L12 Frames and Vision

Question 1

Box System of Measurements

Answer 1

• 2 sets of number used
• horizontal lens size + bridge size = box centre
• bridge size is the distance between both lens frames

Question 2

Effective Lens diameter

Answer 2

Distance from centre of lens to widest part of lens multiplied by 2

Question 3

Field of View

Answer 3

=total area visible through the lens when the eye is in its primary position

• positive lens means FOV is less
• negative lens increases FOV

Question 4

Apparent FOV

Answer 4

Angle subtended by the empty frame aperture at the eyes centre of rotation

Question 5

Real FOV

Answer 5

FOV obtained when a spectacle lens is glazed into the frame

Question 6

FOV is influenced by:

Answer 6

• Frame and therefore lens shape
• vertex distance
• power of the lens (real FOV only)

Question 7

Near Vision Effectivity Error

Answer 7

-mainly effects people who wears positive lens
- back vertex power of lens - vergence which leaves the lens when light originates from a distant object
near vision - light not arriving from a distant object but at a known distance from the lens
-vergence leaving the lens at near also depends on thickness and form of lens
- lenses with same back vertex power but of diff forms are not interchangeable
NVRE = diff bet the image vergence produced by a thick lens at near and the expected vergence from a thin lens

Question 8

Lens Mountings

Answer 8

can be:

• full rim
• rimless
• supra

Question 9

Lens edge

Answer 9

will be different depending on the frame mounting type, to allow the lens to remain securely in the frame:

• bevel = with a high minus lens can ask for the bevel to be pushed towards the front so any thickness is pushed towards the back so its less noticeable for people looking at the px
• supra = (with groove for cord) Some materials are better than others eg polycarbonate, lens will be thicker due to a required min thickness (1.8mm), lens is unprotected at the bottom, can chip easily
• flat = used on rimless frames, strong materials - need a hole drilling in (polycarbonate - high impact resistance)
• mini bevel

Question 10

Frame Bridge

Answer 10

= the bridge forms the main connection bet the right and left rims. It may or may not have pads
- diff designs for plastic and metal frame s

Question 11

Frame Pads

Answer 11

=extensions or attachments to the bridge which form the bearing surface of the frame for the nose
-pads for a fixed pad bridge frame are moulded into the rim and are the same material as the frame itself

Question 12

Bridge types

Answer 12

Fixed pad bridge - plastic frames only, moulded pad shapes

Regular bridge - plastics frames only, no moulded pad shapes

Question 13

In Metal frames, pads are:

Answer 13

pads are on arms

• pads on arms can be screw in or push in
• made from silicone (flexible lightweight, soft) or vinyl (lightweight, rigid, cheaper)
• can be saddle/comfort/strap bridge with same material and attachment

Question 14

Sides

Answer 14

can be drop end, loop end, curl sides