DEMO — fictional range. Local brief only, NOT SENT. No personal data is collected or transmitted.
LENS
In most markets the lens is a regulated article.
A frame is a fashion item and a lens is not. The lens carries a power, a material and coatings, and the combination is subject to a standard and to who is qualified to dispense it.
- Power
- Sphere, cylinder and axis · a prescription rather than a product choice
- Material
- Mineral or organic · weight, thickness and impact behaviour
- Coating
- Hard coat, anti-reflection, hydrophobic · layered, and each layer can fail
- Tint
- Fixed, gradient or photochromic · a category, not a colour
- Impact
- Which standard the lens has to pass, and for which use
- Centration
- Where the optical centre sits, which depends on the fit
Illustrative lens record
| Property | Set by | What changes it |
|---|---|---|
| Thickness | Power and material refractive index | A higher index makes a thinner lens |
| Impact | The standard for the intended use | Material and thickness together |
| Centration | The fit: lens width, bridge and tilt | Moving the frame on the face moves the centre |
| Coating life | How the lens is cleaned and stored | Grit in a case lining, or a dry cloth |
Why centration depends on the frame.
Because the optical centre has to sit in front of the pupil, and where the pupil is depends on how the frame sits. The same prescription in two frames is two different lenses.
The question worth asking instead.
Which standard the lens is dispensed to, and who is qualified to sign it off. A frame can be sold by anyone; a lens usually cannot.