The monofocal lens: the benchmark for optical quality

The monofocal lens: the benchmark for optical quality

Among the types of intraocular lenses, the monofocal lens is the most established and, still today, the one that offers the best image quality. It is not the “entry-level” lens sometimes thought to be: it is the starting choice in many cases and, in some eyes, simply the best option available. In this article I explain how it works, what it offers and what it asks in return, and I go into a detail that makes real differences between models: asphericity.

How a monofocal lens works

A monofocal lens has a single focal point. We usually calculate it for distance vision, so that the patient sees the street, the television or the road clearly without glasses, and uses reading glasses for near work. All the light that enters is concentrated on that single focus, without being split across several distances as multifocal lenses do.

That is precisely its optical advantage: by not dividing the light, it uses all the luminous energy for a single image.

Advantages

• Maximum image and contrast quality. It is the lens with the best contrast sensitivity, especially in low light.
• Almost no night-time phenomena. It produces virtually no halos or glare, which makes it very comfortable for night driving.
• Predictable result. It is the most stable design, with the fewest refractive surprises.
• Suitable in eyes with pathology. When there is retinal disease, advanced glaucoma, an irregular cornea or other conditions that make diffractive lenses inadvisable, the monofocal remains a safe option.

Disadvantages

There is only one trade-off: it focuses at a single distance. If we calculate it for distance, glasses are needed for reading and, to a large extent, also for the intermediate distance (the computer). It does not provide freedom from glasses for near vision.

Who are they the first choice for?

The monofocal lens is the starting choice for several profiles:

• Those who prioritise image quality over freedom from glasses.
• Night or professional drivers, and people who are very active at night.
• Eyes with associated pathology: macular degeneration or other retinal diseases, advanced glaucoma, irregular cornea, severe dry eye or amblyopia. In these cases diffractive lenses are inadvisable and the monofocal is what is indicated.
• Patients who do not mind wearing reading glasses, or who already use them regularly.
• When we are looking for the most predictable result possible.

Asphericity: why not all monofocals correct the same

Here is the detail that sets one model apart from another.

Let’s start with the intuitive part. An ideal lens would gather all the light rays at a single perfect point. But the real cornea is not ideal: it bends the light entering through its central zone slightly differently from the light entering at the edges. As a result, those rays do not meet at exactly the same point and the image loses a little sharpness. We call that small mismatch spherical aberration.

During the day it is barely noticeable: in bright light the pupil narrows and the eye uses only the central part of the cornea, the “cleanest” one. The problem appears at night, when the pupil dilates to capture more light and starts to use the edges as well; that is when this aberration reduces contrast and contributes to the sense of poorer night vision.

In a young eye there is a natural balance: the crystalline lens is designed to cancel out precisely that corneal distortion, and the whole system works almost like a perfect lens. When we operate on cataracts we remove that lens, so compensating —or not— for this aberration becomes the job of the lens we implant. And that is where each model’s design makes the difference.

In more precise terms, the human cornea has on average a positive spherical aberration of approximately +0.27 microns (measured over a 6 mm pupil). Depending on how much of that aberration it compensates for, each modern monofocal adopts a different asphericity profile:

• Aberration-correcting aspheric (negative aberration): they carry a negative spherical aberration that compensates for the cornea’s and aims to leave the eye close to zero, improving contrast especially at night, when the pupil dilates. The J&J TECNIS Monofocal (PCB00) almost fully compensates the average cornea (about −0.27 µm). The HOYA Vivinex XC1 and the Alcon Clareon correct somewhat less (around −0.18 / −0.20 µm), leaving a small positive residual that is more forgiving.
• Neutral aspheric (aberration-free): they neither add nor subtract aberration; the eye keeps that of its own cornea. The ZEISS Asphina 404 is a good example. They are the option of choice when we want to avoid decentration problems.
• Conventional spherical: they add positive aberration. Today they are practically out of use in quality surgery.

Matching the lens to the cornea

The interesting part is choosing the profile according to each patient’s cornea, and eyes that have previously undergone refractive surgery are the clearest example:

• After myopic LASIK, the cornea becomes flatter in the centre and its positive spherical aberration increases. In these eyes a strongly correcting lens, such as the TECNIS, is useful to offset that excess.
• After hyperopic LASIK the opposite happens: the cornea becomes more curved in the centre and its spherical aberration turns negative. Here a correcting lens would overcorrect, so a neutral lens (or even a spherical one) is preferable, to avoid pushing the eye into excessive negative aberration.

Eyes at risk of decentration: neutral is better

Correcting lenses only deliver their benefit if they remain well centred and aligned with the cornea. If the lens decentres or tilts —because of compromised capsular support, zonular weakness, sulcus placement, very long eyes or vitrectomised eyes—, a negative lens introduces new aberrations (coma) that degrade vision more than they improve it. A neutral aspheric lens, by not adding aberrations of its own, tolerates that decentration much better. That is why, when I anticipate that the lens may not sit perfectly centred, I prefer a neutral one such as the Asphina over a correcting one.

In summary

The monofocal lens is not a “basic” starting point but a precision tool: well chosen —the right asphericity profile for each cornea and each situation— it offers a visual quality that is hard to beat. Freedom from glasses for near vision is its limit; sharpness and safety are its great strength. Which lens is the right one is always decided after an individualised assessment.