1.02 Transposition, additions

What is against the rule astigmatism

When the axis is closer to 90 (usually in older people)

What is back vertex distance

Distance between posterior surface of lens and cornea

How to transpose between positive and negative cylinder forms

1. Add sphere and cylinder to get new sphere power

2. Change sign of cylinder - keep power the same

3. Change axis by 90 degrees

Transposing from crossed cylinder to sphero cylinder

1. Cylinder = most neg - most pos

2. Axis = for negative form choose from most negative sphero

3. Sphere = choose from the other one (axis that you didnt use)

Transposing from sphero cylinder to crossed cylinder

1. One of the crosses - use the sphere and its axis from the prescription given

2. For the other cross - Add the sphere and cylinder in the prescription and the axis is 90 degrees away

What are additions and what are they used for

They are positive spherical power added to the distance prescription for near or intermediate vision to help presbyopia

What is presbyopia

Loss of accomodation so cant focus on close objects - reduces near vision

How do we add adds to the prescription

You add it to the sperical power only

Duane Hoffstetter formulae (to calculate amplitude of accomodation based on age)

Max amplitude = 25 - (0.4 x age)

Average amplitude = 18.5 - (0.3 x age)

Min amplitude = 15 - (0.25 x age)

What is amplitude of accomodation

The maximum ability of the eye to change its focus from distant to near objects by accomodation which decreases with age

Determining additons formula

Addition = working distance (Dioptres) - 0.5 x amplitude of accomodation

Why do we multiply amplitude of accomodation by 0.5 when determining additions

Because amplitude of accomodation is full power so we x0.5 for sustained periods (long time)

How to calculate range of clear vision

1. Near point in the add = sphere power / (add + amplitude of accomodation)

2. Far point in the add = sphere power / add

   These numbers give you the total range of clear vision

How to calculate SUSTAINABLE range of clear vision

1. Far point in the add = sphere power / add

2. Near point in the add = sphere power / ( add + (0.5 x amplitude of accomodation)

   These numbers give you the range