Choosing contact lenses

A prescription is not enough

Contact lenses sit on the eye, so they have to match its curvature and diameter as well as correcting the focus. That means a fitting, not simply a glasses prescription converted into lens form — and it applies equally to lenses worn for appearance with no correction in them at all.

A fitting establishes whether lenses suit the eye, which design and material to use, and how long they can comfortably be worn. It ends with a specification, and in the UK lenses are supplied against that.

Not every eye suits lenses. Dry eye, certain lid conditions and some corneal irregularities make it harder, though harder is not the same as impossible and rigid lenses sometimes succeed where soft ones fail.

Soft lenses

These account for most lenses worn. They are flexible, they drape over the cornea, and they are comfortable almost immediately, which is why they suit people who wear lenses occasionally as well as those who wear them daily.

Conventional soft lenses are hydrogels, which hold a good deal of water, and oxygen reaches the cornea mainly through that water. Their limitation is that water is not a particularly good carrier of oxygen, and a lens that dries out through the day becomes less comfortable and passes less of it.

Silicone hydrogels changed that. Silicone transmits oxygen far better than water does, so these lenses carry more oxygen with less water in them — which also makes them less prone to drying out and less hospitable to bacteria on the surface. They are now the common choice.

One claim worth resisting: better oxygen transmission is what made sleeping in lenses possible, and it does not make sleeping in them risk-free. That question is dealt with in wearing contact lenses safely, and it is the single most consequential decision in the whole subject.

Rigid gas permeable lenses

Rigid lenses are smaller than soft ones and hold their shape on the eye rather than conforming to it. That gives them two advantages. Vision is often sharper, because the lens provides its own regular front surface regardless of what the cornea underneath is doing. And because they contain almost no water, they resist deposits and last a long time.

They are far from obsolete. For keratoconus and other irregular corneas they are frequently the only thing that gives good vision, and they are used in orthokeratology for slowing short sight in children.

The catch is adaptation. Rigid lenses feel noticeable at first and take days or weeks to settle, and that tolerance is lost if they are not worn regularly — someone who wears them intermittently can find themselves repeating the adjustment each time. They suit consistent daily wearers considerably better than occasional ones.

Hard lenses

The original hard lenses were made of PMMA, a rigid plastic that transmitted no oxygen at all and limited how long they could be worn. They are genuinely obsolete and are no longer fitted.

The term causes confusion because “hard” is still used loosely for rigid gas permeable lenses, which are a different and much better proposition.

What the lens corrects

A plain spherical lens corrects short sight or long sight.

Toric lenses correct astigmatism. They have different powers in different meridians and so have to stay oriented correctly on the eye, which is managed by weighting the lens. Soft torics are now routine, which was not the case when rigid lenses were the only answer for astigmatism.

Multifocal and bifocal lenses carry more than one power to cover both distance and near, for presbyopia. They are covered in more detail under bifocal contact lenses, including the honest point that a proportion of people never get on with them.

Monovision takes a different approach to the same problem: one eye corrected for distance, the other for near, leaving the brain to favour whichever is sharper. It works well for some people and not at all for others, and it is worth trialling rather than committing to.

Some soft lenses are photochromic, darkening in bright light. They reduce glare but they cover only the cornea, so they are not a substitute for sunglasses or for the protection a wide-brimmed hat gives the skin around the eye.

How often they are replaced

This is a separate axis from material and design, and it is where most of the practical difference lies.

Daily disposables are worn once and discarded. No cleaning, no solution, no case — which removes most of the routes to infection and most of the opportunities to get the routine wrong. They suit occasional wearers, sport, travel and children particularly well.

Reusable lenses are replaced fortnightly or monthly and cleaned nightly in between. They need the full routine, properly done.

Older writing describes disposables as “maintenance-free”, which is true only of the daily kind. A fortnightly disposable needs cleaning every night like any other reusable lens; what is disposed of is the lens at the end of its life, not the routine.

Lenses worn for appearance

Coloured and decorative lenses are medical devices whatever they are sold as, and they need a fitting and a specification exactly as corrective lenses do. Buying them without one is the subject of its own article, and the short version is that an ill-fitting lens on an unexamined eye is how people lose sight over a novelty.

Making the choice

Most of this is settled at the fitting, from the measurements and from how the eye behaves with a trial lens. What is worth bringing to that appointment is the practical information an optometrist cannot measure: how often lenses will actually be worn, for how many hours, for what — sport, screen work, occasional evenings — and how realistic a nightly cleaning routine is.

That last one deserves an honest answer rather than an optimistic one. Someone who will not clean lenses reliably is better served by daily disposables than by a cheaper lens they will look after badly.

Whether lenses are the right answer at all, rather than glasses, is covered under contact lenses or glasses.