DISP - Bifocals - Week 6

Question 1

What are the two formulae for normal accommodation?

Answer 1

18 - (age/3)
15 - (age/4)

Question 2

What are two possible causes of presbyopia?

Answer 2

Loss of flexibility in the lens
Loss of ciliary muscle strength

Question 3

What is the near addition?

Answer 3

The difference between the distance and near Rx

Question 4

Describe the franklin split type bifocals, what are they joined and held together by?

Answer 4

They are two separate lenses that are joined by a bevel and held together by the frame.

Question 5

Name an advantage and three disadvantages of the franklin split bifocals.

Answer 5

Advantage
-independent control over distance and near horizontal centration
Disadvantages
-poor cosmesis
-heavy
-poor vision at the lens junction

Question 6

List four types of bifocal lens type designs.

Answer 6

Round segment
Executive - half distance, half near
A segment - upcurve or downcurve, depending if a wide reading area is needed
D segment - half-moon near lens design

Question 7

How are D segment bifocals measured?

Answer 7

Across the widest part of the segment

Question 8

List the three construction types for bifocal lenses. Which is not common?

Answer 8

Cemented - not common
Fused
Solid

Question 9

In a cemented bifocal, where is the near segment attached?

Answer 9

It is cemented onto the back surface of the distance lens

Question 10

Consider fused bifocals. What is the segment typically made of?

Answer 10

Higher RI glass

Question 11

Where is the near segment attached to in fused bifocals?

Answer 11

Always the front surface

Question 12

Where is the cyl powere worked onto in fused and cemented bifocals?

Answer 12

Fused - back surface
Cemented - front surface

Question 13

List three advantages and one disadvantage of fused bifocal lenses.

Answer 13

Advantage
Cheap
Good for unusual shapes
Cosmetically good
Disadvantages
Increased chromatic aberrations with high RI segments (not a problem in clinical practice)

Question 14

Describe how plastic and glass solid bifocals are made.

Answer 14

Glass - distance and near is ground
Plastic - distance and near is moulded

Question 15

What construction type do all plastic bifocal lenses have?

Answer 15

Solid

Question 16

Which of the three construction types of bifocals is optically superior?

Answer 16

Solid is superior to fused and cemented

Question 17

How are solid bifocals generated?

Answer 17

By having two curves on one surface

Question 18

List 4 advantages of bifocals.

Answer 18

Allows quick change of focus with small eye movements
Less likely to lose anternative spectacles
No need to constantly switch spectacles
Cheaper

Question 19

What are 6 disadvantages of bifocals?

Answer 19

Restricted reading area compared to SVN (larger segment sizes are possible though)
Adaptation - initial discomfort with walking, especially down stairs
Cosmetic issues
Poor intermediate vision
Prismatic jump
Hearsay

Question 20

Name 4 kinds of patients that are unsuitable for bifocals.

Answer 20

Unsteady on their feet
Neck/posture problems
Bedd-ridden patients
No distance refraction

Question 21

What do golf glasses look like and what do they allow users to do?

Answer 21

Any regular bifocal placed extemely low and in the corner of one lens of a pair.
Allows golfers to read and write on the scorecard without compromising distance vision

Question 22

Describe E-D trifocal design.

Answer 22

Has an executive lens design where the bottom half is an intermediate segment.
A smaller near segment is placed within the intermediate segment towards the bottom on the frame.

Question 23

What is an inherent defect of bifocals?

Answer 23

Prismatic jump at the segment edge (base down)

Question 24

Describe the image jump in bifocals.

Answer 24

Objects become displaced at the lens junction, the light does not reach the eyes from this region and the image appears to jump.

Question 25

What kind of prismatic effect do positive and negative lenses exert at the NVP (two each)?

Answer 25

Positive - BU and BO
Negative - BD and BI

Question 26

Where is a bifocal segment usually fitted?

Answer 26

Placed on the lower limbus.

Question 27

What two pieces of advice whould be given to patients with bifocals?

Answer 27

Keep head straight, moving eyes up and down
-move chin up for near, down for distance