PRESBYOPIA/PPLS/BIFOCALS Flashcards
Explain presbyopia
- hardening of crystalline lens
- due to age
- loss of elasticity reduces ability of accommodation
- prescribed add
- around their mid 40’s
How may a myope have no issues managing near tasks when an add is required
- wearing their distance specs lower down nose to increase magnification (vertex distance)
- hold things at a further away distance
- remove distance specs
Explain PPL’s
- multifocal lens
- provides smooth transition
- all in one lens
- convenience
Advantages of PPL’s over bifocals
- no physical line, cosmetically appealing
- blended so no jump
- wide selection of extras, less restrictions
Disadvantages of PPL’s over bifocals
- smaller field of view
- distortion at edges
- longer adaption period
Temporary markings on PPL’s
- prism reference point
- distance reference point
- near reference point
- fitting cross position
Define vertex distance
Distance from the back surface of a lens, to the front surface of the cornea
British standard for BVD
BS 2738
States that any prescription over +/-5.00 must require the vertex distance to be measured
Why may the vertex distanxe be significant in frame fitting?
- increasing/decreasing make the lenses either more or less positive
- could end up under or over corrected
- leads to visual discomfort/ headaches
Meaning of bifocal specifications eg. 38x4x2
38 = seg diameter
4 = seg top position
2 = geometrical inset
Near centration distance (NCD)
Horizontal distance between visual axis with measurements taken in the plane of the specs with eyes fixating on a near object
Optical centre of near position (ON)
The point on or off the lens where the prismatic effect induced by the main lens is neutralised by the prismatic effect induced to the segment
Vertical differential prismatic effect tolerance
1 prism base
Methods of controlling differential prismatic effect in bifocals
- fresnel prisms
- presto bifocal lenses
- slap off
- Franklin split
