Dispensing - Complex bifocals

Question 1

What are the two causes of complexity in bifocals?

Answer 1

• anisometropia
• heterophoria

Question 2

what is anisometropia and why does it cause complex bifocals?

Answer 2

difference of 2 diopters in power between both eyes. Complex because each dioptre difference in power between lenses induces 1PD of differential prism

Question 3

what is vertical differential prism tolerance

Answer 3

• vertical = 1PD differential prismatic effect binocularly

Question 4

what is horizontal differential prism tolerance for distance and near?

Answer 4

• distance = up to 10 PD OUT and 4 PD IN
• near = up to 7 PD OUT and 7 PD IN

Question 5

What is SV differential prismatic effect?

Answer 5

when vertical diplopia is induced due to differing amounts of vertical prism when looking away from the OC causing the patient to complain of seeing 2 images.

Question 6

What are 4 solutions to SV differential prismatic effect?

Answer 6

• Having 2 pairs of specs where RX is the same but 1 has OC for DV and one has OC for NV
• Slab off prism where most negative eye is slabbed off (as this induces the most BD prism) and slab off line is to be positioned in line with lower limbus
• Head position where px is advised to drop head for NV so they look through OCs but this is not good for long periods of reading
• Drop spex down noise for reading but not ideal

Question 7

What is bifocal prismatic effect? How do you calculate it? (check google doc for worked answer with chat)

Answer 7

when lenses of unequal power cause differing amounts of vertical prism to be induced between the two eyes when looking down in bifocals creating vertical image displacement

To calculate first find the near viewing point (normally 10mm below and 2mm in from distance OC) and then find P usinc P=cF (prentice’s rule)

Question 9

name the 10 solutions for bifocal prismatic effect

Answer 9

• slab off bifocal
• franklin split
• prism controlled/ solid prism segment BIFs
• bonded prism segment
• prism controlled bifocal
• grossly decentred D segs
• different sized rounded segs
• fresnel prism
• separate pairs
• contact lenses (last resort)

Question 10

how can slab off be used as a solution for bifocal prismatic effect?

Answer 10

• Minimum is 2PD base down
• Ideally add an A/R
• Slab off line is to be positioned in line with seg top

Question 11

What is franklin split and how can it be used as a solution for bifocal prismatic effect?

Answer 11

• Separate halves, top is distance and bottom is near so OCs can be places as needed for DV and NV
• Prisms can be added to either portion or both to eliminate unwanted prismatic effects

Question 12

How can prism controlled/ solid prism segment BIFs be used as a solution for bifocal prismatic effect? what material and type of bifocal segment are they avaliable in?

Answer 12

• Up to 6 PD prism
• Larger segment indentation denotes base of prism
• Available in glass with a solid visible R30 bifocal segment

Question 13

What is a prism controlled bifocal?

Answer 13

• Similar to franklin split however distance portion has D28 shape cut out which is bonded with the D28 shape cut from the near vision lens
• Cr39 used
• Available from Birchgrove Optical

Question 14

How can grossly decentered D segs be used as a solution for bifocal prismatic effect?

Answer 14

as D seg is shifted based on P=cF

Question 15

How can different sized rounded segs be used as a solution for bifocal prismatic effect?
how do you calculate differences in seg size?

Answer 15

• Use of different sized segs for each eye where largest is dispensed to the eye with the most positive Rx as this induces more BD prism in the reading segment to eliminate the BU of the distance portion
• Difference in seg sizes = (20 x differential prism) / add

Question 16

For a fresnel prism, what is the max thicknes, what is the expected VA improvement with it and what is the range of PD it is avaliable in?

Answer 16

• Max thickness ~2mm
• Will reduce px VA by approx 2 lines on snellen
• May be used for prism in any direction and available from 1PD to 30PD+

Question 17

What is a bonded prism segment?

Answer 17

Where extra prism is worked onto the front surface of a segment and is bonded onto the SV main lens (made of CR39)

Question 18

when dealing with PPL differential prismatic effect, what is the solution and how is it done?

Answer 18

slab off
- Slab off most negative eye as it induces the most base down prism
- Slab off line to be positioned in line with lower limbus so DV is not affected

Question 19

What are the symptoms of anisikonia?

Answer 19

• eye strain
• diplopia
• distortion
• depth perception problems

Question 20

what are the 2 dispensing solutions for aniseikonia?

Answer 20

• Use higher powered +ve lenses to make image more magnified
• Use iseikonic lenses (where les is thicker and more curved to increase spectacle mag)

e.g. RE: +5.00D
LE: +1.00D
, dispense iseikonic lens in the LE and normal lens in RE to balance thickness and weight and fix magnification issue causing aniseikonia