The Human Eye Focus and Vision Correction

Path of Light Cornea Cornea Pupil Pupil Lens Lens Vitreous humor Vitreous humor Retina Retina Optic Nerve (electrical signals) Optic Nerve (electrical signals) 4 “Snell” interfaces 4 “Snell” interfaces

The Eye – Close-up of the Cornea

Fun with Illusions!

The Lens Transparent Transparent Flexible Flexible Changes shape to focus on objects at different distances Changes shape to focus on objects at different distances Spherical for nearby objectsSpherical for nearby objects Flatter for distant objectsFlatter for distant objects

The Eye – Operation, cont. Accommodation Accommodation The eye focuses on an object by varying the shape of the pliable crystalline lens through this processThe eye focuses on an object by varying the shape of the pliable crystalline lens through this process An important component is the ciliary muscle which is situated in a circle around the rim of the lensAn important component is the ciliary muscle which is situated in a circle around the rim of the lens Thin filaments, called zonules, run from this muscle to the edge of the lensThin filaments, called zonules, run from this muscle to the edge of the lens

The Eye – Focusing, cont. The eye can focus on near objects The eye can focus on near objects The ciliary muscle tensesThe ciliary muscle tenses This relaxes the zonulesThis relaxes the zonules The lens bulges a bit and the focal length decreasesThe lens bulges a bit and the focal length decreases The image is focused on the retinaThe image is focused on the retina

The Eye – Focusing The eye can focus on a distant object The eye can focus on a distant object The ciliary muscle is relaxedThe ciliary muscle is relaxed The zonules tightenThe zonules tighten This causes the lens to flatten, increasing its focal lengthThis causes the lens to flatten, increasing its focal length For an object at infinity, the focal length of the eye is equal to the fixed distance between lens and retinaFor an object at infinity, the focal length of the eye is equal to the fixed distance between lens and retina This is about 1.7 cm This is about 1.7 cm

The Retina Collection of rods and cones on back surface of the eye Collection of rods and cones on back surface of the eye Light induces chemical reaction, electrical impluse sent to brain Light induces chemical reaction, electrical impluse sent to brain Cones – color/detail Cones – color/detail Rods – low light Rods – low light

The Eye – Seeing Colors Only three types of color-sensitive cells are present in the retina Only three types of color-sensitive cells are present in the retina They are called red, green and blue conesThey are called red, green and blue cones What color is seen depends on which cones are stimulated What color is seen depends on which cones are stimulated

Retina as the Focal Point

The Eye – Near and Far Points The near point is the closest distance for which the lens can accommodate to focus light on the retina The near point is the closest distance for which the lens can accommodate to focus light on the retina Typically at age 10, this is about 18 cmTypically at age 10, this is about 18 cm The average value is about 25 cmThe average value is about 25 cm It increases with ageIt increases with age Up to 500 cm or greater at age 60 Up to 500 cm or greater at age 60 The far point of the eye represents the largest distance for which the lens of the relaxed eye can focus light on the retina The far point of the eye represents the largest distance for which the lens of the relaxed eye can focus light on the retina Normal vision has a far point of infinityNormal vision has a far point of infinity

Conditions of the Eye Eyes may suffer a mismatch between the focusing power of the lens-cornea system and the length of the eye Eyes may suffer a mismatch between the focusing power of the lens-cornea system and the length of the eye Eyes may be: Eyes may be: FarsightedFarsighted Light rays reach the retina before they converge to form an image. Person can only focus on distant objects Light rays reach the retina before they converge to form an image. Person can only focus on distant objects NearsightedNearsighted Light rays converge before they reach the retina. Person can focus on nearby objects but not those far away Light rays converge before they reach the retina. Person can focus on nearby objects but not those far away

Farsightedness Also called hyperopia Also called hyperopia The near point of the farsighted person is much farther away than that of the normal eye The near point of the farsighted person is much farther away than that of the normal eye The image focuses behind the retina The image focuses behind the retina Can usually see far away objects clearly, but not nearby objects Can usually see far away objects clearly, but not nearby objects

Correcting Farsightedness A converging lens placed in front of the eye can correct the condition A converging lens placed in front of the eye can correct the condition The lens refracts the incoming rays more toward the principal axis before entering the eye The lens refracts the incoming rays more toward the principal axis before entering the eye This allows the rays to converge and focus on the retina

Nearsightedness Also called myopia Also called myopia The far point of the nearsighted person is not infinity and may be less than one meter The far point of the nearsighted person is not infinity and may be less than one meter The nearsighted person can focus on nearby objects but not those far away The nearsighted person can focus on nearby objects but not those far away

Correcting Nearsightedness A diverging lens can be used to correct the condition A diverging lens can be used to correct the condition The lens refracts the rays away from the principal axis before they enter the eye The lens refracts the rays away from the principal axis before they enter the eye This allows the rays to focus on the retina

Diopters Unit used to measure curvature of corrective lenses is the Diopter, which is equal to a m -1 Unit used to measure curvature of corrective lenses is the Diopter, which is equal to a m -1 Negative for myopes (diverging lens) Negative for myopes (diverging lens) Positive for hyperopes (converging lens) Positive for hyperopes (converging lens) The power of the corrective lens P = 1/f, where f must be in meters. The power of the corrective lens P = 1/f, where f must be in meters.

Diopter vision ranges Mild Myopia, 0 to D Mild Myopia, 0 to D Moderate Myopia, to D Moderate Myopia, to D Severe Myopia, to D Severe Myopia, to D Extreme Myopia, < D Extreme Myopia, < D

M.C. Escher’s Stairs

Alternative Vision Correction Orthokeratology (ortho-k) Orthokeratology (ortho-k) Night use contact lenses flatten out corneaNight use contact lenses flatten out cornea Cornea retains this shape for the dayCornea retains this shape for the day LASIK LASIK Surgically reshapes corneaSurgically reshapes cornea

LASIK Most useful for myopia Most useful for myopia More intense reshaping for hyperopia More intense reshaping for hyperopia Other criteria Other criteria Cornea thickness Pupil diameter

Presbyopia and Astigmatism Presbyopia (literally, “old-age vision”) is due to a reduction in accommodation ability Presbyopia (literally, “old-age vision”) is due to a reduction in accommodation ability The cornea and lens do not have sufficient focusing power to bring nearby objects into focus on the retinaThe cornea and lens do not have sufficient focusing power to bring nearby objects into focus on the retina Condition can be corrected with converging lensesCondition can be corrected with converging lenses In astigmatism, light from a point source produces a line image on the retina In astigmatism, light from a point source produces a line image on the retina Produced when either the cornea or the lens or both are not perfectly symmetricProduced when either the cornea or the lens or both are not perfectly symmetric Can be corrected with lenses with different curvatures in two mutually perpendicular directionsCan be corrected with lenses with different curvatures in two mutually perpendicular directions