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Clinical Optics and Dispensing > Week 9: Prescribing for High Ametropia & Magnification Aniseikonia > Flashcards

Week 9: Prescribing for High Ametropia & Magnification Aniseikonia Flashcards

1
Q

Describe ‘lens form’

A

The relationship between the front & back curves

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2
Q

Describe Best form lenses

A

Minimise spherical aberration while still using spherical surfaces

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3
Q

Describe Point focal lenses

A

Correct for oblique astigmatism

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4
Q

Describe Percival lenses

A

Mean oblique power (sagittal & tangential power equal and opposite)

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5
Q

Describe Minimum tangential form

A

Creates minimum tangential error

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6
Q

What is Monochromatic Aberration/Third order/Seidel Aberration?

A

Blur that occurs when a single wavelength of light passes through a lens/lens system and does not come to a point focus

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7
Q

List some assumptions for Monochromatic Aberration

A
  • Spherical lens produces a point image for a point object
  • A line object consists of a series of point objects
  • Position of the image is determined by tracing a pencil of light through the lens
  • Position is determined by the principles of conjugate foci
  • Paraxial rays
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8
Q

What are the types of Seidel Aberrations?

A
  1. Spherical aberration
  2. Coma
  3. Oblique astigmatism
  4. Curvature of field
  5. Distortion
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9
Q

What are some assumptions of Seidel Aberrations?

A
  • All surfaces are spherical
  • Consider marginal rays
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10
Q

Describe Spherical Aberration

A
  • Affects images of objects on and off axis
  • It represents the difference in the radii of the blur circles
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11
Q

Describe Spherical Aberration

A
  • A rotationally symmetric aberration in which the light rays that pass through the paraxial zone of the pupil focus at a different distance than the rays that pass through the marginal pupil
  • Positive when the marginal rays focus ahead of the paraxial rays
  • Negative when the paraxial rays focus ahead of the marginal rays
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12
Q

How do you correct spherical aberration?

A
  • Shape of the lens governs the amount of spherical aberrations
  • Effect of bending a lens on spherical aberration
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13
Q

Describe Coma

A
  • Applies to rays entering the lens at an angle
  • Dependent on lens shape
  • Rays from periphery focus closer to axis & produce a larger blurrier spot than paraxial rays
  • Can be considered as an oblique spherical aberration or off-axis spherical aberration
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14
Q

How do you correct coma?

A
  • For a single lens, coma can be partially corrected by bending the lens
  • Can be corrected by an appropriately placed aperture stop
  • Zero coma for a given object distance
  • Magnitude of coma in a spectacle lens is theoretically large
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15
Q

Describe Field Curvature

A
  • Petzval surface is a surface free of any astigmatism
  • Principally a problem with optical instrumentation (particularly cameras) where image plane is not curved
  • Less of a problem with the eye, because the retina is curved
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16
Q

Describe Distortion

A
  • Image produced is sharply defined
  • Lies in a single plane (e.g. no curvature)
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17
Q

What are the different distortion and describe them briefly

A
  1. Positive Distortion (barrel distortion)
    - Peripheral image point is closer to the centre than ideal image
    - Anterior aperture stop produces minification of marginal rays
  2. Negative Distortion
    - Peripheral image point is further from the centre than ideal
  3. Pincushion distortion
    - Posterior aperture stop produces magnification of marginal rays
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18
Q

How does Spherical Lens Result in Oblique Astigmatism?

A
  • Oblique astigmatism is created by an OFF AXIS pencil of light
  • Unequal refraction at spherical surface causes the pencil of light to become astigmatic & focus as two line images, called tangential & sagittal images, instead of as a single point
  • Two image lines form, perpendicular to each other and separated by the Interval of Sturm
  • Distance between the two line foci in oblique astigmatism is called the astigmatic difference
  • An oblique pencil will exhibit aberration coma, which is minimised by the pupil
  • Oblique astigmatism may be reduced by finding the optimum base curve using Tschernig ellipse graph
19
Q

Describe Oblique Astigmatism

A
  • Off-axis rays that causes radial and tangential lines in the object plane to focus sharply at different distances in the image space
  • Arises from asymmetry in the nature of optical paths followed by rays in the tangential & sagittal planes
20
Q

Describe Best form spherical lenses

A

Eliminates oblique astigmatism

21
Q

Describe aspheric lens

A
  • One which does not have the same radius of curvature throughout the entire surface
22
Q

What is the purpose of aspheric lens?

