Louis Clearkin
Tel: 0044 151 604 7047 L.Clearkin@liverpool.ac.uk


Focus Zones chart

Hover your cursor over the chart to view a pop-up larger version. Click on the chart to download both the chart and a questionnaire.

(photographic simulation only, as an aid to understanding)

An example of near vision.

What the near-to-mid eye sees...

An example of fear vision.

What the mid-to-distance eye sees...

An example of near and far vision.

The effect of neuroadaptation in monovision, when the brain 'evens out' the two messages.

Key points for all patients considering lens surgery

  • There is a risk involved: lens surgery can – and sometimes does – go wrong. Patients need to understand that some problems that arise cannot be fixed.
  • None of us welcomes ageing and many people seek to overturn its effects, particularly where deteriorating vision is concerned. But… the fact that something can be done to an eye, is not in itself a good argument for doing it.
  • For most of us, presbyopia (the natural deterioration in eyesight around the age of 40) is the starting gun of middle age.
  • Laser treatment or implantable contact lenses are a “solution” to this but both can potentially store up problems for the future – particularly achieving accurate biometry on eyes that have undergone such treatments.
  • Dealing directly with the problem - ie the middle-aged lens - is the logical, and I believe the safest, route.
  • My preference for monovision (where one eye is adjusted towards near vision, the other towards the distance) as a good solution to presbyopia and in cataract surgery is receiving support at ophthalmological conferences.  This is because:
    - where treatment works, monovision has been shown to be at least as good as premium lenses
    - where treatment goes awry, the monovision route carries the lowest penalty.
Focus Zones chart
Focus zone chart
Zone 1
0.3 - 1m
Zone 2
0.75 - 1.5m
Zone 3
2 - 5m
Zone 2
5 - 30m
Zone 2
Price tags
Card games
Driving (local)
Driving (night/motorway)
Stadium sports
This chart of is helpful in determining a patient's expectations for their vision following surgery. Decide which three neighbouring zones you would want to be able to see in without the aid of spectacles. For the great majority of people this would be the mid three zones.

Lens and Cataract Surgery


Surgery to the lens of the eye is probably the most commonly-performed operation worldwide. It is safe and reliable with very high success and it has transformed the lives of many people. A very easy-to-understand explanation is also available at the end of this section.

The starting point is to consider what the normal crystalline lens does in a healthy eye. It ensures clear, sharp-focussed, aberration-free vision in almost all lighting conditions, across a wide degree of pupil dilation and for a range of activities - from reading to watching TV to playing sport. It can focus seamlessly and accurately across all distances.

This process of changing our viewing distance is termed accommodation. When we accommodate (look from the distance to near) the crystalline lens alters its shape to increase its power, simultaneously our pupils constrict increasing depth of field (the distance between the nearest and farthest objects in focus) and both eyes converge (point towards the object of regard ). To learn more about depth of field and focus, click here.

Presbyopia is  the failure of accommodation due mainly  to changes in the crystalline lens as we age. From age 40 onwards the inevitable and unavoidable deterioration in function of the lens heralds the onset of middle-age; to add insult to this injury, the term presbyopia translates as “old-man eye”.

The amplitude of accommodation at age 30 is about 14 diopters, falling to about 10 diopters at age 40, 5 diopters at age 60 and plateaus out at about 2 diopters at age 70. Read more about presbyopia here.

Reading spectacles are a convenient and risk-free solution, but one that many people now consider unpalatable and so they seek a surgical solution. Logically, any solution ought to be directed at the problem – which, in presyopia, is the crystalline lens.

For that reason I do not recommend presbyopia treatments such as implantable contact lens or laser refractive surgery. This is because the crystalline lens remains in place so there can be no permanent solution. The crystalline lens will inevitably alter with time, changing the refractive outcome and eventually lens surgery will become unavoidable and more complex because of issues resulting from the earlier surgery.

For an explanation, written in plain language, of cataracts, their treatment and the risks - please click here (31kb PDF).

Refractive Lens Exchange

This is an established option for patients who are becoming presbyopic - particularly if contact lens wear is becoming difficult.

With refractive lens exchange surgery, a patient opts to have lens surgery performed without any visual reduction from cataracts, solely to reduce their dependence on spectacle wear.

The crystalline lens is removed by phako-emulsification and replaced by an intraocular lens implant. Currently the accommodative function of a healthy crystalline lens cannot be replicated by any available intaocular lens  implants - all of which involve some degree of compromise. These can be a solution, but not one that I recommend.

