Ocular refraction questions

Question 1

radius along 30. = +5.475

radius along 20 = +5.125
axial length = 23.75mm

calculate K (ocular refraction)

fsp at 13mm

spectacle correction (BVD)
classify the astigmatism

Answer 1

step 1 - workout f eye along each meridian

f eye = n eye - 1 / r1 = 1.33-1 / r1

f eye - n eye -1 / r2 = 1.33- 1 / r2

calculate axial length in diopters =

n eye / axial length

1.33/ 0.02375 (remember mm to m ) = divide by 1000

K’ = K + feye
K= K’- f eye

subtract the axial length diopters from f eye for each principle meridian

k30 =+ 56.13 - (+60.82) = -4.69 d

k 120 =+ 56.13 - (64.98) = -8.85d

then transpose = most positive of the sphere

difference between the two is the cylinder

axis of the most positive (except on the focimeter you use direction of the most negative lines)

most positive was the -4.69

so it would be transpose
to

-4.69/ -4.16x30

spectacle perception at 13mm

f’sp = k + d

need fsp we have d

change ocular refraction you just worked out into k (axial length)

k = 1/K

do that for 30 and along 120

f’sp = k + d

add k to 0.013

1/ -4.69 = -0.213

1/ -8.85= -0.113

f’sp = -0.213 + (0.013)

= -0.200

f’sp = -0.113- +0.013= -0.1

fsp = 1/ f’sp = 1/ -0.2=-5.00d

fsp = 1 / f’sp = 1 / -0.1= -10.00d

then transpose this using rules

most positive = sphere

difference between the two = cylinder

axis = axis of the most positive

= -5.00/-5.00 x 30

this is regular compound oblique myopic astigmatism

Question 2

how to classify astigmatism

Answer 2

90 degree difference between the two meridians = regular

more than that = irregular due to pathology e.g. due to kerekatonus / trauma , progressive degeneration of cornea , cannot correct with specs

treatment = hard contact lenses , it gives us a spherical correction at the cront

with or against the rule - with the rule = steepest curve = vertical negative correcting cylinder will be 180

with or against the rule are 180 - 90

with 20 added or - either side
outside of those two = oblique

Question 3

what type of astigmatism is this - +8.00/ -4.00 x 90

Answer 3

against the rule because -4.00 is on 90 if it it was on 180 it would be with the rule astigmatism

and both meridians are hyperopic so it would be compound hyperopic

Question 4

what is best vision sphere and how is it calculated

Answer 4

most positive sphere which places the circle of least confusion on the retina in the absence of accomodation

calculation = sphere + cylinder divided by 2

circle of least confusion lies 1/2 way along the astigmatic interval