Being one of the most fascinating aspects of the human body, the human eye is a sense organ that allows vision to occur. The eye behaves in many ways like a camera – we ‘see’ when the lens of our eye starts to focus on an object, whereby the image is then transmitted to the light-sensitive membrane at the back of our eye known as the retina. We now know that neurological impulses are carried from the retina to various parts of the brain. The visual association cortex – which is a disproportionately large area of the brain – then pieces together what we ultimately perceive as a visual scene.

Ancient Greek studies had two main schools of thought about how the human eye ‘sees’. One of them described how rays would be emitted from our eyes, bouncing off objects and the movement of the rays would then allow us to see. Aristotle and his followers believed that something entered our eyes and presented us somehow with a representation of an object – therefore this idea was not so far from what we understand today.

It was Leonardo da Vinci however who first began to grasp the wonderful optical nature of the human eye.

Now, can you not see that the eye embraces the beauty of the whole world? He draws the cosmos, he advises all the human arts and corrects them, he moves humanity to the diverse parts of the world. This is the king of mathematics, whose knowledge is certain; he has measured the distance and size of the stars; he has found the proper place for the elements; he has predicted the future by means of the stars’ course; he has begotten architecture, perspective, and divine painting.” (Manuscript from the Vatican Library, MS Urb. Lat. 1270.)

Leonardo was fascinated by eyes and this was represented in his sketches and paintings. He believed that the eyes were the gateway to the soul and even examined a skull at one point to see if he could fathom how the eyes could see.

It was not until around the 17th century that the real mechanisms of the eye truly began to be understood. For example, at this point, it was deduced that the retina (the light-sensitive layer of tissue lining the inner surface of the eye) was responsible for transmitting nerve impulses to the brain via the optic nerve. Johannes Kepler from Germany and René Descartes from France were two well-known physicists of the time that made great strides in understanding how our eyes work.

It was, in fact, Descartes, in an experiment involving an eye from an ox, who determined that an image was being inverted after being focused on to the retina by the lens of the eye. It was Thomas Young, however, at the beginning of the 19th century, who correctly described how the lens focuses images onto the retina. He also was able to demonstrate astigmatism where an improperly curved cornea cannot focus light correctly on to the retina.

Understanding Our Eyes Today

This leads us on to today’s understanding of our eyes. Light is reflected off objects and projected onto our retina by the lens. The retina then detects this light and sends impulses along the optic nerve to our brain where the end result is decoded into what we know as vision.

Perfect vision (known as emmetropia) is achieved when the lens at the front of the eye projects light rays onto the back of the eye (the retina) in sharp focus, helping us to see images clearly. However, just as with a camera, if the lens is not adjusted properly these images can appear blurred. The extent of this blurring is known as refractive error, consisting of short sight, long sight, and astigmatism.

Other conditions include presbyopia (a condition related to age, where the eye beings to lose its ability to focus on near objects) and of course cataracts.

  • The Eye Test Chart - also known as the Snellen Chartarrow

    We often measure the standard of vision achieved with or without glasses using the familiar eye test (Snellen) chart, which is viewed from a distance of 6 metres (20 feet). It should be noted that this is not a particularly accurate way of recording visual standards – other methods are used in laboratory and research settings. The high contrast lettering on a Snellen chart is also unrepresentative of more difficult visual tasks, such as driving at night or in conditions of poor visibility.

    The top line on the chart is designated 6/60. The second line up from the bottom (on most charts) is 6/6 and represents ‘normal’ sight – commonly known as 20/20 vision, particularly in the USA, as 6 metres is equivalent to 20 feet. In the UK, the minimum standard of vision permissible for driving is between 6/9 and 6/12, equating to the ability to read a number plate from a distance of 20 metres (20.5 metres for older-style plates). The Royal Air Force requires its pilots to have an unaided vision of 6/5 in each eye (usually the bottom line on the chart).

