Subjective Refraction: Optics PDF

OTMS2613 N RAWAT SUBJECTIVE REFRACTION: OPTICS  OUTCOMES FOR THE UNIT Define ametropia and emmetropia Explain the classification of refractive error in Optometry Describe the classification of ametropia...

OTMS2613 N RAWAT SUBJECTIVE REFRACTION: OPTICS  OUTCOMES FOR THE UNIT Define ametropia and emmetropia Explain the classification of refractive error in Optometry Describe the classification of ametropia Describe the difference between power and axis and the effect on the line focus Explain the symbols used to describe the Standard Reduced Emmetropic Eye Describe how astigmatism can be measured State the values of the Standard Reduced Emmetropic Eye Explain far point distance for myopic and hyperopic patients Describe the Non-Standard Reduced eye Describe Spectacle refraction and how it can be calculated Define Spherical ametropia and its components Compare Spectacle and Ocular refraction Describe Refractive ametropia and how it can occur Describe the classification of refractive error as low, medium and high Describe axial ametropia and how it can occur Explain the impact of pupil size on visual acuity and refractive error Define astigmatic ametropia Describe the distortions that can be experienced based on pupil size Describe myopia and its classification into axial and refractive myopia Define depth of field and depth of focus Describe hyperopia and its classification into axial and refractive hyperopia Explain how other factors influence depth of focus Describe the course of myopia and hyperopia Degradation of vision caused by sphere vs. Cylinder Explain the formula for thin lens system and its application Explain the circle of least confusion and how it relates to subjective refraction 5/4/2023 2  Standard Reduced Emmetropic Eye SREE Standard = Ideal (Emmetropia) Single refracting surface One refractive index used When referring to SREE, values have the suffix ‘o’ 5/4/2023 3  Standard Reduced Emmetropic Eye SREE CONSTANT SYMBOL VALUE Fo Refractive power +60,00D External refractive n 1 index Internal refractive n' 1,33 index Derived constants: Radius of ro +5,55mm curvature Axial length k'o +22,22mm 5/4/2023 4  Non-Standard Reduced Eye NSRE n=1 ne = 1,33 The non-standard eye does not have to be EMMETROPIC When referring to NSRE, the suffix used is ‘e’ k'e Fe re 5/4/2023 5  Non-Standard Reduced Eye ORIGINAL POWER FORUMULA FOR THIN LENS SYSTEM n=1 ne = 1,33 𝑛𝑒 − 1 𝐹𝑒 = 𝑟𝑒 If the refractive index (n) of the reduced eye is kept constant, then the total refractive power of the eye is changed by a change in ‘r’. So, if the radius of curvature is decreased, the power of the eye will increase. k'e (P.S. does a decrease in radius of curvature mean Fe that the eye became more steep or more flat?) Relate this to what you understand about re refractive myopes, for example. 5/4/2023 6  Emmetropia Ametropia 5/4/2023 7  Ametropia Spherical ametropia Astigmatic ametropia 5/4/2023 8  Ametropia Spherical ametropia Astigmatic ametropia 5/4/2023 Point image of a point object is formed BUT image is not formed on the retina Point image of a point object is NOT formed INSTEAD two line foci are formed 9  Ametropia Spherical ametropia Astigmatic ametropia Point image of a point object is formed BUT Point image of a point object is NOT formed image is not formed on the retina INSTEAD two line foci are formed Which of these causes poorer vision? 