Retininocopy for astigmatism

Question 1

how to transpose a power cross

Answer 1

• most positive is the sphere
• the difference between the two is the cylinder
• the axis is in the direction of the most positive meridian

Question 2

how to work out the distance correction for a ret result

Answer 2

distance correction is always more negative than the ret result

distance correction is sphere - working distance

I/2 a m is equivalent to 2 diopters

cylinder dosnt change

Question 3

what is the distance correction of this result assuming we have a working distance of 1/2 a m

+7.25 / - 4.75 x 90

Answer 3

distance correction= sphere - working distance

1/2 m = 2 diopters

distance correction

= +5.25 / -4.75 x 90

Question 4

how to perform retinoscopy for astigmatism

Answer 4

see with add +

start with the beam vertically and you move it horiziontally

so you are moving it along 180 because you are moving it side to side

if you are moving the beam and it dosnt move it tells you are not moving along one of the principle meridians

if you want to neutralise one of the Principle meridians you have to move the ret beam in the same direction as one of the principle meridians

next step = to find the orientation of the principle meridian , in order to do that you rotate the collar

change the orientation of the beam

change the direction your moving the beam as the beam changes

the orientation of the beam = the same as the orientation of the reflex

important to write the direction you are moving the beam not the orientation - if the beam is vertical but you are moving it side to side the principle meridian is horizontal

see with add +

see against add -

Question 5

how to draw a power cross

Answer 5

draw a line in the direction in the direction you are moving the beam

and you write down the strength of the lens that is giving you reversal

the other meridian will be perpendicular and then we will find the other Lense that gives reversal

the other meridian won’t be too far away so move the beam around 90 degrees - if you have a with movement add + 1ds and make it reverse

Question 6

how to transpose a power cross into a prescription

Answer 6

most positive = sphere

the difference between the two = the negative cylinder

the direction of the most positive is the negative cylinder axis

Question 7

transpose this ——

the power meridian along 130 requires -2.75 to neutralise it

the power meridian along 40 requires -4.75 to neutralise it

Answer 7

sphere = the most positive so that will be

-2.75

the difference between the sphere and the cylinder will be 2.00 so that will be the cylinder

the axis is the direction of the most positive so will be 130

therefore prescription

-2.75 2.00 x 130

for the working distance you just have to change the sphere

cylinder and axis remains constant only the sphere needs to change just

Question 8

how to get from negative to positive cylincder form

Answer 8

to get to positive cylinder form

you add the sphere to the cylinder

and change the axis by 90

cylinder and axis remain the same