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Sight correction

We see by forming an image on the retina - a carpet of light sensitive cells, each acting like a pixel on a computer screen, that lines the inside of the back of the eye. Images are focused on the retina by the cornea (the clear part of the front of the eye) and the natural lens (suspended within the eye just behind the pupil). About half the UK population (50%) requires spectacles or contact lenses to focus a clear image of a distant object on the retina.

Spectacles and contact lenses either add or subtract focusing power to help form a clear image on the retina where the natural focusing power of the eye is incorrect. Defects in natural focusing power are called refractive errors. Refractive errors are measured in units of lens power ("dioptres" or D) and represented for each eye in your spectacle or contact lens prescription by a number prefixed by a sign (e.g. +1.00D or –12.50D). The sign indicates whether the spectacle correction required is for long (+) or short (-) sight.

Short sight (myopia) and long sight (hypermetropia) are often accompanied by an element of uneven focusing power (astigmatism). Imagine that the cornea is more rugby ball shaped than football shaped. The extent of the difference in focusing power between the smallest and largest radius of curvature for this uneven (toric) surface would be the amount of astigmatism.

This is represented in your spectacle prescription by a second number and an angle (e.g. –4.00D at 80°) indicating the focusing power and orientation of the lens required to correct the astigmatic component of your refractive error.

The younger eye is able to increase focusing power, or accommodate, to see near objects clearly. This flexibility of focus is provided by flexibility in the shape of the natural lens. As we get older, the natural lens becomes less flexible (presbyopia), and the ability to accommodate diminishes. This is why normally sighted people need reading glasses from their mid forties on. The final component of your spectacle prescription describes the reading addition, or the difference between your prescriptions for distance and reading glasses. Typically, this varies from +1.00D in your mid-forties to a maximum level of +3.00D by your late fifties.


Sample spectacle prescription



Refractive surgery


Refractive surgery is not normally performed until the spectacle prescription has been stable (no change greater than 0.50D) for 2 years. Spectacle prescriptions typically stabilise in the late teens or early twenties for myopic patients (-ve prescriptions), and later for hypermetropic patients (+ve prescriptions). Hypermetropic patients are able to compensate by accommodation (as in focussing for near vision) whilst younger, and often only become spectacle dependent in mid life.

Refractive surgery techniques are available to reduce or eliminate the need for spectacles in most patients with a stable spectacle prescription. For younger patients, good distance vision in both eyes is usually the aim. For patients in the reading glasses age group, a spread of focus in which better distance vision is targeted in one eye and better near vision in the other is commonly employed to minimise spectacle dependence over a range of activities.

Lower refractive errors (in the range +4.00 to -10.00D) are usually corrected using laser techniques (LASIK or PRK) to reshape the cornea. Higher errors are corrected using lens implant based methods (RLE or ICL implantation). Astigmatism and age are also influential in determining the most appropriate technique. Many patients over 60 are better suited to lens exchange (RLE) than laser refractive surgery, particularly if early signs of cataract (lens opacity) are present. The summary table below is designed as a rough guide; but the age and refractive range cut offs given for each technique are not absolute, and the risks and benefits of appropriate alternative approaches to correcting your refractive error will be discussed with you at your consultation. 




    -1.00 to -6.00

    -6.00 to -10.00

    Above -10.00

    +1.00 to +4.00

    Above +4.00
  • 21 - 50 years
  • 50 - 60 years
  • over 60 years






Refractive surgery procedures


LASIK (Laser in situ keratomileusis) - range +4D to -10D with up to 6D astigmatism

Principle = wavefront guided excimer laser corneal reshaping beneath a protective corneal flap which is replaced at the end of surgery. This protective flap is most commonly formed using a femtosecond laser (intralasik). Because the surface tissue damage is minimised in LASIK, visual recovery is rapid and virtually pain free.

