Bifocals

Question 1

What is needed for a bifocal to be successful?

Answer 1

Almost invisible line of seg top
Seg permanently fixed
Seg wide and deep enough for px’s needs
Both parts of vision equally good
Minimal TCA (if diff materials used)
Tolerable prismatic effect

Question 2

What seg types work best for minus rxs?

Answer 2

D or C

Question 3

What seg types work best for plus rxs?

Answer 3

C or R

Question 4

What are the types of segment attachment?

Answer 4

Fused
Solid
Moulded

Question 5

How is a fused bifocal made?

Answer 5

Glass lens
Seg has a higher RI
Seg is fused into the lens - can’t feel line

Question 6

How is a solid bifocal made?

Answer 6

Glass lens
Seg ground into lens - can feel line

Question 7

How is a moulded bifocal made?

Answer 7

Plastic lens
Seg ground into lens - can feel line

Question 8

Why does an E-line bifocal not have any jump?

Answer 8

Distance and near OCs overlap

Question 9

What is jump?

Answer 9

Image jumps up due to introduction of BD prism

Question 10

What type of segment experiences the worst jump?

Answer 10

R

Question 11

What is the calculation for jump in an R seg?

Answer 11

Jump = seg radius (cm) x add
P = c x F

Question 12

What is the calculation for jump in an C or D seg?

Answer 12

Jump = distance from seg OC to seg top (cm) x add
P = c x F

Question 13

What measurements are needed for bifocals?

Answer 13

Binocular PD (D&N for inset)
Heights to lower limbus
BVD if >5D

Question 14

What is the inset formula?

Answer 14

Inset = (DPD-NPD) /2

Question 15

What is the minimum seg depth for a bifocal?

Answer 15

14mm

Question 16

If heights are asymmetrical for bifocals, which should you use for better cosmesis?

Answer 16

Lower

Question 17

If a px naturally sits with their head back, how should you adjust the heights?

Answer 17

Lower

Question 18

If a px naturally sits with their head forward, how should you adjust the heights?

Answer 18

Higher

Question 19

How much vertical prism can be tolerated?

Answer 19

1^

Question 20

How much horizontal prism can be tolerated at distance?

Answer 20

10^ OUT
4^ IN

Question 21

How much horizontal prism can be tolerated at near?

Answer 21

7^ OUT and IN

Question 22

How much differential prism is induced for every dioptre of rx difference between the eyes for an anisometrope?

Answer 22

1^

Question 23

What are the solutions to vertical diplopia when reading in SV lenses?

Answer 23

Slab off (remove BD prism from most minus eye)
or
Make sure looking through OC when reading:
Sep pairs with OCs for D&N
Move specs down nose
Move head lower (chin down)

Question 24

What causes vertical diplopia when reading in SV lenses?

Answer 24

If anisometropic: differential prismatic effect caused by the difference in rx

Question 25

What must you work out before using a bifocal solution for prismatic effect?

Answer 25

Prismatic effect at NVP

Question 26

What are the bifocal options to solve prismatic effect?

Answer 26

Franklin Split Different sized R segs Bonded/cemented bifs Prism controlled Grossly decentred D segs Solid prism segment Slab-off

Question 27

What are non-bifocal options to solve prismatic effect?

Answer 27

CLs Fresnel prism Separate pairs PPL + slab off

Question 28

What are some solutions for aniseikonia?

Answer 28

Isiekonic lens (thicken weaker lens to increase spec mag) High index in stronger lens to reduce spec mag Both make image size more similar

Question 29

How do different sized R segs help reduce prismatic effect?

Answer 29

Larger seg given to most plus eye Larger seg induces BD prism at near to counteract BU prism at distance

Question 30

How does a Franklin Split help reduce prismatic effect?

Answer 30

Two different lenses so can have OCs where needed and different amounts of prism in each lens

Question 30

What is the formula for working out the difference in seg size when dispensing different sized R segs?

Answer 30

Diff in seg size = (20x differential prism)/Add

Question 31

What is a bonded/cemented bifocal?

Answer 31

Extra prism is worked onto the front surface of one segment

Question 32

How does a prism-controlled bifocal help reduce prismatic effect?

Answer 32

Segment is bonded into cut out portion of distance lens Allows for up to 6^ in any direction

Question 33

What are some features of a prism controlled bifocal?

Answer 33

Seg is more obvious - bigger indentation = base direction Usually D28 but can be any seg type Usually CR39

Question 34

What is the equivalent of prism-controlled bifocals but in glass? What segment is usually used?

Answer 34

Solid prism segment R30 segment

Question 35

How do grossly decentred D segs help reduce prismatic effect?

Answer 35

Decentration in opposite direction to prismatic effect counteracts it

Question 36

How does slab-off help reduce prismatic effect?

Answer 36

Removes unwanted BD prism from most minus lens to neutralise prismatic effct

Question 37

What should be requested when dispensing slab-off bifocals?

Answer 37

Line up line with seg top (E or D-seg) Add MAR coating

Question 38

What is the minimum slab-off you can order?

Answer 38

2^

Question 39

What is slab-on?

Answer 39

Removing unwanted BU prism from most plus lens