5/4/2023 10  Ametropia Spherical ametropia Astigmatic ametropia Which of these causes poorer vision? Vision Equivalent sphere (myopia or manifest hyperopia) VA After Best Sphere Possible Cyl Rx 6/5 Plano 6/6 0.25-0.75 6/6 0.25-0.50DS 6/9 0.75-1.25 6/9 0.50-0.75DS 6/12 0.75-1.00DS 6/12 1.25-1.75 6/18 1.00-1.25DS 6/18 1.75-2.25 6/24 1.25-1.75DS 5/4/2023 6/36 1.75-2.25DS 6/24 2.25-3.00 11  Ametropia Spherical ametropia Astigmatic ametropia Which of these causes poorer vision? Vision Equivalent sphere (myopia or manifest hyperopia) VA After Best Sphere Possible Cyl Rx 6/5 Plano 6/6 0.25-0.75 6/6 0.25-0.50DS 6/9 0.75-1.25 6/9 0.50-0.75DS 6/12 0.75-1.00DS 6/12 1.25-1.75 6/18 1.00-1.25DS 6/18 1.75-2.25 6/24 1.25-1.75DS 5/4/2023 6/36 1.75-2.25DS 6/24 2.25-3.00 12 You could have an 0,75DC Rx but still have 6/6 vision. But an 0,75DS Rx would reduce vision to 6/9 or even 6/12  Ametropia Spherical ametropia Astigmatic ametropia Which of these causes poorer vision? Vision Equivalent sphere (myopia or manifest hyperopia) VA After Best Sphere Possible Cyl Rx 6/5 Plano 6/6 0.25-0.75 6/6 0.25-0.50DS 6/9 0.75-1.25 6/9 0.50-0.75DS 6/12 0.75-1.00DS 6/12 1.25-1.75 6/18 1.00-1.25DS 6/18 1.75-2.25 6/24 1.25-1.75DS 5/4/2023 6/36 1.75-2.25DS 6/24 2.25-3.00 13 Conclusion: An astigmatic error does not degrade an image as much as a spherical error would  Ametropia Spherical ametropia Astigmatic ametropia Myopia Hyperopia SMA or CMA Axial Axial SHA or CHA 5/4/2023 Refractive Refractive Mixed Astigmatism 14  Spherical ametropia Myopia Hyperopia 5/4/2023 15  Spherical ametropia Myopia Hyperopia Nearsighted Farsighted Eye is too ________ Eye is too ________ or or eye is too ________ eye is too ________ 5/4/2023 16  Spherical ametropia Nearsighted Farsighted Eye is too ________ Eye is too ________ or or eye is too ________ eye is too ________ Myopia Hyperopia Axial myope Axial hyperope Refractive myope Refractive hyperope 5/4/2023 17  Spherical ametropia Nearsighted Farsighted Eye is too ________ Eye is too ________ or or eye is too ________ eye is too ________ Myopia Hyperopia Axial myope Axial hyperope Reason for ametropia is the axial length Refractive myope Reason for the ametropia is the refractive surface Refractive hyperope 5/4/2023 18  Spherical ametropia Myopia Hyperopia Axial hyperope: Axial myope: Fe =60,00D Fe =60,00D k’ < 22,22mm k’ > 22,22mm Reason for ametropia is the axial length Reason for the ametropia is the refractive surface Refractive hyperope: Refractive myope: Fe > 60,00D Fe < 60,00D k’ = 22,22mm k’ = 22,22mm 5/4/2023 19  Spherical ametropia Myopia Hyperopia Whether it’s axial or refractive, how do we manage these refractive errors? 5/4/2023 20  Spherical ametropia Myopia Hyperopia Whether it’s axial or refractive, how do we manage these refractive errors? 5/4/2023 21  Causes of Refractive Refractive and Axial Index ametropia ametropia Curvature ametropia Position of element Absence of lens Axial Change in axial length 5/4/2023 22  CASE 1 Consider the following case… After refracting a patient, you find the following Rx: OD -1,75DS (6/6) OS -2,25DS (6/6) 1. How could you determine if the patient is a refractive or axial myope? 2. If the patient is an axial myope, will the k’ be less than or greater than 22,22mm? 3. If the patient is a refractive myope, what is the power of the patient’s OD and OS? 5/4/2023 23  Ametropia Astigmatic Spherical ametropia ametropia Myopia Hyperopia SMA or CMA Axial Axial SHA or CHA 5/4/2023 Refractive Refractive Mixed Astigmatism 24  Ametropia When it comes to the optics of a myopic/hyperopic eye, we also need to talk about the far point. What is the far point? Why is it significant? Spherical ametropia For a myopic eye, where is the patient’s far point located? Myopia For a hyperopic eye, where is the patient’s far point located? Hyperopia 5/4/2023 25  CASE 2 Consider the following case… A patient with an Rx of -10,00DS holds his book very close to his face when he is not wearing his spectacles. When he wears his spectacles, he holds the book at a normal reading distance. 