Focusing Light on the Retina How

Astigmatism Aspherical cornea: light at some orientations is focused, while light

$Refraction Air: 1. 0 Water: 1. 33$

Lens Accomodation: lens changes shape to adjust focusing power

Presbyopia: ciliary muscles can no longer contract as well; lens cannot be made round

Test for Presbyopia Measurement of ‘near point’: -measure with ruler and pin

Length of Eyeball + Curvature of Cornea Emmetropia: objects focused on retina (normal)

Length of Eyeball + Curvature of Cornea Myopia: objects focused in front of retina

Length of Eyeball + Curvature of Cornea Hyperopia: objects focused behind retina

Length of Eyeball + Curvature of Cornea Myopia (near-sighted): Axial myopia: eyeball

Length of Eyeball + Curvature of Cornea Hyperopia (far-sighted): Axial hyperopia: eyeball

$Myopia Correction concave lens: negative diopter; diffraction laser surgery: remove corneal$

$Hyperopia Correction convex lens: positive diopter; refraction laser surgery: remove corneal$

Testing Acuity Retinoscope: -streak of light is projected into eye

Testing Acuity “E” charts; Bailey-Lovie charts

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> Focusing Light on the Retina How is light focused on the retina? Focusing Light on the Retina How is light focused on the retina? Cornea (spherical; curvature) Refraction (air-fluid) Lens (accommodation) Length of eye ball (axial) Focusing problems: Astigmatism; Presbyopia; Myopia; Hyperopia

> Cornea approximately 80% of focusing Cornea approximately 80% of focusing

> Astigmatism Aspherical cornea: light at some orientations is focused, while light Astigmatism Aspherical cornea: light at some orientations is focused, while light at others is not

>Test for Astigmatism Test for Astigmatism

$> Refraction Air: 1. 0 Water: 1. 33$ Refraction Air: 1. 0 Water: 1. 33

> Lens Accomodation: lens changes shape to adjust focusing power Lens Accomodation: lens changes shape to adjust focusing power

>Presbyopia: ciliary muscles can no longer contract as well; lens cannot be made round Presbyopia: ciliary muscles can no longer contract as well; lens cannot be made round enough for close vision

> Test for Presbyopia Measurement of ‘near point’: -measure with ruler and pin Test for Presbyopia Measurement of ‘near point’: -measure with ruler and pin -near point increases with age -average near points: -10 yr. old: 7 cm -40 yr. old: 21 cm -60 yr. old: 100 cm

>Length of Eyeball + Curvature of Cornea Emmetropia: objects focused on retina (normal) Length of Eyeball + Curvature of Cornea Emmetropia: objects focused on retina (normal)

>Length of Eyeball + Curvature of Cornea Myopia: objects focused in front of retina Length of Eyeball + Curvature of Cornea Myopia: objects focused in front of retina

>Length of Eyeball + Curvature of Cornea Hyperopia: objects focused behind retina Length of Eyeball + Curvature of Cornea Hyperopia: objects focused behind retina

>Length of Eyeball + Curvature of Cornea Myopia (near-sighted): Axial myopia: eyeball Length of Eyeball + Curvature of Cornea Myopia (near-sighted): Axial myopia: eyeball too long (shown above) Refractive myopia: cornea too curved

>Length of Eyeball + Curvature of Cornea Hyperopia (far-sighted): Axial hyperopia: eyeball Length of Eyeball + Curvature of Cornea Hyperopia (far-sighted): Axial hyperopia: eyeball too short (shown above) Refractive hyperopia: cornea too flat

$> Myopia Correction concave lens: negative diopter; diffraction laser surgery: remove corneal$ Myopia Correction concave lens: negative diopter; diffraction laser surgery: remove corneal tissue in center to reduce curvature

$> Hyperopia Correction convex lens: positive diopter; refraction laser surgery: remove corneal$ Hyperopia Correction convex lens: positive diopter; refraction laser surgery: remove corneal tissue around sides to increase curvature

> Testing Acuity Retinoscope: -streak of light is projected into eye Testing Acuity Retinoscope: -streak of light is projected into eye and reflected back to ophthalmologist -retinoscope is moved slowly side-to-side -if streak moves in opposite direction: myopia -if streak moves in same direction: hyperopia -if whole pupil fills with light: emmetropia

> Testing Acuity “E” charts; Bailey-Lovie charts Testing Acuity “E” charts; Bailey-Lovie charts 20/20 you emmetrope “What you can read at 20 feet an emmetrope can read at ___feet”