Retinal Image Height, Anisometropia and Aniseikonia

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Retinal Image Height, Anisometropia and
Aniseikonia
Page 11.1
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Spectacle Magnification

• Spectacle magnification produces a new incident
  chief ray angle at the eye (for a given object)

OBJ

• This change in incident CR angle results in a
  proportional change in refracted CR angle.
• A new refracted CR angle will change RI height

OBJ
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Axial Hyperopia
Corrected with a Spectacle Lens
Hyperopia ?S gt ? ? ??S gt ??
MR
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Why is the new incident CR angle (?S) steeper?
Fig 11.3Page 11.6
Plus lens converges parallel rays toward a focus
at the far point
Q
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Why is the new incident CR angle (?S) steeper?
Plus lens like prisms with bases on axis
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Why is the new incident CR angle (?S) steeper?
Prisms deviate light toward their base
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Why is the new incident CR angle (?S) steeper?
Hyperopia ?S gt ? ? ??S gt ??
Q
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Spectacle Magnification - Hyperope
?S gt ? ? ??S gt ??

• Spectacle magnification in hyperopia increases
  the incident chief ray angle from ? to ?S
• MCR remains unchanged (0.75 for all reduced eyes)
• Greater incident chief ray angle ? proportionally
  greater refracted chief ray angle
• Greater refracted chief ray angle means increased
  retinal image height ? RI height increases with
  spectacle correction of hyperopia

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Spectacle Correction at Knapps Plane (Fe)
Hyperope

• A positive spectacle lens placed at the first
  principal focus (Knapps Plane) of the axially
  hyperopic eye increases retinal image height to
  become the same as the (standard) emmetropes RI
  height
• This occurs for all magnitudes of axial hyperopia

OBJ
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Axial Myopia
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Axial Myopia Corrected with a Spectacle Lens
Page 11.7
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Axial Myopia Corrected with a Spectacle Lens
Myopia ?S lt ? ? ??S lt ??
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Spectacle Magnification - Myope
?S lt ? ? ??S lt ??

• Spectacle magnification in myopia decreases the
  incident chief ray angle from ? to ?S
• MCR remains 0.75
• Smaller incident chief ray angle ? proportionally
  smaller refracted chief ray angle
• Smaller refracted chief ray angle means decreased
  retinal image height ? RI height decreases with
  spectacle correction of myopia

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Spectacle Correction at Knapps Plane (Fe) Myope

• A negative spectacle lens placed at the first
  principal focus (Knapps Plane) of the axially
  myopic eye decreases retinal image height to
  become the same as the (standard) emmetropes RI
  height
• This occurs for all magnitudes of axial myopia

OBJ
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RI Height - Axial Ametropia corrected at Fe
Page 11.8
CHIEF RAY (u)
CHIEF RAY (u)
All corrected RI heights equal
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Spectacle Magnification
Page 11.9

• Expressed three different ways (use all three)
• Origin change in incident chief ray path between
  the uncorrected and corrected eye

1. Application change in retinal image height
   between the uncorrected and corrected eye

1. Calculation an equation to determine the value
   for SM in any particular case

?
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Spectacle Magnification - Hyperope
?S
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Spectacle Magnification - Hyperope
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Spectacle Magnification - Hyperope
OBJ
Page 11.10
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Spectacle Magnification - Myope
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Spectacle Magnification - Myope
OBJ
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Axial Ametropia Corrected with a Contact Lens
Page 11.11
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Axial Ametropia Corrected with a Contact Lens

• Ocular Correction (at the reduced surface) ? no
  effect on incident chief ray path. ? SM 1.0
• Contact lens sits 1.67 mm in front of the reduced
  surface ? f ?CL only differs from ?MR by 1.67 mm.
• ? FCL ? FO

LMR ? FCL
? SM ? 1.0
f?CL ? f?O
? ?CL ? ?
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Axial Ametropia Corrected with a Contact Lens

• SM for a positive contact lens, very slightly gt
  1.0SM for a negative contact lens, very slightly
  lt 1.0
• This means that corrected retinal image height
  will be virtually the same as uncorrected retinal
  image height
• ? correcting an axial anisometrope with contact
  lenses will induce aniseikonia (uncorrected RI
  heights differ negligible change with contacts ?
  difference remains)

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Axial Ametropia Corrected with a Contact Lens
h?CL ? h?O ? h?U
Page 11.11
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Variation on Example 11.1 (p. 11.12)

• Axial anisometrope FO 4 D OD 9 D OS
• For a distant object subtending a 3O visual
  angle
• find uncorrected RI heights for each eye and
  compare with the standard emmetropic reduced eye
• calculate corrected retinal image heights for a
  spectacle correction at Knapps Plane
• Calculate corrected retinal image heights for a
  contact lens correction

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Test questions will NOT be multi-part e.g. From
old Practice Test
This is a variant on part (b) of the current
example
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Variation on Example 11.1

• Axial anisometrope FO 4 D OD 9 D OS
• For a distant object subtending a 3O visual
  angle
• find uncorrected RI heights for each eye and
  compare with the standard emmetropic reduced eye

O.D. 4 D axial hyperopia ? Fe 60 D A
Femm ? Fe ? Femm A Fe 4 60
64 D
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Variation on Example 11.1

• Axial anisometrope FO 4 D OD 9 D OS
• For a distant object subtending a 3O visual
  angle
• find uncorrected RI heights for each eye and
  compare with the standard emmetropic reduced eye

O.S. 9 D axial hyperopia ? Fe 60 D Femm
69 D
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Variation on Example 11.1

• Axial anisometrope FO 4 D OD 9 D OS
• For a distant object subtending a 3O visual
  angle
• find uncorrected RI heights for each eye and
  compare with the standard emmetropic reduced eye

h?U (OD) ?0.819 mm h?U (OS) ?0.759
mm h? (SERE) ?0.873 mm
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Variation on Example 11.1

• Axial anisometrope FO 4 D OD 9 D OS
• For a distant object subtending a 3O visual
  angle
• calculate corrected retinal image heights for a
  spectacle correction at Knapps Plane

O.D. spectacle correction at Knapps Plane, d
16.67 mm
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Spectacle Magnification (O.D)
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O.D.
CHIEF RAY (s)
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Variation on Example 11.1

• Axial anisometrope FO 4 D OD 9 D OS
• For a distant object subtending a 3O visual
  angle
• calculate corrected retinal image heights for a
  spectacle correction at Knapps Plane

O.S. spectacle correction at Knapps Plane, d
16.67 mm
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Spectacle Magnification (O.S)
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O.S.
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Variation on Example 11.1

• Axial anisometrope FO 4 D OD 9 D OS
• For a distant object subtending a 3O visual
  angle
• calculate corrected retinal image heights for a
  spectacle correction at Knapps Plane

h?U (OD) ?0.819 mm h?U (OS) ?0.759
mm h? (SERE) ?0.873 mm
h?S (OD) ?0.873 mm h?S (OS) ?0.873
mm h? (SERE) ?0.873 mm ANK 1.0 (no
difference OD vs. OS)