L4 Transposition

Question 1

Transposition

Answer 1

1) Sphere - add sphere and cyl together
2) Cyl - change the power of the cyl (change the sign)
3) Axis - change the axis of the cyl by 90

Question 2

Toric Lenses

Answer 2

Two curved surfaces, but only one is spherical. The one which corrects for astigmatism is known as toridal. Results in lenses with different thickness throughout the lens

Question 3

Bitoric Lenses

Answer 3

Inefficient, not great to look through

Question 4

Toric Prescriptions can be written in:

Answer 4

1) Sphere/cyl form
2) Cross Cyl form
3) Toric Lens form
4) Optical Cross form

Question 5

Cross Cyl to Sph/Cyl form

Answer 5

1) Take one of your cross cyl powers to represent your sphere
2) The difference between the sphere and the second cross cyl is the cylinder power
3) The axis to be used in the sph/cyl form is the one that wasnt chosen as the sphere

Question 6

Optical Cross to Sph/Cyl form

Answer 6

Sphere = take your first value as the sphere - for minus cyl chose the most positive 
Cyl = difference between the 2 values 
Axis = axis direction of the first number

Question 7

Toric Transposition (the curves)

Answer 7

Sphere curve = curve on the spherical surface (always by itself)
Base curve = lowest curve value (closest to 0) and flattest curve on the toroidal surface. Its opposite the the sphere curve, so if sphere is at the front, base will be on the back
Cross Curve = the highest curve value (furthest from 0) and steepest curve on the toroidal surface - 90 degrees to the base curve

Question 8

How are toric transpositions written?

Answer 8

• Front surface (F1) on the top - this will be convex and so positive
• Back Surface (F2) on the bottom - this will be concave and so negative
  Depending on which surface was the toric surface this would be:
• bc/cc divided by sc
  or
• sc divided by bc/cc

Question 9

Toric Transposition from Sph/Cyl form

Answer 9

Sphere curve = Sphere - base curve (DS)
Cross Curve = Cylinder = base curve (DC)
cc axis is axis from sph/cyl form
Base Curve = sphere - sphere curve (DC)
bc axis is perpendicular to cross curve axis

Question 10

Reminder for toric transposition from sph/cyl

Answer 10

• when the base curve is given in the question, transpose the prescription so that the sign of the cyl is the same as the base curve
• when the sphere curve is given , transpose the prescription so the sign of the cyl is opposite to the sign on the sphere curve