W4 Multifocals

What are problems with bifocals?

• Poor control of near optical centration- when you are looking into the segment you aren’t looking through the optical centre due to induced prism- image may be degraded (off axis aberrations). However if no anisometropia no problem

• Sudden jump in image position as the eyes travel across the segment divide (particularly round segment)- this can be eliminated in vertical direction with base down prism

• Poor visual acuity in segment- due to excess induced prism OR wavy interface between lens material

• Chromatic and monochromatic aberrations in segment (not looking through the optical centre, looking off axis)

What is the optical centre?

The point at which there’s no prismatic effect

What happens when you’re not looking through the optical centre?

Image is degraded by off axis aberrations which increase with increased induced prism

What is the near vision point (NVP?)

The point at which the wearer is looking through when using the bifocals to look at near objects

What is the near optical centre (On?)

Optical distance of the combined distance and segment prescription- the point at which there is no prism when the 2 sections of the bifocal are combined (theoretical as the near optical centre doesn’t extend for enough)

What does Y represent?

Separation between the optical centres of the main lens and the segment

What does X represent?

Separation between the optical centre of the main lens and the near optical centre

How do you calculate prism due to distance portion?

XF where F is the distance Rx

How do you calculate prism due to segment?

(Y-X)A where A is the addition (power)

What does c represent in P=Fc

In prentice’s rule c is the distance from the optical centre in cm

How do you get the prism to cancel out?

X = YA / (F+A)

A AND Y ARE ALWAYS POSITIVE

If X is positive or negative what does it mean?

• If X is positive the near optical centre lies below optical centre

• If X is negative, the near optical centre lies above the optical centre- not in the near portion

What does a positive distance give at the near vision point?

The eyes converge when we look down in a bifocal lens as we’re looking at a near object- NVP is always down and in from the optical centre so we get base up and base out

What does a negative distance give at the near vision point?

What does a negative distance give at the NVP?

Base down and base in- NVP will always be down and in for bifocals

What does a positive prescription near portion give in a round segment?

We get base up at the NVP but base down due to segment- due to the distance portion some induced prism is cancelled out- beneficial for Hyperopes.

What does a negative prescription near portion give in a round segment?

Negative distance gives a base down which gives more base down due to the distance portion, round segs therefore increase the induced prism at the NVP

Which segment should a myope use?

D segments tend to add less to the prismatic effect for myopes or may even reduce it

What’s the base direction in a D segment?

• If NVP above segment centre it’s base down

• If NVP below segment centre it’s base up

• If depth is equal to radius we will always get base up at NVP due to segment

How do we eliminate the jump?

Working prism on the seg which is equal and opposite (base up) to the jump

Why is there less jump in a D segment?

Segment centre is closer to segment top

How do you get a no jump on an E segment bifocal?

Segment centre must be on the dividing line- no overall change in sudden prismatic effect

What is constringence?

Abbé number

How can you calculate TCA?

NVP / Constringence (V)

Where is TCA higher?

Higher index materials with low V values

Where is there a large degree of prism at the NVP?

High myopes with a large round segment and high refractive index

Is induced prism always a problem?

Often not as the wearer doesn’t notice the reduced image quality- however in an anisometropic px it can be worse

Why is induced prism worse in a px with anisometropia?

Prism difference between the 2 lenses, differential prism can cause problems, (asthenopia, blur, eye diplopia), which is a problem for differential vertical prism

Whats a prism control bifocal?

Prism is either at distant only, near only, or near and distant have different prisms

What bifocal has the same prism distant and near?

Conventional bifocals

What do centre controlled bifocals do?

• Neutralises prism at NVP- px will be looking through optical centre

• Base-in prism included for poorly controlled near exophoria or convergence insuffiency

• Eliminates differential prism induced at NVP

• Putting prism on segment moves segment optical centre on segment dividing line to avoid bifocal jump

What is a presto bifocal?

A bifocal segment is a separate lens which is glued into a hole cut in the distance lens- better cosmetically than Franklin’s but more visible than conventional bifocals

What are features of unequal segment sizes?

• Larger segment size is more base down

• Larger segment size in the right (most +ve power)

• Larger segment with the most plus/least minus in the vertical meridian

• More + gives base up and larger segment gives base down

Whats the biprism lens used for?

The eye with the least plus or the most minus as more minus gives more base down