  • What is Short-Sightedness (Myopia)?arrow

    If the world beyond your near vision seems foggy or blurred then it is likely you are suffering from short sight or ‘myopia’.

    Short-sightedness is a common eye condition (with up to 1/3 of people affected in the UK) where distant objects appear blurred and close objects are clearly visible. Often short sight is referred to by its medical term ‘myopia’. In everyday life this may mean that if you are short-sighted you will be able to read books clearly but will have problems watching television, driving or being able to visit the cinema and see the screen clearly. In the United States this condition is referred to as near-sightedness.

    Short sight symptoms

    In some instances an individual’s short-sightedness is very mild. They may choose to wear glasses only occasionally if they are visiting the cinema for example or wear contact lenses as and when needed. Therefore the minor inconvenience of having to put in contacts or find a pair glasses when the occasion presents itself is something that some people choose to put up with.

    Example of short sight

    An example of short sight – near objects are in focus whilst distance vision is blurred.

    However, for others the desire to see clearly at all times means they often wish to consider laser eye surgery or refractive lens exchange. Refractive eye surgery for short sight is a simple procedure that can often be life changing. For those who have persisted with blurred vision, refractive surgery can instantly provide a ‘new world’ of clear vision and comfort.

    Causes of short sight

    Short sight is caused when light rays are brought to a focus in front of the retina – usually, because the eyeball is too long, the cornea (the ‘window’ at the front of the eye) too steep, or a combination of the two. When not wearing glasses or contact lenses, those with short sight will have blurred distance vision. The shorter their sight, the more blurred is their view of the world. There has been much debate over what exactly contributes to short sight.

    Even relatively modest short sight – for example, a prescription of only -3D (dioptres) – will cause considerable blurring of distant objects and difficulty in recognising even the largest letter on the eye test chart.

    The point of focus for someone with -3D of short sight is 33cm (12”) in front of the eye. Someone of -10D would have a focal point only 10cm away. Without glasses, this person could read small print clearly at 10cm; however, their distance vision would be extremely blurred.

    Laser eye surgery or lens replacement can be life changing – you might want to check out David Gartry’s most recent reviews at Lasik Eyes.

    There are many causes of myopia although these are often debated – symptoms are very similar. In general, someone who is myopic will always have blurred distance vision but good near vision. It is worth mentioning however that if someone has a high level of myopia, even near vision may be affected and in order to see clearly, objects may have to be moved very close to the eye.

  • What is Long-sightedness (hyperopia)?arrow

    Long-sight, like short-sightedness or myopia, is a refractive error (refraction simply means the bending of light). If you are long sighted you will see objects in the distance clearly but your near vision will be blurred.

    The way in which light is bent and focused by the eye will determine the type of visual problems experienced) and can be corrected by the use of contact lenses, glasses or laser eye surgery. There are many treatments available for long-sightedness – please do get in touch with us if you feel you are suffering from this condition. Being ‘long-sighted’ is also known as the condition ‘hyperopia’.

    Long sight symptoms

    If someone is long-sighted they might struggle to read a menu in a restaurant or a page in a book but will be able to see films at the cinema clearly. In the United States this condition is referred to as being farsighted.

    As with myopia the cause of long sight is attributed to where the lens focusses light rays that enter the eye. In the case of long-sightedness the light rays seem to focus ‘behind’ the retina – of course this cannot happen so the lens tries to accommodate this refractive error by becoming more rounded. In younger people this process of counteracting long-sightedness is known as accomodation. However the resulting effect is that people with hyperopia over time cannot fully ‘accommodate’ this error and their vision for nearby objects becomes blurred.

    Long sight example

    If you are long-sighted you might struggle to read the text on your mobile phone.

    When someone describes the condition of long sight they might often confuse this with ‘presbyopia’ which is something that in time we all suffer from. Presbyopia is where the eye loses it’s elasticity and therefore cannot focus well on near objects. Therefore the symptoms are very similar to long sight. However long sight is a condition that can affect young patients too.

