Choosing glasses

Two separate decisions

Buying glasses means choosing lenses and choosing a frame, and the two are usually discussed in the opposite proportion to how much they matter. The frame is what gets looked at in the mirror; the lenses determine whether the glasses are comfortable, how thick they end up, and what they cost.

Both start from the prescription, which comes from a sight test. The prescription belongs to the person tested, not the practice, and it can be taken elsewhere — including to an online retailer, which has its own considerations.

Lens material and thickness

A stronger prescription needs more lens power, and the straightforward way to achieve that is a thicker lens. The alternative is a material that bends light more for the same thickness, described by its refractive index. The higher the index, the thinner the lens for a given prescription — and the more it costs.

Whether that is worth paying for depends on the prescription. For a mild one the difference is barely visible and the standard material is fine. For a strong one it is the difference between glasses that look and feel normal and glasses that are heavy and obviously thick at the edges. A smaller frame also helps considerably, because less lens means less edge thickness, which is one reason the frame and lens decisions are not really independent.

Polycarbonate is a different proposition: thinner and lighter than standard plastic and far more impact resistant, which is why it is the usual choice for children, for sports and for anyone working where something might hit the eye. It scratches easily, so it is normally supplied with a hard coating.

Coatings

Anti-reflection coating is the one most worth having. It reduces the reflections from the lens surfaces that cause ghosting around lights, the visible glare off the lens in photographs, and some of the strain from oncoming headlights and bright screens. It reduces rather than eliminates them.

Scratch-resistant coating is close to standard on plastic lenses and does not make them scratch-proof.

Coatings marketed for filtering blue light from screens are sold heavily. The evidence that they reduce eye strain or protect the eye is weak, and the ordinary causes of screen discomfort — how long, how close, how dry the eyes get — matter considerably more. Those are covered under digital eye strain.

Reading and distance in one pair

From the mid-forties, presbyopia means near focus no longer looks after itself, and the question becomes how to cover both distances.

Separate pairs is the simplest answer and often the best one, particularly for someone who only needs help close up.

Bifocals have two zones with a visible dividing line: distance above, near below. They are unfashionable but they work, and the line that makes them obvious is also what makes them easy to adapt to.

Varifocals, also called progressives, blend from distance at the top through an intermediate zone to near at the bottom, with no visible line. The trade-off is that the blending narrows the usable field at each distance, so there is a period of learning to point the head rather than the eyes, and some peripheral softness that most people stop noticing. A proportion never get on with them. It is worth establishing before ordering what happens if that turns out to be the case.

Occupational lenses are a less familiar option worth asking about: lenses weighted for a particular working distance, such as a screen at arm's length and a desk below it, rather than for driving. These genuinely are task-specific and not for general wear — which is a distinction older writing tends to blur, sometimes describing all multifocals that way. Bifocals and varifocals are everyday glasses.

The equivalent choices in contact lenses are covered under bifocal contact lenses.

Tints, photochromics and polarisation

These three are often lumped together and do different things.

A tint is permanent, solid or graduated, and is as much a style choice as a functional one.

Photochromic lenses darken in sunlight and clear indoors, which saves carrying a second pair. Two caveats are worth knowing: they respond to ultraviolet light, so they darken poorly behind a car windscreen, which blocks much of it; and they take time to clear, which can be a nuisance going indoors.

Polarisation is the one most often misdescribed. A polarising filter cuts the glare reflected off flat surfaces — water, wet roads, snow, car bonnets. That is what it does. It is not what provides ultraviolet protection, which is a separate property of the lens material or coating; most sunglasses offer both, but polarisation and UV protection are not the same feature and a lens can have one without the other.

For anything worn in sun, the thing to confirm is the UV protection specifically, since that is the part that matters for the long-term health of the eye.

Frames: the part that is a health question

Most of what is written about frame materials is about weight, durability and looks. There is one genuine health consideration in it.

Nickel is the commonest cause of allergic contact dermatitis, and spectacle frames sit against the skin of the nose, temples and behind the ears for the whole day. Anyone who reacts to cheap jewellery or belt buckles should say so when choosing frames. Nickel-free options exist and are worth asking for by name: titanium, pure or near-pure, and nickel-free stainless steel among the metals, and acetate among the plastics.

Otherwise, the material trade-offs are straightforward. Titanium is light, strong and corrosion resistant, and costs more. Stainless steel is light and affordable, and its nickel content varies. Aluminium is light and takes colour well. Memory alloys, sold under various proprietary names, flex and return to shape, which suits anyone who sits on their glasses. Acetate — the standard plastic, also called zyl — is light and inexpensive and comes in the widest range of colours and patterns, but is more brittle and can weaken with heat, so it should not be left on a dashboard.

Older writing sometimes describes beryllium as a titanium alloy. It is not; it is a separate element, used in its own alloys, and corrosion resistant.

Fit matters more than style

A frame that does not fit is uncomfortable whatever it looks like, and with a strong or varifocal prescription a poor fit also degrades the vision, because the optical centre of the lens has to sit in front of the pupil.

What to check: the frame should sit level, the bridge should carry the weight without pinching or sliding, the sides should follow the head without gripping behind the ears, and the eyes should sit roughly in the centre of the lens rather than high or towards the edge. Varifocals in particular need adequate lens depth, which rules out some very shallow frames.

A dispensing optician adjusting a frame properly makes more difference to how glasses feel than any material does.

Style, honestly

There is a long-standing convention in opticians' shops that frame shapes should contrast with face shapes — angular frames on a rounder face, softer frames on a squarer one — and that frame colours should be chosen against skin tone. It is a reasonable starting point and nothing more than that. It is taste rather than fact, and it changes with fashion.

The more useful advice is practical: look at them in natural light, take a photograph rather than relying on a mirror, bring someone whose judgement is trusted, and if two pairs are affordable, two different pairs are more useful than two of the same.

What to ask

What index is being recommended and what visible difference it makes to this prescription; which coatings are included and which are extra; whether varifocals can be exchanged if they do not work out, and within what period; whether a nickel-free frame is needed; and what the total comes to with the lenses in the frame rather than the frame price on the display.

Whether glasses are the right answer at all is covered under contact lenses or glasses, and the lens equivalents under choosing contact lenses.