Instead I recommend targeted myopic monovision; read the reaction of an ophthalmologist who chose this treatment.

With monovision, one eye is targeted for mid-to-distance vision and the other eye for mid-to-near vision: the eye for distance vision will be very slightly blurred up close and the eye for up close will be slightly blurred when looking at distant objects. But with both eyes open, neuroadaptation means that the brain "evens things out", usually creating a result which is acceptably clear, giving comfortable vision at all distances.

You can read more about monovision here.

The mono-focal lenses used reduce the risk of visible lens aberrations that can be troublesome and permanent with some intraocular lenses (IOLs). In randomised control trials, monovision performs at least as well as the best "premium IOLs".

It is a long-established principle that "it's the brain that does the seeing". Neuoadaptation is the process by which specialised cells in the occipital cortex adapt to the new visual environment. See www.aao.org/publications/eyenet/200707/feature.cfm

This process usually takes only a short period of time but dramatic change can overwhelm the process and cause permanent symptoms in about 1 in 10 patients. I feel it is important to be cautious in the present state of knowledge and this explains why I do recommend multifocal lenses. The visual cortex is often adapted to anomalies such as astigmatism and vision can appear worse to a patient if these anomalies are excessively or inappropriately altered, despite vision actually measurably better and there being no other abnormal findings.

Though monovision might sound difficult to adjust to, many contact lens wearers already find it beneficial. Most people adapt very well to it and eventually don't even notice which eye is their "distance eye" and which is their "near eye." Most importantly, additional evaluations undertaken at each stage are used to ensure that patients will be able to tolerate monovision. To facilitate this I arrange additional and often quite lengthy evaluations by a specialist optometrist skilled in achieving accurate biometry (a technique that predicts the refractive outcome). I also optimise the ocular surface prior to biometry and surgery by addressing any Meibomian Gland Dysfunction and any element of Dry Eye that may compromise the accuracy of the biometry or clarity of vision.

Despite all of these precautions accuracy of predicted refractive outcome currently is at best 95%, so cannot be guaranteed. Using monofocal lenses means that the procedure is more robust - and more forgiving. Realistically, patients ought to accept limited spectacle wear as an acceptable outcome. Any potential patient must accept that any surgery can be complicated.

My guiding principles are always patient safety and optimum results, and as a very experienced surgeon with special expertise in complex cases, it is very important to me that anyone considering surgery understands that any surgery on the eye is very finely balanced and is not risk-free. My experience as a medico-legal expert constantly reinforces the value of meticulous planning in order to avoid prejudicing what should be an excellent outcome.

Although very few patients experience problems, they can - and do – happen everyone who undergoes surgery is exposed to ALL risks of that surgical procedure. Even though the likelyhood of suffering any complication is low – and considerable efforts are made to reduce risks as far as possible, my patients need to understand that irreversible damage is a possibility and should never be discounted when they are making decisions about pursuing surgery.


Intermediate age related macular degeneration
Pic by Rakesh Ahuja, MD

If you have cataracts, the time to think about surgery is when the changes in your lens are impairing your vision so much that your daily life is affected.

Cataract in Adults: A Patient's Guide” is a good place to start your decision-making process. Click here to download a copy (71kb PDF).

In cataract surgery the likelihood of problems varies from patient to patient – and by about 75-fold depending on individual circumstances. The level of risk can be predicted by proper pre-operative evaluation, and I always advise my patients on how they can avoid problems. Often this means planning for a longer surgery time, making a general anaesthetic necessary (so that there can be no risk of a local anaesthetic wearing off during a more lengthy procedure, compromising the outcome).

To replace the lens, I now use micro-incision surgery as routine as I am convinced about the improved surgical safety and lens performance. In this operation two 1.8 mm incisions are made into the perimeter of the cornea on the side of the eye. The cataract is removed and the intraocular lens implanted through these incisions. They are self-sealing and require no stitches. Currently my preferred choice is the Akreos MICS IOL which is extremely well-tolerated for patients, even those with a history of uveitis and/or glaucoma. It is designed to provide better vision quality compared to standard lenses and can be compressed easily to fit through a 1.8 mm incision; it unfolds smoothly once implanted into the eye, recovering its correct shape without damage.
(Go to www.bauschsurgical.com/cataract/akreos/akreos-lens.aspx to read more).