    Laser eye surgery can correct long sight and the results can often be more surprising than short sight correction – mainly because when someone suffers from long sightedness they often cannot see very much of the world around them. Long sight is more common in patients older than 35 years old. In some cases, LASIK might not be the most appropriate treatment and instead, we would sometimes recommend refractive lens exchange as an alternative. This has many benefits of treating long sight but it is always advisable to come in for a consultation first so we can determine whether presbyopia is present.

  • What is Astigmatism?arrow

    Astigmatism is a term many of us have heard of but most are unsure as to what it means exactly. While myopia (short-sight) and hyperopia (long-sight) are easier to explain, astigmatism is more complex.

    The reason for this is because astigmatism is measured not simply by a number (a ‘scalar’ term mathematically) but it has a specified orientation, direction or axis and is therefore a ‘vector’.

    To explain: our spectacle and contact lens prescriptions are defined in terms of the basic units of optical power – dioptres (D) – and we might be told, for example, that we are “minus 3 with minus 1 unit of astigmatism”, which means that we have 3 dioptres of short-sight (myopia) and 1 unit of astigmatism. Astigmatism also has a sign (+ or -) and therefore the prescription would be written as follows:

    Right Eye: -3.00DS / -1.00DC axis 100

    The capitals DS mean simple ‘dioptre sphere’ (in this case myopia) and the DC stands for ‘dioptre cylinder’. Mathematically a lens (or cornea) with a cylindrical shape would generate the refractive error ‘astigmatism’ and the terms are often used interchangeably. Note that the axis or angle of the astigmatism is 100 – in degrees. In handwritten prescriptions the degrees sign is usually omitted on purpose to avoid possible confusion with a ‘0’.

    In the case of the human eye, astigmatism occurs when the front surface – the cornea – is not perfectly uniformly curved, like a football. Instead, it is slightly more oval in shape in one direction, like a rugby ball. A football is spherical of course and looks identical from any point of view. A rugby ball, however, has two curves at right angles to each other. End-on it looks circular but side-on it is oval or elliptical. These two distinct curves, that are mathematically ‘orthogonal’ or at perfect right angles to each other, result optically in two points of focus rather than one. If a person with astigmatism were to look at the spokes of a bicycle wheel, for example, some of them would appear in focus but others (at right angles) would be more blurred because lines at a certain angle might be more in focus on the retina at the back of the eye.

    Astigmatism test

    To test for astigmatism you will need to book a consultation at our clinic. This diagram is a simple (but by no means diagnostic) test – if all lines look the same or nearly the same, then you may not need an astigmatism correction.

    Types of Astigmatism

    There are, in fact, two variants of astigmatism, regular and irregular. Regular astigmatism (as described above) is the more common form where the cornea is more curved in one direction than the other in a very uniform or regular way (like the rugby ball). The two curves (or meridians) are at perfect right angles to each other. This type is easily corrected with glasses, contact lenses or laser eye surgery or lens replacement sugery.

    ‘Irregular astigmatism’ is less common and occurs where some irregular distortion, perhaps caused by disease or trauma with scarring, has affected the cornea. This type of astigmatism cannot be corrected fully with glasses but contact lenses (usually rigid gas permeable) and/or corneal surgery may be option.

    Correcting Astigmatism

    LASIK, LASEK or lens exchange surgery can correct regular astigmatism, whilst also treating your short or long sight. The results of laser astigmatism correction have improved very considerably since the early days of PRK and LASIK, especially with the advent of highly sophisticated ‘tracking systems’ which ensure that the correct angle or axis of astigmatism is treated.

    Patients undergoing cataract or refractive lens exchange surgery with significant or high astigmatism can have custom fit Toric (astigmatic) lenses inserted at the time of their surgery to correct their basic refractive error plus their astigmatism. Alternatively, for smaller amounts of corneal astigmatism, limbal (or corneal) relaxing incisions can be created in the cornea by the femto-second laser or by hand for up to 1.5 dioptres.

