Visual Optics 2006/2007

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

2. Visual Optics Assignments Visual Optics II Labs • Topic must be optics-based • Topic must be controversial (conflicting theories, e.g. the bichromatic test) or describe some phenomenon that is not fully understood (e.g. aniseikonia) • Report must be original (your own words) and include references (at least one citation per paragraph) • Will allow 20 minutes for presentations to include discussion of the “controversy.”

3. Which of the following patients would be expected to have the WORST vision with BVS (assume all are capable of 20/20 when fully corrected)? • Patient with 3.50 D BVS • Patient with plano BVS • Patient with +3.50 D BVS • Insufficient information provided

4. Which of the following steps in a subjective refraction is not (by itself) objectively verifiable? • Add 0.25 DS during a fog-defog and the patient says that the letters are clearer • Add 0.50 DS during fog-defog and the patient improves by two lines on the VA chart • Add 0.50 DC during the JCC part of a refraction and simultaneously change the sphere by +0.25 DS • After correctly finding BVS and correctly predicting 2.00 D astigmatism, add +1.00 DS to move the posterior focal line to the retina

5. During a JCC refraction, the cyl power is being refined. If the amount of cyl is changed from 2.50 DC to 1.50 DC, what change should be made in sphere power? • Add 0.50 DS to the current sphere power • Add 1.00 DS to the current sphere power • Add +0.50 DS to the current sphere power • Add +1.00 DS to the current sphere power

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

7. Full correction 6 2 Use 2 4  180 CMA to demonstrate JCC after retinoscopy Assume retinoscopy was “off” for cyl giving 3 DC axis 160 Full correction in SC notation: 2 4 DC axis 180 Page 6.20 Fig 6.3 Page 6.5

8. Page 6.20 Using the JCC to Refine Cylinder Axis after Retinoscopy Focal line positions with 3 DC axis 160 in front of patient’s eye The patient has 1 D residual astigmatism (ignoring the 20 axis error) The clinician then does a modified BVS procedure to move COLC to retina The patient’s vision with COLC on retina will be a little worse than for a 1 D astigmat because of the 20 axis error (slightly irregular COLC)

9. Page 6.20 Using the JCC to Refine Cylinder Axis after Retinoscopy Check that the “partial correction” does maintain COLC on retina The equivalent sphere to the partial correction should equal the equivalent sphere to the full correction (4 DS)  BVS * again, we are ignoring the 20 axis discrepancy

10. Page 6.20 Using the JCC to Refine Cylinder Axis after Retinoscopy Check that the “partial correction” does maintain COLC on retina The equivalent sphere to the partial correction should equal the equivalent sphere to the full correction (4 DS)  BVS 5.50 * again, we are ignoring the 20 axis discrepancy 2.50

11. Page 6.20 Refining Cylinder Axis using Obliquely Crossed Cylinders

12. Refining Cylinder Axis using Obliquely Crossed Cylinders Page 6.20 • Optometrists use negative cylinders for refraction • Two negative cyls crossed at an oblique angle produce a resultant cyl with intermediate power meridian • Clinicians “think” axis during refraction (trial cyl, JCC, etc.) • We can consider obliquely crossed negative cylinder axes, because the resultant cylinder axis will be correct (rotated 90 from the resultant power meridian)

13. Refining Cylinder Axis using Obliquely Crossed Cylinders Page 6.20 Obliquely crossed cylinders of the same sign will produce a resultant cylinder with an axis between the two Like an airplane’s path through the air with wind direction at an acute angle NO WIND

14. WIND Refining Cylinder Axis using Obliquely Crossed Cylinders Page 6.20 Obliquely crossed cylinders of the same sign will produce a resultant cylinder with an axis between the two Like an airplane’s path through the air with wind direction at an acute angle resultant direction

15. Obliquely Crossed Cylinders (same sign) Page 6.21 Resultant cylinder axis is between two obliquely crossed cylinder axes Resultant axis will be closer to the axis of the higher power cyl Fig 6.12 - Two negative cylinders with axes crossed at an oblique angle produce a resultant negative cylinder with an intermediate axis.

16. Resultant axis Obliquely crossed negative cylinders with axes 45 apart Trial cyl axis (TCA) 90 Page 6.19 For JCC axis refinement, we place the handle parallel to the TCA This produces obliquely crossed negative cylinders The resultant axis is in between, and closer to the higher power cyl (axis) WHY DO IT THIS WAY? This mimics rotating the cyl axis a little CW Flipping the JCC over, mimics rotating the JCC axis the same amount CCW

17. Page 6.20 Refining Cylinder Axis for Our CMA Required correcting cylinder axis is 180 We currently have the trial cylinder axis (TCA) set to 160 To refine axis, we set it to the 160 (that we found with retinoscopy), place the JCC handle along 160 and present “first” and “second” views with obliquely crossed cylinders 45 either side of 160

18. Fig. 6.13 Page 6.22 JCC: Refining Cylinder Axis

19. Fig. 6.13 Page 6.22 JCC: Refining Cylinder Axis First and second views are equivalent to rotating the TCA clockwise then counterclockwise from 160 The advantage of the JCC is that the patient sees instant comparisons, not a gradual change in axis The axis “rotation” is also identical between first and second

20. Fig. 6.13 Page 6.22 JCC: Refining Cylinder Axis The patient prefers “second” because the resultant cylinder axis is closer to 180 This prompts the clinician to rotate the TCA from 160 toward 180, e.g. to 170 JCC handle is now aligned with 170. First and second? Patient prefers the view with the resultant rotated toward 180

21. Refining Cylinder Axis using Obliquely Crossed Cylinders Page 6.22 • What happens when trial cylinder axis reaches 180? • First and second should be the same and both should be blurry because they both move the resultant cyl axis away from 180 • Here you should reassure the patient that this is normal • Further refine by rotating beyond 180 • The patient should “push” you back toward 180 • From this point it is a matter of fine-tuning to get the exact axis • For a 3 D (eventually 4 D) cyl, axis should easily be set to an accuracy of 1 • For an 0.50 D cyl, axis cannot be set as accurately

22. Page 6.23 Refining Cylinder Power using the Jackson Cross Cylinder

23. Refining Cylinder Power using the JCC Page 6.23 • Easier to consider power refinement using actual cyl powers in actual meridians because we are now moving the focal lines relative to the retina

24. JCC: Refining Cylinder Power Page 6.23 JCC power meridians are 90 from their axes Negative axis parallel and perpendicular to TCA for power refinement Negative PMs are always 90 away (from axis) Fig 6.14 - To refine trial cylinder power, the cross cylinder is placed with its (a) axes, and therefore also (b) principal meridians parallel to / perpendicular to the trial cylinder principal meridians

25. Refining Cylinder Power – JCC Power Meridians Shown Fig. 6.15 Page 6.24 JCC axes parallel and perpendicular to TCA for power refinement First view moves both FLs closer to the retina. This makes COLC smaller Second view moves both FLs away from the retina. The COLC gets larger