prisms

Question 1

outline a prism image jump (3)

Answer 1

• occurs in bifocals
• caused by the geometry of the seg (segment height)
• optical centre of near zone is offset from the opticalcentre of the rest of the lens

Question 2

describe the effect of prisms (2)

Answer 2

• light is refracted towards the BASE

- image is displaced towards the apex

Question 3

identify the 3 prism base notations

Answer 3

• standard axis
• 360°
• binasal notation

Question 4

define the prismatic effect of lenses (2)

Answer 4

• gradually increasing prismatic effect towards the periphery
• prismatic effect increases the incidence of ghost images (in progressive Add lenses)

Question 5

what is the concept of Fresnel lenses (1)

Answer 5

Fresnel lenses utilises prisms to achieve high powers with lower thickness

Question 6

distinguish between when to use stock lenses or grind jobs (2+2)

Answer 6

1. stock lenses are sufficient when
   - minus lenses do not have excessive MSU
   - plus lenses where MSU is close to stock lens size
2. grind jobs
   - low powers
   - plus lenses where calculated MSU is between stock blank sizes

MSU = minimum size uncut

Question 7

outline the importance of Yoked prisms

Answer 7

Yoked prisms are prisms of equal power positioned in front of both eyes such that their bases are oriented in the same direction.

• used to treat non-refractive conditions
  (eg. shifting horizons or midlines)

Question 8

define esotropia and how is it corrected

Answer 8

condition where one eye turn inwards (cross-eyed)

• corrected by prism positioning and splitting

Question 9

how is differential prismatic effect eliminated at near point vision

Answer 9

slab off bifocal