1. AREA 2: Fitting single-vision lenses Topic 4: Alignment, decentration, and minimum diameter Topic 5: Calculating thickness in spherical lenses Topic 6: Calculating thickness in astigmatic lenses Topic 7: How lenses affect vision Topic 8: Prismatic effects Topic 9: Base rules Topic 10: How alignment affects binocular vision

2. Topic 8: Prismatic effects Prentice's rule Calculating prismatic effect Measuring prismatic effect using a lensometer

3. Prentice's rule When a ray of light passes through a prism, it is bent towards the base of the prism. If an object is viewed through the same prism, however, it is seen to be displaced towards the apex of the prism.

4. Prentice's rule Prism dioptre (Δ): The deviation of a light ray by 1 cm at a distance of 1 metre

5. Prentice's rule Orientation of the prism base at a given point on the lens In plus lenses, the base of the prism is oriented towards the centre of the lens. In minus lenses, the base of the prism is oriented towards the edge of the lens.

6. Prentice's rule UNITS: d (cm) P (D) OC CM The prismatic effect at any point other than the optical centre (CM) can be calculated as follows:

7. Calculating prismatic effect It is easy to calculate the prismatic effect at any point on a lens if you know the distance between this point and the optical centre and the power of the lens in this direction. d

8. Calculating prismatic effect OC CM Example: BVP: +2.00 D distance OC – CM: 10 mm Δ CM =1 cm · 2.00 D = 2 Δ 10 mm

9. Calculating prismatic effect Prismatic effect in astigmatic lenses The prismatic effect of a point on either of the two principal meridians can be calculated using Prentice's rule and the power of the corresponding meridian.

10. Calculating prismatic effect Prismatic effects in astigmatic lenses If the point is not on either of the two principal meridians, we cannot apply Prentice's rule directly, as we do not know the power of the meridian it is on.

11. Measuring prismatic effect using a lensometer

12. When the prismatic effect is oblique, it can be expressed by its compound form, or by its horizontal and vertical components separately. Measuring prismatic effect using a lensometer

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