia
-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.
Presbyopia
-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.
Astigmatism
-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.
Simple Myopia
-Axial Myopia
-Corneal Myopia
-Lenticular Myopia
(Use minus, Spherical lenses to correct)
Myopia and Near Point Vision
-The eye has no need to accommodate and does not converge.
-The myope has a tendency to under accommodate and under converge.
4 ways to deal with uncorrected myopia
-Don’t use one eye.
-Force eyes to converge at near.
-Alternate vision.
-Eyes turn out.
(Leans toward Exophoria)
Simple Myopia continued..
-In many cases a myope has convergence without accommodation.
-This causes asthenopia and sometimes exophoria.
Simple Hyperopia
-Axial Hyperopia
-Corneal Hyperopia
(Use plus, spherical lenses to correct)
Hyperopia and young children
-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.
4 ways to Deal With Uncorrected Hyperopia
-Ignore one image, develop lazy eye.
-Diplopia
-Asthenopia
-Alternate Vision
-Eyes can become cross-eyed
(Leans toward Esophoria)
Simple Hyperopia
-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.
Astigmia
-Focal line instead of focal point.
-Simple myopic astigmatism.
-Simple hyperopic astigmatism.
-Compound myopic astigmatism.
-Compound hyperopic astigmatism.
-Mixed astigmatism.