1. Is a -10,00DS classified as low, medium or high myopia? 2. Why would the patient need to hold his book very close to his face when reading without correction? 3. Can we estimate the value of the distance he holds the book without correction? 5/4/2023 26  Course of MYOPIA If present at birth, normally disappears by one year A small % of children may exhibit before entering school In adolescence phase, myopia increases In adulthood, the trend towards hyperopia exceeds that of myopia until advanced age is reached when the onset of cataracts can cause the myopia to increase again 5/4/2023 27  Course of HYPEROPIA Almost all eyes are hyperopic at birth and during preschool years After school begins, myopia increases During the growth years, the hyperopia changes little, increase/decrease In adults, most hyperopia remains constant until presbyopia and then may increase slightly In advanced age, cataracts develop and decrease hyperopia 5/4/2023 28  Change in refractive error over time 5/4/2023 29  DEGREE OF MYOPIA PRESCRIPTION DEGREE OF HYPEROPIA PRESCRIPTION LOW -0.25 to -3.00D LOW +0.25 to +3.00D MEDIUM -3.25 to -6.00D MEDIUM +3.25 to +5.00D HIGH -6.25 to -10.00D HIGH Greater than +5.00D VERY HIGH Greater than -10.00D DEGREE OF ASTIGMATISM PRESCRIPTION LOW -0.25 to -1.00DC MEDIUM -1.25 to -3.00DC HIGH Greater than -3.00DC 5/4/2023 30  Small pupils: VA limited by Diffraction Large pupils: VA limited by Optical aberrations A mid-size pupil is the ideal as a compromise between optical aberrations and diffraction Effect of pupil size on Refraction 5/4/2023 31  Pupil size also affects a patient’s depth of focus Pupil size is INVERSELY PROPORTIONAL to the patient’s depth of focus The smaller the pupil size, the larger the depth of focus Effect of pupil size Revise: Definition of Depth of Focus on Refraction vs. Depth of Field 5/4/2023 32  Revise: Definition of Depth of Focus vs. Depth of Field Depth of field: distance over which an object can be moved (front or back) while still being perceived as clear Depth of focus: distance over which an image can be moved while still Effect of pupil size being perceived as clear on Refraction 5/4/2023 33  Revise: Definition of Depth of Focus vs. Depth of Field What would happen if we didn’t have a Depth of field/focus? An object not placed perfectly would appear out of focus Additional reading: What factors, Effect of pupil size other than pupil size, can affect one’s on Refraction Depth of focus? 5/4/2023 34  Ametropia Astigmatic Spherical ametropia ametropia Myopia Hyperopia SMA or CMA Axial Axial SHA or CHA 5/4/2023 Refractive Refractive Mixed Astigmatism 35  Astigmatism: power of the refractive surfaces is not the same along all meridians There will be one meridian where the power is greatest and another where it is the least Astigmatic (one meridian steeper than the other). ametropia Typically 90 degrees from each other SMA or CMA SHA or CHA e Mixed Astigmatism 5/4/2023 36  Astigmatism: power of the refractive surfaces is not the same along all meridians There will be one meridian where the power is greatest and another where it is the least Astigmatic (one meridian steeper than the other). ametropia Typically 90 degrees