A

1.Produce a flatter lens (decreasing magnification) & making it more attractive

  1. Produce a thinner, lighter weight lens
  2. To optically correct lens aberrations
23
Q

Describe Aspheric & Prescribing Prism

A
  • Aspheric lens has a spherical centre with an aspheric periphery
  • It is essential that the eye is located at the optical centre of the lens
24
Q

What are the High Plus Lens Designs and describe briefly

A
  1. Regular spherical lenses
    - Will suffer from significant peripheral distortions
    - Can be corrected through asphericity
  2. Lenticulars
    - Lens has a central area with a prescribed power, with a peripheral are of no script
25
Q

What are the following lens designs for extremely high myopia?

A
  1. Minus Lenticular
    - Like plus lenticular, but it has a minus optical, and this time a plus carrier to decrease the edge thickness
  2. Myodisc
    - Identical to the minus lenticular, but the chamfer in the carrier is flat, hence edge thickness will be the same as that of the aperture
26
Q

Describe what is spectacle magnification

A

SM = retinal image size in corrected eye / retinal image in same eye uncorrected

27
Q

What is aniseikonoa?

A
  • Relative difference in the size and/or shape of the images seen by the right & left eyes
28
Q

What are the different types of aniseikonoa and describe briefly

A
  1. Physiologic
    - Occurs naturally
  2. Anomalous Aniseikonia
    - Anatomical: caused by the anatomic structure
    - Optical: caused by the optics of the eye (inherent optical aniseikonia) or optics of the correcting lens (induced aniseikonia)
29
Q

Describe Axial ametropia

A
  • Occurs when the axial length of the eye ball is either too short or too long
  • May use Knapp’s Law to produce a “normal” image size
30
Q

Describe Refractive Ametropia

A
  • Length of the eye ball is normal, but the person is ametropic
  • May use the strategy of contact lenses as the uncorrected image size will be the same size as the image size for a normal emmetrope
  • Introducing spectacle lenses will produce relative magnification differences
31
Q

What is Relative spectacle magnification (RSM)

A

RSM = image size for a corrected ametropia eye / image size for a standard emmetropic eye

32
Q

Describe Knapp’s Law

A

If ametropia is axial, image size is different from normal eye because axial length is different from normal

33
Q

Describe distortion

A
  • Occurs because there is different magnification at different areas of the lens periphery in proportion to the distance of those areas from the lens OC
34
Q

Describe Anomalous Distortions symmetrical

A
  • One eye sees an image symmetrically larger than the other eye
  • Meridional aniseikonia has a meridional size difference in a meridian of one eye compared to the other meridian
35
Q

Describe Meridional Distortions

A
  • If due to a cylindrical lens or meridional magnifying lens, then the image is elongated in the meridian of power
36
Q

What are the breakdown of binocular fusion?

A
  • Diplopia
  • Suppression
37
Q

What are the distortions of stereoscopic space and describe briefly

A
  • Geometric size effect: slanting of objects when horizontal magnification is produced in one eye
  • Induced size effect: opposed slanting induced when vertical magnification is produced in one eye
  • Declination: forward/backwards slant induced when cylinders are induced at oblique axes
38
Q

Who experiences aniseikonia?

A
  • Anisometropes: prevalence of anisometropia > 1D in adults is 5 – 10%
  • Patients who have had cataract surgery: 40% of all pseudophaes had aniseikonia related symptoms
39
Q

How to correct anniseikonia and briefly describe

A
  1. Contact lenses
    - To achieve normal retinal image sizes, this is the best option for both axial & refractive anisometropes
  2. Spectacle modification
    - To equalise retinal image sizes, it is best to use spectacle modification for both axial & refractive anisometropes
    - By modifying; base curves, lens thickness, vertex distances
40
Q

What are the four approaches for correcting anniseikonia

A
  1. First Pass Method
  2. Directionally Correct Method
  3. Estimating Percent Magnification
  4. Measure Percent Magnification
41
Q

Describe First Pass Method briefly

A
  • Short vertex distance
  • Small eye size
  • Aspheric lens design
  • High index lens material
42
Q

How do you Directionally Correct Magnification Changes?

A
  1. Both lenses plus (Anisohyperopia)
    - Frame with minimum vertex distance
    - Small Eye Size
    - Higher Plus Lens
    - Lower plus lens
  2. Both lenses minus (Anisomyopia)
    - Frame with minimum vertex distance
    - Small Eye Size
    - Higher Minus Lens
    - Minus lens
43
Q

What is the Estimating Percent Magnification?

A

1.5% per dioptre of anisometropia for Refractive Anisometropia

44
Q

How do you measure Percent Magnification?

A

Measure using a ‘space eikonometer’ or Aniseikonia inspector

Clinical Optics and Dispensing (10 decks)

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