4. OPTICAL CONSIDERATIONS WITH INCREASING LENS POWER

When dispensing complex or high-powered lenses which points must be considered

1. the prescription and vertex distance

2. lens type, form and material

3. field of view

4. lens thickness

5. reflections and surface treatments

6. frame selection and fitting

7. centration

Define vertex distance

the distance from the corneal apex to the visual point on the lens

what is back vertex distance

distance in millimetres between the front of your eye and the lens of your glasses

Define effective power

the power of the lens that reaches the eye

As a lens is moved away from the eye (increaing vertex distance) what happens to the effective power

becomes more positive

As lenses are moved closer to the eye (decreasing the vertex distant) what happens to the effective power

the effective power becomes more negative

what is the formula for effective power when the vertex distance is moving further away from the eye

spectacle power (Fsp) / 1+ distance in vertex distance x spectacle power

what is the formula for effective power when the vertex distance is moving closer to the eye

spectacle power (Fsp) / 1- distance in vertex distance x spectacle power

Define lenticular lenses

lenses that show different parts of an image when viewed from different angles

Define solid lenticular lenses

a lens that has a carrier convex slope

Define a plano lenticular lens

the carrier is plano and the aperture is either convex or concave

define a cemented lenticular lens

The aperture is a sphere that is cemented onto a carrier lens that incorporates the cylinder

Define a fused lenticular lens

The aperture is manufactured onto the back surface of a plus lens and is filled with a higher refractive index glass and heated. The front surface is manufactured to reduce the same amount of power. Like a fused bifocal

what type of lens is thick at the edge, yet thin in the middle

minus lens (concave)

what type of lens is thick in the centre, and thin at the edge

a plus prescriptions lens (convex)

what are the three factors which are considered when selecting and fitting a frame

1. fitting

2. eyelashes

3. centration