Bifocals L2

Question 1

Why Bifocals?

Answer 1

• For presbyopes
• For children that have problems with accommodation
• Allow clear vision at any 2 distances
• Wide reading area
• More convenient than 2 pairs

Question 2

Disadvantages of Bifocals?

Answer 2

• Visible line bet distance vision and reading area
• Indicative of pxs age
• Smaller seg diameters have limited FOV compared to single vision
• Px over 55 years will start to require intermediate correction on top of distance and near
• Prismatic effect
• Jump

Question 3

Solid Bifocals

Answer 3

• Made from a single piece of material
• Can feel the segment dividing line
• All solid lenses are plastic
• The rx is diff on near on a solid lens due to a change in curvature

Question 4

Cemented

Answer 4

• Made from 2 pieces of material, segment is bonded to the main lens, usually onto the back surface
• Held together by apoxy reisn - but this can yellow with time
• Advantage is you can work diff rx into bifocal as its made from 2 diff materials eg you can work prism into near section

Question 5

Fused

Answer 5

• Both main lens and segment are glass
• Cant feel segment
• Main segment made from crown glass (1.523) and theres a depression curve cut into the glass, and a piece of glass of high refractive index (1.65) is placed within that
• So change of rx comes from change in refractive index of the materials

Question 6

How to work out the add in the seg of a FUSED bifocal?

Answer 6

A = (F1-Fc)/K

F1 = power of distance portion 
Fc = power of depression curve 
K = fused bifocal blank ratio (n-1)(ns-n)

Question 7

Franklin Split

Answer 7

• Composed of 2 sep lenses which are held together by the frame
• Held by apoxy resin
• Can incorporate two diff rxs within distance and near - same as cemented

Question 8

What is the geometrical inset?

Answer 8

Distance between the distance OC (Od) and he midpoint of the segment (Os)

Question 9

What should the distance OC coincide with?

Answer 9

The pxs distance pd

Question 10

Where is the seg OC?

Answer 10

• Approx 4mm below seg top for D/C seg

- Centre of the circle for R seg

Question 11

Where is the Near Vision Point?

Answer 11

Where the px will look for near - this is a vertical and horizontal measurement based on the near pd and the assumption that a px looks 8-10mm down to read and 2 mm in to read

Question 12

What is the Near Optical Centre

Answer 12

• OC of the near portion. Should coincinde with the NVP
• For a plano distance lens it is the centre of the round seg or 4mm below seg top for a D seg
• Horizontally its rarely the case due to the displacement form horizontal prism in the distance lens
• Vertically its often not if the distance rx is minus due to base down prism even if a D seg is used and a plus dist rx causes the OC to rise from the centre of the seg due to base up prism

Question 13

Disadvantage of Bifocal - Prism

Answer 13

• Prism is exerted by the distance lens and ther eading seg
• If its a plus rx, opt for the round seg as it induces less prismatic effect when they look down to read - there is a base up prism exerted when reading in a positive lens

Question 14

Disadvantage of Bifocl - Jump

Answer 14

Jump = sudden intro of base down prism by the prismatic effect of the dividing line

• Causes the image to move up as they move from one image to the other
• the bigger the diff bet the seg top to the OC of the near seg, the more jump there is

Question 15

How to calculate jump experienced by a R seg?

Answer 15

Jump = seg radius (cm) x Add

Question 16

Which type of bifocal has no jump?

Answer 16

E seg as the OC of the near vision seg is on the dividing line

Question 17

Which segs have less jump?

Answer 17

C and D segs as their near OCs are 4-5mm from the top of the seg

Question 18

Which seg has the most jump?

Answer 18

R segs as the near OC of the seg is far from the distance OC as it is in the centre of the seg