Complex Bifocals L5

Question 1

Why do we need to dispense complex bifocals?

Answer 1

• anisometropia = an equal refractve power bet the 2 eyes (1.50D or more)
• results in asthenopia and diplopia due to dfferential prism and aniseikonia

Question 2

What is differential prismatic effect?

Answer 2

Result of diff amounts of prism experienced by each eye when looking away from OC, tolerance of only 2 prism dioptres vertically, but much more horizontally

Question 3

What is anisokonia?

Answer 3

• a diff in retinal image size bet the eyes resulting from diff in rx
• diff in image size is 1% of every dioptre anisometropia
• we can only tolerate 1%
• 1 dioptre = 1% diff i retinal image size

Question 4

How to correct for anisokonia?

Answer 4

ISEIKONIC/ SIZE LENSES

• increase spectacle mag bby maing image larger but dont alter power
• would give size lens to eye with least positive power
• visually comfortable but can end up with thicker lens

Question 5

How can you reduce mag without using spectacle mag?

Answer 5

• Flaten base curve
• High index material
• aspheric lens

Question 6

What are the bifocal soluions?

Answer 6

• slab off prism/ bi prism lenses
• franklin split
• prism controlled bifocals
• diff sized round segs
• fresnel prism

Question 7

What is slab off/ bi prism?

Answer 7

• removes unwanted base down prism from the bottom of the back of the lens
• results in a line across the lens so an E or D seg with the seg top coinciding with the line looks best
• can slab off a varfocal but not good cosmetically as it puts a line across the lens
• can slab on also
• min of 2 prism dioptres base DN required

Question 8

What is franklin split?

Answer 8

• DV and NV are completely separate lenses
• the OCs can be placed were required and prism can be surfaced into either portion to eliminate ay unwanted prismatic effect
• not cosmetically good

Question 9

What are prism controlled bifocals?

Answer 9

• available in glass, with a vsible round 30mm bifocal seg
• up to 6 prism dioptrs of extra prism is worked in any direction in the seg only
• depression curve is worked onto distance portion and small prism dropped in
• so thinnest edge indicates prism base

Question 10

What are diff sized round segs?

Answer 10

• largest seg is dispensed to the most positive eye (induces more base down prism in the seg for reading, to eliminate the base up of the dist portion
  = (20 x differential prism)/ add
• gives diff in seg sizes not size of either seg

Question 11

What seg sizes are available?

Answer 11

r - 22,24,25,28,30,38,40,45
d - 25,28,35,40,45
c - 25,28,35

Question 12

What is a fresnel prism?

Answer 12

• normally used a temp measure
• 2mm thick sheet of plastic to shape
• will reduce px va by approx 2 lines - so give to one eye but not good for drivers around 6/9
• may be used for prism in any direction and available from 1 prism dioptre to 30
• dont split fresnel because it will reduce VA in both eyes
• diff to keep clean as dirt goes in grooves

Question 13

What options are available for anisometropia in single vision?

Answer 13

• contact lenses

- can control decentration by altering their head position so you always look through OC