Bifocal contact lenses

What these lenses are for

From the mid-forties onwards the lens inside the eye loses enough flexibility that close focus fails. This is presbyopia, it happens to everyone, and correcting it means supplying the near focus the eye can no longer produce.

Spectacles do that with reading glasses, bifocals or varifocals. Contact lenses do it by methods that have no spectacle equivalent, and this article covers those.

Bifocal, multifocal, and the words that get muddled

Strictly, a bifocal lens carries two distinct powers, one for distance and one for near, with a boundary between them. A multifocal lens carries a gradual change in power, or several zones, giving intermediate distances as well as the two extremes.

In practice the terms are used loosely, including by manufacturers. Most soft presbyopic contact lenses sold today are multifocal rather than strictly bifocal, because the gradual design copes better with the middle distances — a computer screen, a dashboard, a price label on a shelf — which a two-power lens handles badly.

Anyone searching for “bifocal contact lenses” will find the whole category, and that is the sensible way to read the term here.

The two ways a lens can do it

There are two genuine approaches, distinguished by whether the lens moves on the eye.

Simultaneous vision puts the distance and near powers in front of the pupil at the same time. Light from both reaches the retina together, and the visual system learns to attend to whichever image is sharp and disregard the other. The zones can be arranged as concentric rings — centre-near or centre-distance — or as a smooth aspheric change in power from the middle of the lens outwards. Concentric and aspheric are therefore variations on simultaneous vision rather than alternatives to it.

Alternating vision, also called translating, works like bifocal spectacles. The distance portion sits in front of the pupil in normal straight-ahead gaze. On looking down to read, the lower eyelid catches the lower edge of the lens and pushes it up, bringing the near segment over the pupil. The lens has to be weighted or shaped to stay in the right orientation, since a lens that rotates freely would put the segment in the wrong place.

These are not freely interchangeable with lens material. Simultaneous designs are available in soft lenses, including daily disposables, and in rigid gas permeable. Alternating designs are essentially a rigid gas permeable option: soft translating lenses exist but are not widely used, because a soft lens couples to the eyelid too closely to move reliably.

So in most cases, someone asking an optometrist for presbyopic contact lenses is being offered a soft simultaneous-vision multifocal, and the alternating design only enters the conversation if rigid lenses are on the table.

What simultaneous vision costs

This is the part usually left out, and it explains most of the disappointment.

If two images are present at once and only one is in focus, the out-of-focus light is still arriving. It overlays the sharp image and reduces contrast. The result is vision that is usually perfectly workable but slightly less crisp than a single-focus correction, and the difference is most obvious in poor light, where the pupil is larger and more of the lens is in play.

The reported problems follow from that: ghosting, flare, and haloes around bright lights. Centre-distance designs in particular can throw a halo around distance images, which is at its most unwelcome facing oncoming headlights. These visual effects, rather than discomfort or handling, are the main reason people abandon multifocal lenses.

Performance also depends on pupil size, which varies between people and changes with light levels and age. A design that works well for one person can be unusable for another with the same prescription.

None of this makes them a bad option. It makes them a trade: a wider field of clear vision and no spectacles on the face, in exchange for a degree of crispness and some difficulty at night. It is worth knowing that on measured visual performance, varifocal spectacles do better than soft multifocal lenses — so the choice is not between a modern answer and an old one.

Monovision, the other route

The alternative does not use a multifocal lens at all. One eye is corrected for distance, the other for near, with ordinary single-vision lenses, and the brain learns to use whichever eye is giving the sharper image.

It sounds as though it should not work, and for a substantial number of people it works well. It is quick and straightforward to fit and generally cheaper.

The cost is depth perception. Using the two eyes on different focal distances reduces stereoacuity, and the reduction grows with the strength of the near correction. For most daily activity this goes unnoticed; for anything where judging distance matters, including night driving, it can.

Comparisons do not give a clean verdict. Multifocals often perform better overall, and the loss of stereopsis is the most likely reason. But some studies find better distance acuity with monovision, and preferences divide: in one comparison roughly a third of participants preferred monovision to the multifocal designs tested. Success rates are high among people already used to contact lenses and markedly lower among those who have never worn them.

A frequent compromise is modified monovision, combining a multifocal in one eye with a different correction in the other, which tends to report higher satisfaction than either approach alone.

What to expect from a trial

Three expectations are worth setting before the first appointment.

It may take more than one attempt. Designs differ considerably, and someone who gets on badly with one may do well with another from a different manufacturer. Finding the right one is trial rather than calculation.

Adaptation is not guaranteed. The common reassurance is that the eyes adjust within a few weeks. Evidence for genuine adaptation to simultaneous vision is weaker than that suggests. Visual complaints are more noticeable in the first week and may settle somewhat, but someone who still dislikes the vision after a fair trial is unlikely to be won round by persistence.

Reading glasses may still appear. Plenty of people wear multifocal lenses and keep a cheap pair of readers for small print in bad light. That is a normal arrangement and a perfectly good outcome.

Not getting on with these lenses is not a failure, and it is not a sign of being fussy. Pupil size, visual demands and how much optical irregularity a given visual system will tolerate all vary genuinely between people. The spectacle alternatives are perfectly respectable, and glasses and contact lenses each do things the other cannot — almost everyone who wears lenses needs a current pair of glasses regardless.

Getting fitted, and the overnight question

A contact lens fitting is a separate appointment from a sight test, and a spectacle prescription is not sufficient to supply lenses. Fitting measures the curvature and diameter of the cornea, assesses the tear film, trials a lens to see how it sits and moves, and teaches handling. For presbyopic lenses the trial stage matters more than usual, because the vision has to be judged in real conditions rather than across a consulting room.

In the UK contact lenses may only be supplied by or under the supervision of a registered optometrist, dispensing optician or medical practitioner — a requirement that applies to every kind of contact lens, corrective or not.

One older option deserves a flat warning. Lenses sold for continuous or extended wear, kept in overnight, are no longer regarded as a routine choice. Sleeping in contact lenses is among the clearest avoidable risk factors for microbial keratitis, the corneal infection that can scar the eye and permanently affect sight. Daily disposables have become the default recommendation for most wearers, and presbyopic designs are widely available in that form.

Dry eye is also worth mentioning, because it becomes more common from the same age presbyopia arrives, and it makes contact lens wear less comfortable. Anyone whose eyes are already dry should raise it at the fitting rather than discover the problem afterwards.