Chapter 5 Astigmatic Image Formation

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Chapter 5 Astigmatic Image Formation
Page 5.31
2
Clinical Classification of Regular Astigmatism
Fig. 5.24 Page 5.45
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Example 5.11 Correcting Astigmatism
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Example 5.11
Fig 5.24 Page 5.45
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Fig 5.24 Page 5.45
Example 5.11 Correcting Astigmatism
?5
?3
180? meridian A    Femm  ?  Fe    60  ?  63
    ?3 D myopia (refractive).
90? meridian A    Femm  ?  Fe    60  ?  65 
   ?5 D myopia (refractive).
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Fig 5.24 Page 5.45
Example 5.11
Add ?3 DS in front of eye. What does the patient
then see on the VA chart?
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Fig 5.24 Page 5.45
Example 5.11
Add ?5 DS in front of eye. What does the patient
then see on the VA chart?
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One of the two appearances below is the valid
starting point for an all focal-line based
astigmatic subjective refraction. Which one?
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Clinical Aspects of Astigmatism
Page 5.47

• In real-world vision, looking at complex objects,
  the uncorrected astigmat will not always be
  focusing focal lines or COLCs on the retina
• All other parts of the interval of Sturm are
  elliptical in cross-section
• ? A feature of uncorrected astigmatic vision is
  elliptical elongation or distortion (dont
  confuse with the aberration, distortion) of
  images
• This effect increases with magnitude of
  (uncorrected) astigmatism and with pupil diameter

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Accommodation in the Uncorrected Astigmat
Page 5.47

• To obtain optimum uncorrected vision, astigmats
  will (subconsciously) try to place the COLC on
  the retina
• In orientation-dominant objects, the astigmats
  visual system may try to place one or other focal
  line on the retina especially in city
  environments, where much of the landscape is
  horizontals and verticals
• How do our five clinical types of astigmat fare
  when uncorrected?

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Page 5.47
Take the eyes that we used to define the five
clinical types of astigmatism
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Fig 5.24 Page 5.45

• The uncorrected CHA can accommodate to bring any
  part of the Interval of Sturm to the retina, IF
• Amplitude of Accommodation is sufficient
• The young CHA should ? have reasonable vision
  when uncorrected
• The downside is fatigue, due to constant
  refocusing to obtain optimum clarity

Accommodation
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Page 5.47
Take the eyes that we used to define the five
clinical types of astigmatism
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Fig 5.24 Page 5.45

• The uncorrected SHA already has one focal line on
  the retina without accommodation
• Accommodation can bring any other part of the
  Interval of Sturm to the retina
• The demand on Amp Accom is less than for a CHA
  with the same magnitude of astigmatism
• Fatigue is still an issue because any part of the
  IOS can be moved to the retina

Accommodation
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Page 5.47
Take the eyes that we used to define the five
clinical types of astigmatism
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Fig 5.24 Page 5.45

• For the uncorrected MxA vision depends in part
  where the COLC is located (relative to the
  retina) in distance vision
• If it is in front, vision will be worse because
  only the posterior FL can be moved to the retina
• If it is on or behind the retina, the patient has
  a choice of COLC or posterior FL

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Page 5.47
Take the eyes that we used to define the five
clinical types of astigmatism
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Fig 5.24 Page 5.45

• With the posterior FL on the retina, the SMA
  obtains some clear vision at distance
• Accommodation is no help because it moves the
  entire IOS in front of the retina
• Poor uncorrected vision rather than accommodative
  fatigue is the main symptom for the uncorrected
  myopic astigmat

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Page 5.47
Take the eyes that we used to define the five
clinical types of astigmatism
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Fig 5.24 Page 5.45

• Of all uncorrected astigmats, the CMA will have
  the worst distance vision, but no problem with
  accommodative fatigue

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Vision in Astigmatism (w BVS) vs. Spherical
Ametropia
Fig. 5.25 Page 5.48
Uncorrected spherical myope
Vision in the uncorrected 2D myope is identical
to that of the 4 D astigmat with COLC on the
retina COLC size is the basis for predicting
magnitude of astigmatism Move the COLC to the
retina with sphere. Worse vision correlates with
higher astigmatism
Uncorrected astigmat with COLC on retina
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Clinical Aspects of Astigmatism
Page 5.49
Importance of Axis Direction

• For general distance vision in city/indoor
  environments the wtr and atr astigmat should be
  better off than the oblique astigmat because much
  of the environment is made up of horizontals and
  verticals
• Reading performance in the uncorrected astigmat
  will be better if the patient can move vertical
  focal lines to the retina

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Fig. 5.26, page 5.50
(a) Emmetropia
(b) Uncorrected astigmatism horizontal FLs on
retina
(b) Uncorrected astigmatism vertical FLs on
retina
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Who will have an easier time reading (near
vision) a 2 D wtr CMA or 2 D atr CHA, if for
distance vision both have the COLC 3 D from the
retina?
2 D wtr CMA
2 D atr CHA
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Who will have an easier time reading distant
street signs a young 2 D wtr CHA or 2 D atr CHA,
if for distance vision both have the COLC 3 D
behind the retina?
2 D wtr CHA
2 D atr CHA