from each other This results in the formation of two line foci (instead of a single point focus) SMA or CMA Would the steeper or flatter meridian create a line focus closer to the SHA or CHA lens? e Mixed Astigmatism 5/4/2023 37  Astigmatism: power of the refractive surfaces is not the same along all meridians There will be one meridian where the power is greatest and another where it is the least Astigmatic (one meridian steeper than the other). ametropia Typically 90 degrees from each other This results in the formation of two line foci (instead of a single point focus) SMA or CMA SHA or CHA Separation between two line foci → Interval of Sturm. COLC dioptric midpoint between two line foci e Mixed Astigmatism 5/4/2023 38 Astigmatic ametropia How do we correct astigmatism? Spectacles: spherocylindrical lenses (Write out an e.g. of a spherocylindrical Rx) Contact lenses: Toric lenses 5/4/2023 39  Astigmatism: power of the refractive surfaces is not the same along all meridians There will be one meridian where the power is greatest and another where it is the least Astigmatic (one meridian steeper than the other). ametropia Typically 90 degrees from each other SMA or CMA Simple Myopic Astigmatism vs. Compound Myopic Astigmatism SHA or CHA Simple Hyperopic Astigmatism vs. Compound Myopic Astigmatism e Mixed Astigmatism 5/4/2023 Mixed Astigmatism 40 Draw a diagram showing where line foci are formed in the above instances  Recap of OTMS1512… Classification Location of line foci Power cross Prescription E.g. of Rx 1 on the retina/ 1 in front of the 1 plano / Simple myopic astigmatism Plano/(-) cylinder Plano/-1,00x90 retina 1 minus Compound myopic -2,00/-0,50x90 Both in front of the retina Both minus (-)sphere/(-)cylinder astigmatism -3,00/-4,00x180 1 on the retina/ 1 behind the 1 plano / (+)sphere/(-)cylinder Simple hyperopic astigmatism +2,00/-2,00x45 retina 1 plus (Sphere = Cylinder) Compound hyperopic (+)sphere/(-)cylinder Both behind the retina Both plus +1,00/-0,25x135 astigmatism (Cylinder < Sphere) 1 minus / (+)sphere/(-)cylinder Mixed astigmatism 1 in front and 1 behind the retina +1,00/-1,50x60 1 plus (Cylinder > Sphere) 5/4/2023 41  QUESTIONS: CLASSIFICATION OF PRESCRIPTION QUESTIONS Provide a diagnosis based on the following prescriptions: 1. -3,50/-1,00x90 2. +2,00/-0,75x135 3. +1,00/-2,00x65 4. ∞/-1,50x115 5. +0,75/-0,75x180 5/4/2023 42  Astigmatism Astigmatic Can also be classified according to etiology/cause ametropia Corneal Lenticular Total Astigmatism Astigmatism Astigmatism 5/4/2023 43  Astigmatism Astigmatic Can also be classified according to etiology/cause ametropia Corneal Lenticular Total Astigmatism Astigmatism Astigmatism Instruments to detect corneal Instruments to detect total astigmatism: astigmatism: ✓ Keratometer ✓ Retinoscope ✓ Corneal topographer 5/4/2023 44  Astigmatism Astigmatic Can also be classified according to Orientation ametropia 90 ATR 120 60 OBLIQUE OBLIQUE 30 150 WTR WTR 180 0 5/4/2023 45 WTR astigmatism is more common in young patients with an axis of 180 ± 20 degrees  Astigmatism Astigmatic Can also be classified according to Orientation ametropia 90 ATR 120 60 OBLIQUE OBLIQUE 30 150 WTR WTR 180 0 5/4/2023 46 Write an example of a prescription that will be classified as (a) WTR (b) ATR (c) Oblique  QUESTIONS: CLASSIFICATION OF PRESCRIPTION QUESTIONS Provide a diagnosis based on the following prescriptions: 1. -3,50/-1,00x90 2. +2,00/-0,75x135 3. +1,00/-2,00x65 Now, comment on whether the prescription is WTR, ATR or 4. ∞/-1,50x115 oblique 5. +0,75/-0,75x180 5/4/2023 