L7

Question 1

When is there no prismatic effect in a spectacle lens?

Answer 1

When the centration distance (distance between the optical centres) is equal to the pupillary distance.

Question 2

When does a prismatic effect occur in a spectacle lens?

Answer 2

When the centration distance is not equal to the pupillary distance.

Question 3

How do optometrists typically describe prismatic power?

Answer 3

By resolving the horizontal and vertical prismatic power as separate components (e.g., 5Δ DN 3Δ IN).

Question 4

How do lens manufacturers typically describe prismatic power?

Answer 4

As a single power in one meridian (compounding prism power, e.g., 5.83Δ DN & IN along 121).

Question 5

What is the general rule for decentration to induce prism?

Answer 5

For a positive lens, decentre the lens in the same direction as the base; for a negative lens, decentre in the opposite direction as the base.

Question 6

What must you do if you want to avoid a prismatic effect in a spectacle lens?

Answer 6

Place the optical centre of the lens over the pupil centre.

Question 7

What must you do if you want to induce a prismatic effect in a spectacle lens?

Answer 7

Decentre the spectacle lens.

Question 8

What is decentration in practical dispensing?

Answer 8

The movement of the optical centre of a lens, either intentionally (to induce prism) or unintentionally (if not aligned with the pupil).