    In the longer term, patients with significant astigmatism following cataract or refractive lens exchange surgery may benefit from laser eye surgery to eliminate or at least minimise their refractive error.

  • What is Presbyopia or Reading Vision Correction?arrow

    Presbyopia (From the Greek for ‘old eyes’ – unfortunately!) is the age-related condition characterised by the need to wear reading glasses later in life, usually from mid-40s onwards.

    It is often mistakenly called ‘long sight’ but presbyopia is caused by the eye’s natural lens losing its ability to focus and the focusing muscles becoming weaker. Those who wear glasses or contact lenses already for short or long sight will require additional vision correction as presbyopia develops, usually in the form of bifocal or multifocal (progressive, Varilux, varifocal) glasses.

    It is very important to understand that presbyopia affects us all, whether or not you have had laser eye surgery to correct your distance vision. Therefore, once you reach your mid- to late-40s, it is highly likely that you WILL require reading glasses – even former RAF pilots, for example, will need reading glasses from their mid-40s onwards.

    A small number of prospective laser eye patients of presbyopic age choose not to have laser eye surgery, believing that they’ll simply end up swapping one pair of glasses for another. This is understandable – however, most prefer to have clear distance vision and accept the need for reading glasses along with all normally-sighted people of their age.

    Blended Vision and Monovision

    If you are short-sighted, it is possible to correct the dominant eye fully for good distance vision and partially correct the non-dominant eye to leave some residual short-sightedness to help with reading vision. This strategy is known as ‘mono’ or ‘blended’ vision and provides freedom from spectacles most of the time. Many patients have already, very successfully, used the blended vision / monovision strategy with their contact lenses. If not, we would always advise a contact lens trial period intended to simulate the ‘mono’ or ‘blended’ vision effect before their laser surgery.

    Other solutions

    There is another solution to help with your reading addition – multifocal intraocular lens implants or IOLs. They are given the name ‘multi’-focal as they aid distance, intermediate and close reading vision (to a degree). They are premium correcting lens implants which are not available on the NHS and have become popular in recent years for the treatment of presbyopia. These lenses can usually be well tolerated and can give reasonably good distance and close vision, with a greater depth of focus than monofocal (‘mono’ focal – singular correction for distance only). However, this is sometimes at the expense of image quality. Glare and haloes may be troublesome. We would not recommend multifocal IOLs for professional drivers, those who drive frequently at night or those with very exacting visual requirements.

  • What is Cataract?arrow

    A cataract exists when the natural crystalline lens of the eye becomes cloudy or opaque over time, leading to blurring and a gradual loss of visual clarity.

    cataract surgery

    Cataract surgery is the most commonly performed elective procedure in the world, with around 300,000 treatments undertaken every year in the UK alone. David Gartry now performs laser-assisted cataract surgery, known as femto-phako, which is the most advanced cataract surgery technique currently available.

    Most cataracts develop in later life, although they can sometimes be present at birth, be drug-induced or develop after injury to the eye. Mild clouding of the lenses is common in people of all ages, although surgery is not required if vision remains unaffected. Some people are unaware that cataracts are developing – it is often a very slow process that starts at the edge of the lens and may not initially affect vision.

    Surgery is performed when clouding becomes noticeable and starts to impede the patient’s daily life. There are a number of factors that will affect the timing of surgery, such as occupation, night-driving difficulties, general glare or scattering of light, and how well the patient can read a standard eye-test chart.

    Typical early symptoms of cataract are light scatter, (e.g. dazzle from oncoming car headlights during night driving) and a progressive shift towards short sight (changing spectacle prescription). Later, vision becomes blurry and colours are dulled.


    A cataract in a patient characterised by the blurred, grey opaque appearance

    It is no longer considered necessary or desirable to wait until a cataract has fully developed (by which time the patient would have been eligible for registration as blind) before undergoing surgery. However, waiting for cataract surgery does not harm the health of your eye, and equally good results can be obtained from cataract surgery performed at an early or a later stage.