47  Astigmatism Astigmatic Can also be classified according to Orientation ametropia WTR astigmatism 5/4/2023 48  WTR astigmatism Astigmatic ametropia WTR astigmatism Horizontal meridian is flatter Produces vertical line focus further away from the lens Vertical meridian is steeper Produces a horizontal line focus closer to the lens Minus cylinder axis 180, power Needed to neutralize the steeper meridian 5/4/2023 along the 90 (V) 49  Astigmatism Astigmatic Can also be classified according to Orientation ametropia ATR astigmatism 5/4/2023 50  ATR astigmatism Astigmatic ametropia ATR astigmatism Horizontal meridian is steeper Produces vertical line focus closer to the lens Vertical meridian is flatter Produces a horizontal line focus further away from the lens Minus cylinder axis 90, power along Needed to neutralize the steeper meridian 5/4/2023 the 180 (H) 51 Astigmatic ametropia REMEMBER: POWER ALONG 90 (AXIS IS 180) → PRODUCES A HORIZONTAL LINE FOCUS POWER ALONG 180 (AXIS IS 90)→ PRODUCES A VERTICAL LINE FOCUS 5/4/2023 52  CASE 3 Consider the following case… The following is found during Subjective refraction: OD +1,00/-0,50x180 OS -0,50/-0,50x180 1. Describe the location of the horizontal and vertical line foci for the right eye when uncorrected 2. Describe the location of the horizontal and vertical line foci for the left eye when uncorrected 3. Draw a diagram showing the location of the line foci when the right eye is fully corrected 5/4/2023 53  +1,00/-0,50x180 First step, POWER CROSS 5/4/2023 54  +1,00/-0,50x180 First step, POWER CROSS +1,00 +0,50 -0,50 +1,00 5/4/2023 55  +1,00/-0,50x180 First step, POWER CROSS +1,00 +0,50 -0,50 +1,00 Now, consider where will these images be formed? In front of the retina or behind the retina? 5/4/2023 56  +1,00/-0,50x180 First step, POWER CROSS Now, consider where will these images be formed? +1,00 +0,50 -0,50 In front of the retina or behind the retina? +1,00 Both will form images behind the retina? But which will be closer to the retina? 5/4/2023 57  +1,00/-0,50x180 First step, POWER CROSS Now, consider where will these images be formed? +1,00 +0,50 -0,50 In front of the retina or behind the retina? +1,00 Both will form images behind the retina? But which will be closer to the retina? The smaller Rx will be formed closer to the retina. 5/4/2023 58  +1,00/-0,50x180 First step, POWER CROSS Now, consider where will these images be formed? +1,00 +0,50 -0,50 In front of the retina or behind the retina? Both will form images behind the retina? But which will +1,00 be closer to the retina? The smaller Rx will be formed closer to the retina. But, will it be horizontal or vertical line focus? 5/4/2023 59  REMEMBER: POWER ALONG 90 (AXIS IS 180) → PRODUCES A HORIZONTAL LINE FOCUS POWER ALONG 180 (AXIS IS 90)→ PRODUCES A VERTICAL LINE FOCUS 5/4/2023 60  +1,00 Will be behind the retina AND will produce a +0,50 HORIZONTAL line focus (closer to the retina) -0,50 Will be behind the retina AND will produce a VERTICAL line focus (further from the retina) +1,00 REMEMBER: POWER ALONG 90 (AXIS IS 180) → PRODUCES A HORIZONTAL LINE FOCUS POWER ALONG 180 (AXIS IS 90)→ PRODUCES A VERTICAL LINE FOCUS 5/4/2023 61  +1,00 Will be behind the retina AND will produce a +0,50 HORIZONTAL line focus (closer to the retina) -0,50 Will be behind the retina AND will produce a VERTICAL line focus (further from the retina) +1,00 FINAL STEP most times will be to DRAW the line foci. You should label the diagram using +0,50 the powers of the meridian. +1,00 5/4/2023 62  What if the patient was wearing