SURFACE LASER TREATMENTS (Transepithelial PRK)– range +4D to –10D with up to 6D astigmatism

Principle = the corneal skin layer is removed by the excimer laser (transepithelial PRK) and regenerates in 4-5 days after wavefront guided laser reshaping of the corneal surface to correct vision. Recovery time is longer for surface laser treatments but they have safety advantages for some patients with thinner corneas. Although recovery time is slower, final results for LASIK and surface treatments are similar.

SMILE (Small Incision Lenticule Extraction)– range: -2 to -10D with up to 6D astigmatism

Principle = a femtosecond laser is used to trace the 3D template for a lens shape (the lenticule) through the tissues just beneath the corneal surface. Optical reshaping and focus correction is achieved by extracting this lens shaped layer of corneal tissue through a small incision. SMILE combines some of the advantages of LASIK (painless recovery) and TransPRK (safe early return to contact sports), but treatment is deeper in the cornea (limiting the range of suitable patients) and may be less easy for the surgeon to centre and align accurately. We have decided against continuing to use the current version of SMILE at Moorfields, but we are watching developments in SMILE with interest.  

RLE (Refractive Lens Exchange) - range: myopia/hypermetropia at any level can be treated with RLE.

Principle = replacement of the natural lens with an intraocular lens (IOL) delivered through a self-sealing micro incision which does not affect eye wall strength. Incisional techniques or specialized toric IOLs can be used in tandem with RLE to reduce astigmatism, and multifocal IOLs can be implanted to reduce spectacle dependence for near vision. RLE is identical to modern cataract surgery in which a new lens is implanted within the capsule of the natural lens which shrink wraps the implant and stabilises it in the natural position. Initial stages of the operation are now commonly performed with a specialised femtosecond laser.

The natural lens becomes more misty with age, and patients over 60 years old are often more suited to RLE than laser refractive surgery.



ICL (Intraocular Collamer Lens) implantation - range: up to -17D myopia; up to +10D hypermetropia.

Principle = implantation of a soft flexible artificial lens which is seated just in front of the natural lens and behind the iris. Preservation of the flexible natural lens helps to avoid reading glasses and toric ICLs can be used to correct astigmatism. This approach is particularly suited to younger patients who are out of range for laser refractive surgery.


More information

Our approach to each of these procedures is described in more detail in our procedure specific information on this site.

Useful standardised patient information on laser eye surgery, ICLs, and refractive lens exchange is now available on the Royal College of Ophthalmologists website here.


Refractive surgery consultation

Your suitability for refractive surgery is determined in a refractive surgical consultation. The consultation includes a multi-staged examination of your eyes for which you should allow 1–2 hours (some waiting between key stages is inevitable). First, you will see Mr Allan's team for scanning and refraction tests. You will then see Mr Allan to review information from these tests, examine your eyes, and discuss your procedure choice.

Having read the relevant patient information packs (these are normally sent out to you when you initially enquire about refractive surgery in Moorfields) it is useful to make a list of any particular questions you may have and bring this with you to the consultation. Key stages are:

  • Corneal scanning and refraction check
  • Wavefront scanning
  • Ocular examination and review with Mr Allan

Before attending the consultation, you should:

  • Leave your contact lenses out (1 week for soft and 2 weeks for rigid contact lenses)
  • Continuous contact lens wear can produce temporary changes in the shape of your cornea. It is important to leave contact lenses out prior to the consultation to ensure that your corneal scanning and refraction tests are accurate.
  • Bring a record of your spectacle prescriptions over the last two years
  • Refractive surgery is normally postponed until the spectacle prescription is stable. Small variations in your spectacle prescription are normal, but if there has been a change of more than 0.5D over the last 2 years, we would normally wait 12 months before proceeding with surgery after a repeat check to ensure stability.
Travelling to Moorfields

Nearest Tube – OLD STREET on the NORTHERN LINE

Examination involves the use of drops to dilate the pupil. Pupil dilation causes temporary visual blurring. You should not drive to the consultation. If you are being driven, limited NCP car parking is available near the hospital.


If you wish to arrange a preliminary consultation, please telephone 020 7566 2156 or 07484 081815 (or from outside the UK +44 20 7566 2156 or +44 7484 081815) or email