spectacle magnification and differential prism Flashcards
what is spectacle magnification ?
- is the ration of the retinal image size in the corrected eye, to that in the uncorrected eye
how does a positive lens affect magnification ?
positive lens increases magnification
how does a negative lens affect magnification ?
negative lens decreases magnification
what does spectacle magnification depend on ?
. power factor=power of the lens
. shape factor=form and thickness of the lens
. BVD
.refractive index
where is thickness in positive lens ?
positive lens is thickest in the middle
what does spectacle magnification not have ?
spectacle magnification has no units
what does it mean if spectacle magnification is greater then 1 ?
- for a hyperope it is greater than 1 (magnification)
what does it mean if spectacle magnification is less than 1?
- for a myope it is less than 1 (minification)
what are ways to increase spectacle magnification ?
- increase power of F1
- increase thickness
- decrease n
why can’t we change overall power of lens to increase spectacle magnification ?
- because you will be altering patient’s prescription
what happens if you increase d(vertex distance )?
- it will make image larger
- use for hyperope
what happens if you decrease d(vertex distance)?
- it will make image smaller
- use for myope
what is equation of spectacle magnification ?
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
why is it important to calculate spectacle magnification?
- due to anisometropia which is an unequal refractive power between the two eye
what does anisometropia result in ?
- 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
what does anisometropia cause ?
- double vision
- eye strain/asthenopia
- headache
when are we mostly concerned about differential prism ?
- when a patient is reading
- because they are looking away from optical centre
what is near vision point?
- near vision point - is point where eyes go to read
where is near vision point?
8-10mm below
2mm in - to do with convergence
what is 1.00DS of anisometropia equivalent to ?
for every 1.00DS anisometropia there will be 1 prism dioptre of differential prism
how can we solve problems of anisometropia in single vision lenses ?
- 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
what are size lenses?
- they only alter magnification
- these lenses have a back surface power of 0.00DS( they do not change the prescription )
when can we use a size lens?
- if there is an intolerance in the variation in spectacle magnification between the two eyes then we can dispense a size lens
what are we considering for size lens ?
- for size lens we consider only the shape factor
what is equation is used for size lenses?
SM = [1/(1-t/n F1)]
how can the magnification of the shape factor for an afocal size lens be expressed?
- △ = -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
what is is creating the problem with differential prism ?
- patient is experiencing different amounts of prism in each eye
- where the px eye are sitting
