Bifocals

Question 1

3 advantages of bifocal?

Answer 1

*Clear vision at 2 distances.
*Wider reading area than in a varifocal.
*More convenient than using 2 separate pairs.

Question 2

7 possible disadvantages of bifocals?

Answer 2

*Visible line between distance vision and reading area.
*Indicative of patient’s age.
*Reflection from seg top can be a problem.
*Smaller seg diameters have limited field of view compared with single vision.
*Patients over 55yrs will start to require intermediate correction on top of distance and near.
*Jump causes image to be displaced upwards as eye crosses dividing line from distance to near – caused by sudden introduction of base down prism.
*Problems can occur when tinting if using 2 different RIs.

Question 3

6 Requirements of a bifocal?

Answer 3

*Should have barely visible dividing line between DV and NV.
*Seg needs to be permanently fixed to main lens.
*Seg should be wide and deep enough to meet px’s needs.
*Vision should be equally good through DV and NV areas.
*Any prismatic effect induced when px looks through DVP or NVP should be tolerable i.e., vertical differential prism less than 1Δ.
*Chromatic aberration should be minimised – especially when using materials of differing v-values as this will only induce TCA.

Question 4

What causes jump?

Answer 4

Sudden change in prismatic effect.

Question 5

Why is jump a concerning factor?

Answer 5

It causes displacement of the px’s visual world.

Question 6

How is jump calculated for round seg vs C or D seg?

Answer 6

*For a round seg: Jump = Add x seg. radius (cm)
*For a C or D seg: Jump = Add x (distance from OC of seg to top of seg) (cm)
Answer is always base down.

Question 7

No jump bifocals are available as?

Answer 7

Executives

Question 8

What segments are selected for -ve vs +ve RX?

Answer 8

*-ve prescriptions – dispense D or C segs
*+ve prescriptions – dispense C or R segs up to +3D, R segs only over +3D

Question 9

What are the important considerations required when selecting a seg?- 5

Answer 9

*Identify needs and requirements of patient
*Identify patient’s occupation and hobbies
*What size seg does patient need?
*Previous wearers – check they are happy with size and position of seg
*Check patient’s posture

Question 10

3 methods of manufacture for bifocals?

Answer 10

*Fused bifocals – Glass lenses where the seg fused into the main lens is made in a higher refractive index than the main lens. The seg dividing line cannot be felt with a fingernail.
*Solid bifocals – Glass lenses where the whole lens is ground from one piece of glass. The seg dividing line can be felt with a fingernail.
*Moulded bifocals – Plastic lenses. The seg dividing line can be felt with a fingernail.

Question 11

Binocular vs monocular measurements for bifocals?

Answer 11

Binocular PD- unless px is asymmetrical or has high RX.

Question 12

Asymmetrical measurements of PDs leads to?

Answer 12

Asymmetrical positions of seg which is not cosmetically ideal.

Question 13

How is inset calculated?

Answer 13

(Distance PD- Near PD)/ 2

Question 14

Minimum depth of seg should nor be below approx?

Answer 14

14mm

Question 15

How do the following postures affect the placement of a seg?

Answer 15

*If head held back – segs to be set lower to prevent segs impinging on DV.
*If px has a stoop – segs to be set higher to ensure useful vision through seg.

Question 16

If a px is a previous seg wear how are they dispenses?

Answer 16

*Previous wearers – always see old BIFS on px as px may be happy with segs set low for a reason – if px happy, match segs during dispense.

Question 16

How do the seg placement of the following occupations varry?
BUS driver vs writer?

Answer 16

*Bus/truck driver – set segs lower
*Architect/writer – set segs higher

Question 16

How to make it look cosmetically better - for asymmetrical px’s?

Answer 16

Set heights equally at lower height of the two to avoid poor cosmetic outcome.

Question 16

Seg position for Down’s Syndrome px’s?

Answer 16

Set seg at pupil/ iris margin

Question 17

Describe how the various occupations seg varies? Pilot? VDU users? Golfers? Driving mask?

Answer 17

*Pilots – Double D segs (DD28)
*VDU users – IV in main lens with NV seg
*Golfers – stick-on Adds outset
*Diving masks – sometimes have Adds outset so divers can read gauges

Question 18

How is vertical and horizontal prismatic effect calculated for main vs segment lenses?

Answer 18

Main lenses:
PV = vertical dist. between OD and NVP (cm) x FV
PH = horizontal dist. between OD and NVP (cm) x FH
Segment Lenses:
PV = vertical dist. between OS and NVP (cm) x Add
PH = horizontal dist. between OS and NVP (cm) x Add

Question 19

When is a minus lens with a flat-top segment or a plus lens with a round segment used?

Answer 19

It is used to reduce the amount of induced base down prism in a bifocal

Question 20

How is jump calculated?

Answer 20

Jump= Os to segment dividing line (cm) X Add

Question 21

How are DD28 (double D) designed?

Answer 21

*Intermediate zone on top of lens so higher instruments can be seen clearly as well as the regular near zone on the bottom.