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Excellent tutorial by Dr. Tim Root
- The power of the cylinder is indicated in plus cylinder form
- (Retinoscopy) Streak = Beam of light from the retinoscope
- Light reflex = light reflex in the patient’s pupil
Example without astigmatism
1. Always start with “with” motion: add minus power until you have “with” motion in all axes
2. Add plus power until you neutralise the reflex in all axes
- 1. Number: This value is used for the sphere (in this example +3.00dpt)
3. Calculate
-
- Subtract working distance: 1.5dpt at 66cm arm length
- Sphere = 1. Number – 1.5dpt (normal working distance)
- In this example: +3dpt – 1.5dpt = +1.5dpt Sphere
Example with astigmatism in main axes (90° / 180°)
1. Add minus power until you have “with” motion in both axes
- Light reflex bigger at 180° -> sign for astigmatism
2. Add plus power until you neutralised the reflex in one axis
-
- 1. Number: This value is used for the sphere (in this example +3.00dpt)
3. Add plus power until you neutralise the reflex of the other axis
-
- 2. Number: This value is used for the cylinder (in this example +4.00dpt)
- 3. Number: Axis of the second reflex that you neutralised (in this example 90°)
4. Calculation
-
- Subtract working distance: 1.5dpt at 66cm arm length
- Sphere = 1. Number – 1.5dpt (normal working distance)
- In this example: +3.0dpt – 1.5dpt = +1.5dpt Sphere
- Cylinder = 2. Number – 1. Number
- In this example: +4.0dpt – 3.0dpt = +1.0dpt Cylinder
- Axis = 3. Number
- In this example: 90° Axis
5. Converting Plus to Minus Cylinder
-
- Add the Cylinder power to the Sphere -> new Sphere
- Cylinder: Change the Cylinder positive / negative sign
- Axis (+/- 90°) -> Change the axis by 90°
- Example: +1.5dpt /+1.0dpt/90° becomes +2.5dpt/ -1.0/180°
- Sphere: +1.5dpt + (+1.0dpt) = +2.5dpt
- Cylinder: +1.0dpt becomes -1.0dpt
- Axis: 90° + 90° = 180°
Example with Astigmatism in oblique Axis
0. Rotate the streak until you are parallel to the light reflex, then continue with the normal steps
1. Add minus power until you have “with” motion in both axes
2. Add plus power until you neutralised the reflex in one axis
- 1. Number: This value is used for the sphere (in this example +1.00dpt)
3. Add plus power until you neutralise the reflex of the other axis
-
- 2. Number: This value is used for the cylinder (in this example +2.00dpt)
- 3. Number: Axis of the second reflex that you neutralised (in this example 160°)
4. Calculation
-
- Subtract working distance: 1.5dpt at 66cm arm length
- Sphere = 1. Number – 1.5dpt (normal working distance)
- In this example: +1.0dpt – 1.5dpt = -0.5dpt Sphere
- Cylinder = 2. Number – 1. Number
- In this example: +2.0dpt – 1.0dpt = +1.0dpt Cylinder
- Axis = 3. Number
- In this example: 160° Axis
5. Converting Plus to Minus Cylinder
-
- Add the Cylinder power to the Sphere -> new Sphere
- Cylinder: Change the Cylinder positive / negative sign
- Axis (+/- 90°) -> Change the axis by 90°
- Example: -0.5dpt /+1.0dpt/160° becomes +0.5dpt/ -1.0/70°
- Sphere: -0.5dpt + (+1.0dpt) = +0.5dpt
- Cylinder: +1.0dpt becomes -1.0dpt
- Axis: 160° + 90° = 70°