L16 Introduction To Bifocals Flashcards
Why do you need Bifocals?
- Presbyopia= lens becomes too stiff to move around, you can’t move the lens as much and so begin to lose accommodation
- small print becomes difficult to read, feel like they have to hold something further than arms length to read clearly
- presbyopia coincides with existing rx
Glasses options for presbyopia
1) Single Vision
2) Bifocals
3) Trifocals
4) Varifocals
5) Enhanced readers/ occupationals
Single Vision
- full lens dedicated to rx
- two separate pairs
- good for people doing fine work
- not convenient as they have to keep changing glasses
Bifocals
Focuses at distance and near - has a visible segment
Trifocals
Focuses at distance, intermediate and near
Varifocals
- don’t have a visible segment
- have distance at top, near at bottom, has a smooth transition in middle for intermediate
- some distortion at the periphery
Enhanced readers/ occupationals
- px doing lots of intermediate work
- more near/inter than distance
Why pick bifocals?
- allow clear vision at any 2 distances
- wide reading area
- more convenient than 2 pairs
Disadvantages of bifocals
- visible line bet distance vision and reading area = cosmetic
- indicative of px age
- small seg diameters have limited FOV compared with single vision
- prismatic effect - segment will also exert prismatic effect on top of main rx
- jump
Bifocal shapes
Described by their segment shape and size in my eg D28
- D seg
- R seg
- C seg
- E seg
D segs
- cosmetics - reasonable - straight line is noticeable tho
- vision - seg exerts base up prism - good for minus lenses as when reading, you may experience some base down prism wearing this lens
- availability - excellent, available in large and small sizes
R segs
Cosmesis - good, segment not as noticeable
Vision - seg exerts base down prism so good for positive lenses
Availability- excellent, available in large and small sizes
C segs
Cosmesis - better than d segs, curved top not as noticeable
Vision - seg exerts base up prism, food for minus lenses
Availability - reasonable selection of sizes
Executive lenses
Cosmesis - poor - straight line across
Vision - good optics, no jump
Availability - harder to get hold of
Advantages and disadvantages for diff segs
Cosmesis - r segs are less noticeable
Cost - larger segments more expensive
Availability - e segs harder to get hold of
Vision- less jumps with D segs, none with E segs
