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Subjective Refraction Eye As a Camera

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Subjective Refraction Eye As a Camera 9/4/2015 Optics of human eye Subjective refraction Eye as a camera Components Dr. Ali Abusharha Schematic eye and reduced eyes Axes and visual angles Optical aberrations 1 2 Eye as a camera Components Eyelids- shutter The cornea Cornea- focusing system Lens- focusing system The anterior chamber Iris- diaphragm The iris and pupil Choroid- dark chamber Retina-light sensitive film The crystalline lens The retina 3 4 Cornea Vertical diameter slightly less than horizontal Reasons of refraction: Front apical radius 7.5 - 7.7 mm Curvature. Back apical radius 6.4 - 6.8 mm Significant difference in refractive indices of air and cornea. Actual refractive index cornea= 1.376 Power of cornea +43D (2/3 of total eye power) 5 6 1 9/4/2015 Iris and Pupil The anterior chamber •Regulate amount of light entering the eye • At 2.4mm pupil size, best retinal image obtained, Cavity between cornea and iris as aberration and diffraction are balanced. Filled with aqueous humor. Average Depth of AC – about 2.5-4.0 mm • 2-4mm Change in AC depth change the total power. 1mm size: forward shift of lens- increase about 1.4D in power Refractive index of aqueous humor= 1.336 • depth of focus increases Small pupil • Concept used as pin hole test in refraction Large pupil • Retinal image quality improves 7 8 The crystalline lens Lens accounts for about one third of the refraction • Birth 3.5 – 4 mm of the eye. Thickness • Adult life 4.75 – 5 mm ACCOMODATION Provides a mechanism of focusing at different • Ant surface 10 mm Radius of curvature • Post surface 6 mm distances. • Nucleus 1.41 Refractive index of lens • Pole 1.385 OPTICAL CHANGES IN CATARACTOUS LENS • Equator 1.375 Visual Acuity reduction. Total power • 15 -18 d. Myopic shift. Monocular diplopia. • At birth- 14-16 D Accommodative power • At 25yrs- 7-8D Glare. • At 50yrs- 1-2D 9 Color shift. 10 Retina Axes and visual angles Maximum resolving power at fovea. A concave spherical surface with r =-12 mm. Advantages of curvature of retina over plane image forming surfaces of cameras and optical instruments: The curved images formed by the optical system is brought in the right order. A much wider field of view is covered by the steeply curved retina 11 12 2 9/4/2015 Optical aberrations OPTICAL AXIS: line passing through centre of Diffraction of light cornea, lens and meets retina on nasal side of Spherical aberrations fovea Chromatic aberrations VISUAL AXIS: line joining fixation point, nodal Decentering point and fovea Oblique aberrations FIXATION AXIS: line joining fixation point and Coma centre of rotation 13 14 REFRACTIVE ERRORS Emmetropia • Ametropia: a refractive error is present • Myopia: Near sightedness 43 diopters • Hyperopia (Hypermetropia): Far sightedness 24-25mm • Presbyopia: Loss of accommodative ability of the lens resulting in difficulties with near tasks 18 diopters • Astigmatism: the curvature of the cornea and/or lens is not spherical and therefore causes image blur on the retina Accommodation at rest 15 16 REFRACTIVE ERRORS Myopia • Anisometropia: a refractive power difference between the 2 eyes (> 2D) A form of refractive error in which parallel rays of light entering the eye are focused in front of retina • Aniseikonia: a difference of image size between the 2 eyes as perceived by the patient with accommodation being at rest. • Aphakia: (Phakos=lens), aphakia is no lens • Pseudophakia: artificial lens in the eye 17 18 3 9/4/2015 Etiological types Axial(MC)-increased AP length of eyeball Curvatural-increased curvature of cornea, lens or both Index-increased refractive index of lens with nuclear sclerosis Positional-anterior placement of lens Myopia due to excessive accommodation 19 20 Clinical types of myopia Clinical features - Symptoms Congenital Distant blurred vision Simple or developmental Half shutting of eyes Asthenopic symptoms Degenerative or pathological Night blindness Acquired Divergent squint Assignment: write an essay about clinical types of myopia discussing the difference between these types and mechanism of each type 21 22 Signs Optical treatment Prominent eyeballs Concave lenses Large cornea (Minus lens) Anterior chamber is deep Large & sluggishly reacting pupil Fundus examination-changes seen only in pathological myopia Contact lenses 23 24 4 9/4/2015 Optical treatment Adults: Surgical treatment <30years-full correction >30years-less than full correction with which patient is Radial keratotomy comfortable for near vision. Lamellar corneal refractive procedures Laser based procedures HIGH MYOPIA PRK under correction is done to avoid LASIK near vision problem LASEK magnification of images C-LASIK contact lenses are better(to