L15 Single Vision Dispensing

Question 1

Spectacle Magnification

Answer 1

Ratio of retinal image size in the corrected eye, to that in the uncorrected eye

Question 2

Spectacle Magnification depends on

Answer 2

• power factor = power of the lens
• shape factor = form and thickness of the lens
• BVD

Question 3

SM EQUATION

Answer 3

SM=[1/(1-dF’V)] x [1/(1-t/nF1)]

d is from the back of the glasses to the entrance point of the pupil

Question 4

SM for myope/Hypermetrope

Answer 4

• for a hyperope it is greater than 1 = magnification
• for a myope it is less than 1 = minification
• has no units

Question 5

How to increase spectacle magnification

Answer 5

• increase power of F1 - but can’t really change it
• increase thickness - by altering curves
• decrease n
• increase d for hypermetrope ( increase vertex distance)
• decrease d for a myope ( decrease vertex distance)
• a thin lens makes magnification less

Question 6

Why calculate spectacle magnification?

Answer 6

• anisometropia

- differential prism

Question 7

Anisometropia

Answer 7

= an unequal refractive power ( around 2D) bet the 2 eyes

- results in aniseikonia - a diff on the retinal image size bet the eyes resulting from the difference in rx

Question 8

Differential prism

Answer 8

If prism encountered by each eye is different then the px will experience differential prismatic effect
- can cause Diplopia, eye strain, headaches and general discomfort

Question 9

Near vision point

Answer 9

=point to which eyes go to read
- 8 to 10mm down and 2mm in - in is due to vergence

P=cF

• from this relationship we can see that for every 1Ds of anisometropia there will be 1 prism diopter of differential prism
• we can cope more vertically with differential prism than horizontally

Question 10

Size/ orthogonal lenses for Anisokonia

Answer 10

• if there is an intolerance in the variation in SM bet the 2 eyes then we can dispense a size lens
• these lenses have a back surface power of 0Ds so don’t change the rx
• here, the back vertex power and the vertex distance are fixed, meaning we can’t change the power factor
• alters the shape factor

Question 11

Magnification if shape factor of an a afocal size lens

Answer 11

-tF2/10n

Question 12

Solutions for differential prism

Answer 12

1) separate pairs
2) franklin splint
3) bonded Plano prism
4) slab off

Question 13

Franklin split

Answer 13

• not usually dispensed
• 2 half lenses cemented together in the frame
• ugly, thick
• info in first half of the lens could be diff to second half

Question 14

Bonded Plano prism

Answer 14

• looks like a bifocal

- stuck on top of lens

Question 15

Slab off

Answer 15

• way of working prismatic effect into one half of the lens
• works vertically and not horizontally
• can remove/ add prism to parts of lenses