The eye’s lens has the ability to change shape.  This allows it to focus light on the retina whether the light is coming from far away (the lens becomes thin), from a near object (the lens becomes thick) or from anywhere in between.

This process is called accommodation, and is strongest in early life.  However, the lens’s ability to accommodate declines with time because of a natural loss of its elasticity.  Presbyopia is the result of these changes and is noticed when we reach our 40’s and 50’s. This is why reading glasses become necessary – they provide the extra focusing power required to see near objects.

When the natural lens becomes cloudy with the passage of time, this is called cataract.  During cataract surgery or clear lens extraction, the natural lens is removed and replaced with a clear plastic lens.  This new lens is monofocal, and is best able to focus light on the retina from one distance.  Spectacles are then required to see clearly at other distances.  Usually, patients like to see clearly in the distance and use spectacles for intermediate (computers, painting, sheet music) and near (reading) work.  However, there are some exceptions.

Refractive Lens Exchange (RLE) utilises the same surgical techniques to remove the eye's natural lens, but this is done in the absence of a cataract and so is called RLE.  The natural lens is then replaced with a lens implant.
Whenever the eye's natural lens is removed, whether because of cataract, RLE, or presbyopia correction, the replacement lens implant can be either monofocal or multifocal.

Multifocal Intraocular Lenses (mIOL’s) employ new technology to mimic the process of accommodation – they are sometimes called pseudoaccommodative IOL’s. 
The three main mIOL’s currently in use are illustrated below.  They create focal points for light coming from far, intermediate and near objects – but the patient only sees the object of interest.

It is inadvisable to implant a monofocal IOL in one eye and a multifocal IOL in the other. The eyes should receive the same kind of lens. Depending on an individual patient’s requirements, the same mIOL may be used for both eyes or a “mix-and-match” approach to mIOL implantation may be advised. This may involve, for example, placing a ReSTOR lens in one eye to achieve good distance and near vision and a ReZoom lens in the other eye to give better intermediate vision. Different surgeons use other combinations.

No.

Many, but not all, patients are able to see satisfactorily at all distances.  Some may need to wear spectacles for some tasks, such as any form of reading, or reading for prolonged periods of time, or driving at night
It is important to understand that there is a difference between “satisfactory” vision and “crystal clear” vision.  For example, a patient may have vision that is “crystal clear” (or 20/20 vision) for distance and near tasks, but only “satisfactory” (or 20/40 vision) for intermediate distance.
When analysing reports about these lenses, it is also important to be aware that some studies give the proportion of patients who “never wear spectacles” whereas others give the proportion who “either never or only occasionally wear spectacles” – these are different measures and should not be compared as though they are the same thing.

Glare, halos and reduced contrast sensitivity.

Glare and halos may occur after implantation of regular monofocal IOL’s as a result of lens design.  They are more likely with mIOL’s because, in addition to lens design effects, mIOL’s create in-focus images for distance and near objects simultaneously and this overlay of images can cause symptoms.
Contrast sensitivity is the ability to differentiate between an object and its background.  A reduction is inevitable following implantation of mIOL’s, and the tolerability of this depends on the patient’s expectations.  Unrealistic expectations often lead to disappointment in the visual outcome. The following table is provided as a brief summary of outcomes.  Please note that a great number of studies have been performed to look at the outcomes of lens surgery and implantation of monofocal as well as multifocal IOL’s.  You should ask any questions you have before you make a decision regarding your desire to have mIOL implantation.

A small proportion of patients (between 0 and 2% in different studies) have experienced symptoms of such severity that they have requested the mIOL be removed.

No.

People who are long-sighted (hyperopic) or emmetropic (no spectacle correction required) tend to have favourable outcomes more often than those who are short-sighted  (myopic).  Astigmatism may need to be corrected before mIOL implantation, depending on the examination findings.

Most surgeons would advise against mIOL implantation for the following categories of patients:

Patients that are hypercritical with unrealistic expectations

Patients with excessive complaints about their prescription

Patients who drive at night for a living or whose occupation or hobbies require night vision

Patients who are amateur or commercial airline pilots

Patients who have life long complaints about glare Patients who are happy wearing glasses Patients who want guarantees on surgical outcomes