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Visual Optics 2006/2007

Visual Optics 2006/2007. Chapter 6 Astigmatism & Subjective Refraction. Dioptric separation of FLs = 3 D. Image produced by +/+ Spherocylinder. Page 6.1. Astigmatism = 3 D.

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Visual Optics 2006/2007

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  1. Visual Optics 2006/2007 Chapter 6 Astigmatism & Subjective Refraction

  2. Dioptric separation of FLs = 3 D Image produced by +/+ Spherocylinder Page 6.1 Astigmatism = 3 D Fig. 6.1 - Formation of focal lines by a sphero-cylindrical lens. For parallel incident light the focal lines fall at the second principal foci.

  3. Spherical Equivalents to Spherocylinders Page 6.2

  4. Fig 6.1, Page 6.1 Page 6.2 • Spherical equivalent to an astigmatic lens produces a point image at the dioptric midpoint of the (original) astigmatic image • For parallel incident light  average of astigmatic meridional powers • For the above lens and parallel incident light: OBJ OBJ

  5. Fig 6.1, Page 6.1 Page 6.2 • The dioptric midpoint of the astigmatic image defines the COLC plane • This is the plane of optimum image quality for an astigmatic lens or eye • This is one of the main reasons that we are interested in spherical equivalents OBJ OBJ

  6. Define Best Vision Sphere (BVS). Single, most complete, correct, answer: • Spherical lens giving the astigmatic patient best possible vision prior to correcting any of their astigmatism • Equivalent sphere to a patient’s full astigmatic correction • Spherical lens that places the COLC on the retina of the astigmat prior to correcting any of their astigmatism • All of the above

  7. The Astigmatic Eye and Equivalent Spheres

  8. The Astigmatic Eye and Equivalent Spheres Page 6.3 • The equivalent sphere to the full astigmatic correction places the COLC on the retina, giving the patient best possible vision with a spherical lens • Call this lens the Best Vision Sphere (BVS) or Best Sphere • No other spherical lens will give the astigmat better vision, so vision with BVS is as good as it gets prior to shrinking the Interval of Sturm with cylinder • Can find BVS power either from: • the equivalent sphere to the full astigmatic correction (Visual Optics) • the spherical lens power needed to move the COLC to the retina of the uncorrected astigmatic eye (“Full” Subjective Refraction) OBJ OBJ OBJ

  9. Fig 5.24 Page 5.45 Page 6.3 Take the eyes from Chapter 5 that we used to define the five clinical types of astigmatism and find the BVS for each

  10. Full ametropic correction Fig 5.24 Page 5.45 • Equivalent sphere to full astigmatic correction (VO)

  11. Spherical lens power to move COLC to retina (Subj Ref) Fig 5.24 Page 5.45 FS = +4 D In terms of COLC position:  BVS = +4 D Full ametropic correction

  12. Fig 5.24 Page 5.45 Page 6.3 Take the eyes from Chapter 5 that we used to define the five clinical types of astigmatism and find the BVS for each

  13. Full ametropic correction Fig 5.24 Page 5.45

  14. Fig 5.24 Page 5.45 Page 6.3 Take the eyes from Chapter 5 that we used to define the five clinical types of astigmatism and find the BVS for each

  15. Full ametropic correction Fig 5.24 Page 5.45

  16. Fig 5.24 Page 5.45 Page 6.3 Take the eyes from Chapter 5 that we used to define the five clinical types of astigmatism and find the BVS for each

  17. Full ametropic correction Fig 5.24 Page 5.45

  18. Fig 5.24 Page 5.45 Page 6.3 Take the eyes from Chapter 5 that we used to define the five clinical types of astigmatism and find the BVS for each

  19. Full ametropic correction Fig 5.24 Page 5.45

  20. When an astigmatic patient’s astigmatism has been fully corrected, the image of a point source will appear as follows (according to paraxial optics): •  • undefined

  21. Examples of Equivalent Spheres using ocular power and image vergences Page 6.4

  22. +3 Full correction 0 LCOLC = +58.5 D Example 6.1: Uncorrected Simple Hyperopic Astigmatism Femm = +60 D Page 6.4 Equivalent sphere? +1.50 DS Fig. 6.2 - Reduced eye example of simple hyperopic astigmatism. Focal line positions correspond to the uncorrected eye

  23. Full correction 6 2 Example 6.2: Uncorrected Compound Myopic Astigmatism Femm = +60 D Fig 6.3 Page 6.5 Equivalent sphere? 4.00 DS LCOLC = +64 D

  24. 2 D 2 D Example 6.2: Compound Myopic Astigmat with BVS Page 6.6 BVS produces symmetrical Mixed Astigmatism Fig. 6.4 - Focal line positions for case of compound myopic astigmatism (Example 6.2) with 4.00 DS best vision sphere in front of eye .

  25. Example 6.2: Fully Corrected Compound Myopic Astigmat Page 6.6 Full correction produces a point focus at the retina

  26. Page 6.7 Use of the BVS in Clinical Refraction

  27. Sphere-only Refraction (Donder’s Method) Page 6.7 NOTE: this is a fully subjective refraction. In practice, it is modified because you start with retinoscopy/autorefraction (objective) findings.However, the principles behind each step remain the same. • Subjective methods must have verifiable outcomes • BVS is verified by the patient’s vision: “best vision” with a spherical lens • Reason: the COLC is the most compact part of the astigmat’s IOS • Factor in accommodation by calling BVS the most positive (least negative) sphere that gives the patient optimum vision. • This is important because it is easy to inadvertently overminus a patient if this “push plus” approach is not used • By placing the dioptric midpoint of the IOS on the retina, BVS produces symmetrical mixed astigmatism – this provides a common starting point for all subjective astigmatic refractions OBJ OBJ OBJ

  28. Vision with BVS Page 6.7 • Diameter (h) of the COLC is directly proportional to the amount of astigmatism • Assuming constant pupil diameter (y), vision with BVS should be a systematic function of the amount of astigmatism • F1 and F2 are so much larger than the normal range of astigmatism that (F1 + F2) in the denominator is not really a factor • NOTE: BVS power and vision with BVS are totally unrelated. BVS power simply moves the COLC to the retina. COLC size then depends only on the amount of astigmatism (assuming constant pupil diameter, y) OBJ

  29. Expected VA for an uncorrected healthy 1 D myope is 20/60. If an astigmatic patient with BVS only in front of the eye is found to have 20/60 VA, what is the predicted amount of astigmatism? • 0.50 D • 1.00 D • 1.50 D • 2.00 D

  30. Predicted Vision in Uncorrected Ametropia and Astigmatism Page 6.8

  31. Vision in Astigmatism (ŵ 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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