spectacle magnification and differential prism

Question 1

what is spectacle magnification ?

Answer 1

• is the ration of the retinal image size in the corrected eye, to that in the uncorrected eye

Question 2

how does a positive lens affect magnification ?

Answer 2

positive lens increases magnification

Question 3

how does a negative lens affect magnification ?

Answer 3

negative lens decreases magnification

Question 4

what does spectacle magnification depend on ?

Answer 4

. power factor=power of the lens
. shape factor=form and thickness of the lens
. BVD
.refractive index

Question 5

where is thickness in positive lens ?

Answer 5

positive lens is thickest in the middle

Question 6

what does spectacle magnification not have ?

Answer 6

spectacle magnification has no units

Question 7

what does it mean if spectacle magnification is greater then 1 ?

Answer 7

• for a hyperope it is greater than 1 (magnification)

Question 8

what does it mean if spectacle magnification is less than 1?

Answer 8

• for a myope it is less than 1 (minification)

Question 9

what are ways to increase spectacle magnification ?

Answer 9

• increase power of F1
• increase thickness
• decrease n

Question 10

why can’t we change overall power of lens to increase spectacle magnification ?

Answer 10

• because you will be altering patient’s prescription

Question 11

what happens if you increase d(vertex distance )?

Answer 11

• it will make image larger

- use for hyperope

Question 12

what happens if you decrease d(vertex distance)?

Answer 12

• it will make image smaller

- use for myope

Question 13

what is equation of spectacle magnification ?

Answer 13

SM=[1/(1-dF’V)] x [ 1 / (1-t/n F1)

• the first bit is the power factor
• the second bit is the shape factor which is influenced by refractive index and thickness

Question 14

why is it important to calculate spectacle magnification?

Answer 14

• due to anisometropia which is an unequal refractive power between the two eye

Question 15

what does anisometropia result in ?

Answer 15

• results in aniseikonia -difference in the retinal image size between the eyes resulting from the difference in prescription
• differential prism-if the prism encountered by each eye is different then the patient will experience differential prismatic effect

Question 16

what does anisometropia cause ?

Answer 16

• double vision
• eye strain/asthenopia
• headache

Question 17

when are we mostly concerned about differential prism ?

Answer 17

• when a patient is reading

- because they are looking away from optical centre

Question 18

what is near vision point?

Answer 18

• near vision point - is point where eyes go to read

Question 19

where is near vision point?

Answer 19

8-10mm below

2mm in - to do with convergence

Question 20

what is 1.00DS of anisometropia equivalent to ?

Answer 20

for every 1.00DS anisometropia there will be 1 prism dioptre of differential prism

Question 21

how can we solve problems of anisometropia in single vision lenses ?

Answer 21

• anisokonia - size lenses/isogonal lenses
• differential prism -
• sep pairs

-franklin split - 2 half lenses cemented together- held in frame together
. information in top half and bottom half of lens are different

-bonded plano prism- stick an extra bit of lens onto the point where px is looking through
. can put extra power or prism

• slab off - way of working prismatic effect into one half of the lens
• we can remove or add prism into parts of lens

Question 22

what are size lenses?

Answer 22

• they only alter magnification

- these lenses have a back surface power of 0.00DS( they do not change the prescription )

Question 23

when can we use a size lens?

Answer 23

• if there is an intolerance in the variation in spectacle magnification between the two eyes then we can dispense a size lens

Question 24

what are we considering for size lens ?

Answer 24

• for size lens we consider only the shape factor

Question 25

what is equation is used for size lenses?

Answer 25

SM = [1/(1-t/n F1)]

Question 26

how can the magnification of the shape factor for an afocal size lens be expressed?

Answer 26

• △ = -tF2 / 10n
  to see what lens we need to order we need to see if we will change the refractive index or curvature
• this can be rearranged to find the curves F2
  F2= ( -10n △ ) /t
• or to find thickness
  t= (-10 △ n ) F2

△= is prism - tells us the percentage increase the lens will give us

Question 27

what is is creating the problem with differential prism ?

Answer 27

• patient is experiencing different amounts of prism in each eye
• where the px eye are sitting