Weight & Thickness of Lenses

Identify the plus and minus lenses

Plus lenses are thickest in the centre and get thinner towards the edge

Minus lenses are thin in the centre and get thicker towards the edge

PLUS lenses

MINUS lenses

Frames – fitting

HCL

Properties of a well fitting frame:• Appears straight on face• Well fitting on the nose• A large enough size so that person does not aware of edges of frame• Eyes fairly well centred

– horizontally– vertically (usual for eyes to be a little higher in frame)

• Lenses frame not too far away from the eyes• well fitting behind ears

Frames – fitting

HCL

Frame types

Metal OR Plastic

Full frameRimless framehalf-rimless frame

1. Too narrow for face, bending back at hinge to get to ears2. Eye size of frame not big enough for this person, sitting

quite high3. Frame sitting slightly tilted on face4. Coming up off the ear on second photo

Frame too small

1

2

3

1. Pupils very high in this frame2. Eye size of frame too big/deep for this person 3. Again, frame slightly tilted on face4. Actually quite good width overall

Frame too big

12

3

1. Frame good width and shape on bridge of nose2. Eye size of frame quite deep, but person has

varifocal lenses, and frame has enough height above pupils

3. Frame level

Well fitting Frame 1

2

3

1. Frame good width on face 2. Frame eye size has sufficient depth & is

good size over bridge of nose3. Frame level – but facial asymmetry - Right

eye higher than left4. Good fit over ear & appropriate tilt of

frame on second photo

Well fitting Frame 2

1

3

4

#1 Spher

ical Part

Myopia Short-sighted Minus numberse.g. -2.50D,

-6.25D

Moderate values +/-2.00 to 4.00D

High valuesGreater than

+/-5.00Hypermetropiaor Hyperopia

Long-sighted Plus numbers e.g. +1.75D,

+5.75D

#2 Cylindrical part

Astigmatism Oval or ‘rugby ball’ shaped eye

(cornea)

Cylindrical ortoric lenses

Number & Axiswritten after

‘Spherical’ part

Moderate value / -1.50DC *α

High values greater than

/ -2.50DC *α

R +2.25 / -2.00 * 180L +1.75 / -1.50 * 170

Spheric

al

Cylindric

al

Spectacle PrescriptionsAnalysing a spectacle prescription1. Is it a myopic, hyperopic and/or

astigmatic prescription?2. Is it a moderate/high prescription?

4 Spectacle PrescriptionsLens Materials

•Ideally we want a lens to be light, thin and clear with little reflectance and that they are robust

•Nowadays, lenses are mostly made from Plastic material. This is safer and lighter than glass

•Basic lens material is called CR39

•Lenses can be made thinner and lighter if they are made out of denser materials. These are called ‘high-index’ materials. There are a variety of types. They have slightly different properties.

•If prescription is moderate/high, advantageous that lenses are made from high-index material. Lenses will be thinner and lighter.