correction? Both line foci lie on the retina 5/4/2023 63  Ocular vs. Spectacle refraction Spectacle refraction: power of Ocular refraction: power of correcting lens needed at the correcting lens needed at the surface spectacle plane to refract light to be of the eye to refract light to be focused clearly on the retina focused clearly on the retina 5/4/2023 64 Spectacle refraction: power of correcting lens needed at the spectacle plane to refract light to be focused clearly on the retina Ocular refraction: power of correcting lens needed at the surface of the eye to refract light to be focused clearly on the retina 5/4/2023 65 Spectacle refraction: power of correcting lens needed at the spectacle plane to refract light to be focused clearly on the retina Ocular refraction: power of correcting lens needed at the surface of the eye to refract light to be focused clearly on the retina Spectacle refraction (Fsp) Ocular refraction (K) 5/4/2023 66  Spectacle refraction: power of correcting lens needed at the spectacle plane to refract light to be focused clearly on the retina Ocular refraction: power of correcting lens needed at the surface of the eye to refract light to be focused clearly on the retina Spectacle refraction (Fsp) Vertex distance (d) Ocular refraction (K) Vertex distance is taken as the distance in which you performed refraction 5/4/2023 67 Spectacle refraction: power of correcting lens needed at the spectacle plane to refract light to be focused clearly on the retina Ocular refraction: power of correcting lens needed at the surface of the eye to refract light to be focused clearly on the retina Spectacle refraction (Fsp) Vertex distance (d) Ocular refraction (K) 5/4/2023 68 Spectacle refraction: power of correcting lens needed at the spectacle plane to refract light to be focused clearly on the retina Ocular refraction: power of correcting lens needed at the surface of the eye to refract light to be focused clearly on the retina Spectacle refraction (Fsp) Patient who uses spectacles, requests contact lenses OR Patient who knows contact lens Rx, requests spectacles Therefore, we need to know How to calculate Fsp if we know K AND How to calculate K if we know Fsp Vertex distance (d) Ocular refraction (K) 5/4/2023 69 Spectacle refraction: power of correcting lens needed at the spectacle plane to refract light to be focused clearly on the retina Ocular refraction: power of correcting lens needed at the surface of the eye to refract light to be focused clearly on the retina Spectacle refraction (Fsp) How to calculate Fsp if we know K AND How to calculate K if we know Fsp 𝐾 𝐹𝑠𝑝 = 1 + ⅆ𝐾 𝐹𝑠𝑝 𝐾= 1 − ⅆ𝐹𝑠𝑝 Vertex distance (d) Ocular refraction (K) 5/4/2023 70  How to calculate Fsp if we know K AND How to calculate K if we know Fsp 𝐾 𝐹𝑠𝑝 𝐹𝑠𝑝 = 𝐾= 1 + ⅆ𝐾 1 − ⅆ𝐹𝑠𝑝 Important: ‘d’ must be entered in meters ‘d’ is assumed to be 12mm, unless otherwise stated Difference between Fsp and K is significant for prescriptions greater than 5,00D This can be easily checked by the following e.g. Fsp=-2,00D, K=-1,95D The difference is thus negligible 5/4/2023 71  𝐾 𝐹𝑠𝑝 𝐹𝑠𝑝 = 𝐾= QUESTIONS: SPECTACLE VS. OCULAR REFRACTION 1 + ⅆ𝐾 1 − ⅆ𝐹𝑠𝑝 1. Calculate Fsp if K=+7,50D 2. Calculate K if Fsp = -6,50D 3. Comment on whether a myopic prescription increases or decreases when moving from spectacles to contact lenses 5/4/2023 72  𝐾 𝐹𝑠𝑝 𝐹𝑠𝑝 = 𝐾= QUESTIONS: SPECTACLE VS. OCULAR REFRACTION 1 + ⅆ𝐾 1 − ⅆ𝐹𝑠𝑝 4. Calculate Fsp if K=-10,00D and d=14mm 5. Calculate K if Fsp = +9,75D and d=13,5mm 6. Calculate Fsp if K= +6,50/-2,00x90 5/4/2023 73