avoid image magnification) E-LASIK 25 26 Hypermetropia Etiological types It is the refractive state of eye where in parallel rays of Axial(m.c)-decreased AP diameter of eyeball light coming from infinity are focused behind the sensitive layer of retina with accommodation being at Curvatural-flattening of cornea, lens or both rest Index –old age, diabetics under treatment Positional-posteriorly placed lens Absence of lens-aphakia 27 28 CLINICAL TYPES TOTAL HYPERMETROPIA It is the total amount of refractive error,estimated after SIMPLE HYPERMETROPIA complete cycloplegia with atropine PATHOLOGICAL FUNCTIONAL HYPEROPIA Divided into latent & manifest Assignment: write an essay about clinical types of hyperopia discussing the difference between these types and mechanism of each type 29 30 5 9/4/2015 MANIFEST HYPERMETROPIA LATENT HYPERMETROPIA Remaining part of total hypermetropia Correct by accommodation and convex lens Consists of facultative & absolute Corrected by inherent tone of ciliary muscle High in children FACULTATIVE HYPERMETROPIA Decreases with age Corrected by patients accommodative effort Revealed after abolishing tone of ciliary muscle with atropine ABSOLUTE HYPERMETROPIA Residual part not corrected by patients accommodative effort 31 32 NORMAL AGE VARIATION At birth +2+3D HM Slightly increase in one year of life, Gradually diminished by the age 5-10 years Facultative hyper metropia In old age after 50 year again tendency to Manifest hyper HM metropia Total hyper Absolute hyper . Tone of ciliary muscle decreases metropia metropia . Accommodative power decreases Latent hyper metropia . Some amount of latent HM become manifest . More amount of facultative HM become absolute 33 34 SYMPTOMS Principal symptom is blurring of vision for TREATMENT close work BASIS FOR TREATMENT Symptoms vary depending upon age of No Treatment patient & degree of refractive error Error is small Asymptomatic Asymptomatic Visual acuity normal Asthenopic symptoms No muscular imbalance Defective vision only (particularly near vision) 35 36 6 9/4/2015 Young children(<6 or 7yrs) Adults Some degree of hypermetropia is physiological so no correction If symptoms of eye-strain are marked, we correct as much of the total hypermetropia as possible, Treatment required if error is high or strabismus is trying as far as we can to relieve the present accommodation working in school small error may require When there is spasm of accommodation we correction correct the whole of the error In children error tends normally to diminish with Some patients with hypermetropia do not initially growth so refraction should be carried out every six tolerate the full correction indicated by manifest month and if necessary the correction should be refraction so we under correct them reduced, ortherwise a lens which is overcorrecting their error may induce an artificial myopia No deduction of tonus allowance in strabismus 37 38 PRESBYOPIA The physiologic loss of MODE OF TREATMENT accommodation in the SPECTACLES eyes in advancing age OPTICAL TREATMENT CONTACT LENS SURGICAL 39 40 Physiologic loss of accommodation in SYMPTOMS advancing age deposit of insoluble proteins in lens in The need to hold reading material at advancing age-->elasticity of lens progressively arm's length. decrease-->decrease accommodation around 40 years of age , accommodation Blurred near vision become less than 4.00 D, causing difficultly with reading fine print , headache , visual Headache fatigue Fatigue Symptoms worse in dim light. 41 42 7 9/4/2015 SPECTACLES Surgery Monovision LASIK Plus lens Monovision & CK (or) IntraCor Convex lens Refractive lens exchange Corneal Inlays & Onlays 43 44 Types ASTIGMATISM Regular astigmatism – change in refractive power is uniform from one meridian to another A defect of an optical system causing light rays With-the-rule astigmatism from a point source to fail Against-the-rule astigmatism to meet in a focal point resulting in a blurred and Oblique astigmatism imperfect image. Bi-oblique astigmatism Irregular astigmatism –Irregular change of refractive power in different meridians 45 46 Types of regular astigmatism Simple astigmatism Simple hyperopic astigmatism Simple myopic astigmatism Compound astigmatism Compound hyperopic astigmatism Compound myopic astigmatism Mixed astigmatism 47 48 8 9/4/2015 • Symptoms : Regular Astigmatism : Blurring of vision Correctable by Spherocylindrical lenses Asthenopic symptoms Etiology : Tilting of head 1. Corneal - abnormalities of curvature [common] Squinting [Half closure of eyelid] 2. Lenticular is rare. It may be: i. Curvatural - abnormalities of curvature of lens as seen in lenticonus. ii. Positional - tilting or oblique placement of lens , subluxation. 3. Retinal - oblique placement of macula [rare] 49 50
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