Types of lens: a guide to what you are being offered

When somebody orders a pair of glasses, they are really making three decisions in a row: how many distances the lens has to cover, what material it is made of and what coatings go on top. Almost always only the third gets explained, because it is the one on display in the window.

This guide covers all three, in order, and in plain English.

Decision 1: how many distances

Single vision

One power across the whole lens. It is the simplest lens and, within its own territory, the one that sees best. The entire lens is optimised for one distance.

It works if you only need correcting for distance, or only for near. And it remains the best answer for fixed tasks: sewing, reading, working with your hands at one particular distance. Many people with presbyopia who struggle with progressives for reading would be better off with a well-calculated pair of single-vision readers.

Bifocal

Two zones separated by a visible line: distance on top, near below. There is no transition.

They are rarely prescribed today, but they have two real advantages. The near zone is wide and always in the same place, with no corridor to hunt for. And the brain has nothing to learn. For an older person who has never adapted to a progressive, and who only alternates between reading and looking straight ahead, it is still a defensible option. What it does not cover is the intermediate distance: the screen and the metre and a half are left out.

Progressive

The power changes continuously from top to bottom, with no lines. It covers distance, intermediate and near with a single pair.

It is the lens of everyday life. In exchange, the intermediate corridor is narrow and at the sides there are zones of distortion that the brain learns to ignore. That is why with progressives you move your head, not your eyes.

One point nobody makes at the counter: not all progressives are the same. They differ in the width of the corridor, the smoothness of the transition, and how far the lens is calculated with your specific fitting measurements rather than a standard design. That is where most of the price difference lies between two progressives that look identical to you.

Occupational

It gives up distance vision and uses that space to widen the intermediate and the near. It is the lens of the desk, the workshop, the music stand.

With them the screen is sharp from side to side and your chin stays where it should be. With them you do not drive and you do not go out in the street.

Decision 2: the material and the index

What the lens is made of

• Standard plastic (CR-39). The classic. Good optical quality, cheap, somewhat thick in high prescriptions and easier to scratch if it has no hard coating.
• Polycarbonate. Very impact-resistant. The American Optometric Association recommends it alongside Trivex for sports and hazard eyewear. It is the sensible option for children and for manual work.
• Trivex. Resistance similar to polycarbonate with better sharpness. It costs more.
• Glass. The best optical quality and the best behaviour against scratches, but it is heavy and it can break into fragments. It is little used today and there are frames that will not take it.

What the refractive index is

The index measures how much the material bends light. The higher it is, the less thickness is needed to achieve the same power. The usual values are 1.5, 1.6, 1.67 and 1.74.

Here lies the most expensive misunderstanding in an optician’s. The index is not a scale of quality. It is a scale of thickness.

In a low prescription, going up in index barely changes anything visible: the lens was already thin. You are paying for tenths of a millimetre. In a high myopia or hyperopia, it does change the look of the glasses and the weight on your nose.

And the index is not the only thing that decides thickness:

• The prescription. It is the main factor, by a long way.
• The size of the frame. A large frame forces the lens to be cut far from the centre, where the edge is thicker. Going down from a wide frame to a smaller one thins the edge more than going up two steps of index.
• The shape. A round or oval frame makes better use of the lens blank than a very wide rectangular one.
• The mounting. In a rimless or nylon-cord frame the edge is on show; in an acetate one it is hidden.

An honest detail about high indices: the higher the index, the more the material tends to split light into colours at the periphery of the lens. Some people perceive it as a slight coloured halo when looking through the edge. It does not happen to everybody and it is not a manufacturing fault, but it is better to know beforehand than afterwards.

Decision 3: the coatings

Anti-reflection

It is the only one we recommend virtually always.

An untreated lens reflects part of the light reaching it. Those reflections translate into haloes around headlights when driving at night, into glints that hide your eyes in photographs, and into slightly less contrast all day long. Anti-reflection removes them almost entirely.

They are not all the same. The good ones include a water-repellent layer and are far easier to clean. The cheap ones craze after a while, and then the glasses look worse than with no coating at all.

Hard coating

A layer that increases scratch resistance. On plastic lenses it is all but obligatory, and these days it usually comes as standard. No plastic lens is unscratchable, whoever tells you otherwise.

Water- and grease-repellent

They repel water and grease. They sound like a dispensable extra and on the coast they are not: they reduce the mark left by salt spray and sun cream, and they make the lens clean in one wipe instead of three. Every extra clean is a chance to scratch the lens.

Photochromic

The lens darkens with ultraviolet radiation and clears again indoors. It is convenient, it saves carrying two pairs and it gives permanent ultraviolet protection.

It has two limits worth knowing before ordering. Inside a car they darken little or not at all, because the windscreen already filters the ultraviolet that activates the lens, except in models made specifically for driving. And in cold weather they take longer to clear. Here, with our climate, that second limit is barely noticeable.

Polarised

It absorbs light reflected off horizontal surfaces: the sea, wet tarmac, a car bonnet. On the Costa Cálida it is the coating people are most grateful for, especially driving west along the coast road or fishing.

Two warnings. First: polarising and filtering ultraviolet are different things, you have to insist on both. Second: polarised lenses make some screens and some car dashboards hard to read.

Tint

A fixed tint provides no ultraviolet protection in itself. The degree of protection depends on the lens material, not on the colour. Brown and grey tints are the ones that best respect colour perception; blues and reds worsen contrast sensitivity.

What to ask before paying

Four questions. If they answer all four, you are in good hands.

1. “How much thinner is it with the index you are proposing, in millimetres?” It is a specific figure the optician can calculate. If the difference is half a millimetre, save yourself the money.
2. “Is this progressive calculated with my fitting measurements or is it a standard design?”
3. “What exactly does the anti-reflection coating include and what guarantee does it carry?”
4. “If I cannot adapt, what happens?” Ask for the answer in writing on the quotation.

And one more thing

The best lens mounted in a frame that opens out, that slips or that rests badly stops being the best lens. The prescription is calculated for a particular position of the eye relative to the lens, and that position is set by the frame.

That is why it is worth coming back to have them adjusted every few months, even if nothing needs changing. It takes five minutes and it is what makes a pair of glasses last three good years instead of one bad one. In Puerto de Mazarrón you can drop in and have it done without an appointment.