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The crystalline lens in the eye has the ability to alter its dimensions and therefore changing its refracting power.

The crystalline lens in the eye has the ability to alter its dimensions and therefore changing its refracting power. This change allows us to alter our focus from a distant object to a near object and have the object focused on the retina and therefore be seen clearly (barring all other corrections the eye may require.) This mechanism is known as Accommodation. Accommodation decreases with age and becomes noticeable as we enter our 40’s – this change is known as Presbyopia.


A material commonly used in the manufacture of spectacles frames.

It is lightweight, hypoallergenic and can be produced in a large variety of styles and colours.


If the retina is distorted or does not function properly or the brain is unable to interpret the image efficiently, the sense of light or form will be affected and the eye is said to be amblyopic.

If the retina is distorted or does not function properly or the brain is unable to interpret the image efficiently, the sense of light or form will be affected and the eye is said to be amblyopic. Except in the case of correctable squints the visual acuity of this eye will always be poorer even though the optical performance may be good. This is sometimes referred to as a Lazy eye.


Astigmatism is due to the eye not being spherical or other irregularity in one or more of its refracting surfaces.

The eye does not form a point image on the retina but instead blur ellipses or focal lines. Consequently objects are not seen clearly. Astigmatism can be corrected with spectacle lenses and some contact lenses.

Bifocal Lenses

Bifocal spectacle lenses have two distinct sections with a line separating them, usually a distance correction in the upper section and a reading correction in the lower ‘segment’.

They are available in many different materials and size and shape of the reading segment can be varied.

Binocular Vision

When the light entering both eyes is fused and perceived as a single image by the brain we binocular vision.

Blur Circle

This is the patch of light that falls on the retina when myopic or hypermetropic eye is uncorrected.

Correcting spectacles or contact lenses will allow the light to form a sharp point focus on the retina.


The crystalline lens in the eye is a transparent structure allowing light to refract and pass through to the light sensitive retina.

With age natural changes within the crystalline lens can cause the development of areas of opacity, which can impair a persons vision. Cataracts can be described as Cortical, Nuclear or Subcapsular and each type can affect the vision in different ways. Increased exposure to damaging UV rays, especially in the young eye, can bring about the early onset of cataracts, as can certain systemic diseases such as Diabetes.

Cataracts can also be congenital. Cataract surgery to remove the crystalline lens and implant a replacement lens is a very common and quick procedure in the UK now.

Colour Vision

The retina has certain cells sensitive to the varying wavelengths of light allowing us to perceive our world in a whole range of subtle colour differences.

For some people this does not function as well in their visual system and they can be described as having defective colour vision. There are varying types of defective colour vision, which depend of the specific colours a person may not be able to clearly distinguish between. Colour vision is usually congenital and cannot be cured. It can also be acquired in some circumstances.

Contact Lenses

These are correcting lenses worn on the surface of the eye.

Contact lenses are mainly available as Soft lenses, including silicone hydrogel lenses, and Gas Permeable lenses. There are many different lenses available for different requirements including daily disposable lenses, extended wear lenses that may be kept in the eye for a number of days and nights, Ortho K lenses that are worn only a t night and allow clear vision with no correction during the day. These days most prescriptions can be corrected with contact lenses including myopia, hypermetropia, astigmatism and presbyopia.


As we look at a distant object the eyes both are directed straight forward, assuming the eye is normal and healthy.

When we then go to look at an object that is near to us both eyes must turn inwards towards each other to allow the image from each eye to be fused as a single image in the brain. The movement of the eyes inwards is known as Convergence.


This is the transparent part of the eye that the light entering the eye passes through before it enters the pupil.

It is approximately the size diameter as the iris, which we can see through the cornea.

Crystalline Lens

This is the lens within the eye, which is positioned behind the iris.

The crystalline lens changes shape to allow the eye to focus for reading and close work. Over time, the lens may become cloudy and less transparent; this is known as cataract.


This is the unit of measurement used in Optics to measure the correcting lenses the eyes require to have an object focused on the retina.

Your spectacle prescription or contact lens specification will be written in Dioptres.


If distant objects are imaged sharply on the retina with no correction required the eye can be described as emmetropic.

In emmetropia the fovea coincides with the eyes 2nd focal point.


This is an area of the macula on the retina where our central vision is concentrated, the centre of the fovea is known as the foveola.

Damage to this area of the retina can cause a loss in our central vision but does not affect the peripheral vision.


This is a pathological condition of the eye usually caused by an increase in the intra-ocular pressure of the eye.

If left untreated it causes damage to the optic disc and peripheral field loss. There are often no symptoms in the early stages and therefore regular eye test are required to detect the onset of glaucoma before permanent damage occurs. Most frequently glaucoma is a chronic condition but can be found in an acute form known as acute closed angle glaucoma.

Graduated Tint

A tinted lens can be dyed to create a darker tint in one region of the lens than another.

Usually the upper portion of the lens is dark with a gradual lightening of the colour to the lower edge of the lens. Graduated tints are available in a wide range of colours.


Here the light entering the eye from a distant object does not hit sharply on the retina but forms a blur circle, in theory the eyeball or globe is too short.

The eye can be described as Hypermetropic and this condition is also known as long sightedness. Plus powered lenses are prescribed to correct hypermetropia.

Index – Refractive index

This refers to the density of a transparent optical medium, such as a spectacle lens, and its ability to bend light.

Spectacle lens materials are available in a variety of refractive indices and to be able to make a lens thinner as often required with higher prescriptions we may choose a higher refractive index. Plastic lenses are available in 1.53, 1.6, 1.67 and 1.74. Glass lenses are available in 1.5, 1.7, 1.8 and 1.9 and Polycarbonate is available in 1.59.


This is the term for the perceived change in the position of a viewed object as the eye moves from the distance portion to the reading portion of a bifocal lens.


This is inflammation of the Cornea. There are numerous possible causes.

Contact lens patients must be compliant in the routine care and handing of their contact lenses and cases to avoid possible Microbial Keratitis, an infection caused by bacteria, fungi or a protozoa known as Acanthamoeba. Microbial keratitis is a very painful and sight threatening condition.


Technically termed Hyperopia and also known as Far-sightedness. A long sighted patient will require plus powered lenses to correct their vision.

Long-sightedness (hyperopia)

Long-sighted people will see far objects more clearly than near objects.

This is because light from near objects is focussed behind the retina. Often in children and young people the eyes are able to compensate for this by re-focussing so vision may not be blurred. Long sight will often cause symptoms of tired eyes or headaches. In very young children severe long sight can cause one eye to turn, commonly known as a squint. Glasses may be needed to correct a squint or to ease discomfort when reading or performing near vision tasks. In older people, as the eyes’ ability to refocus reduces, long-sight can also begin to affect distance vision. Lenses to correct long sight will be convex, or thicker in the centre.

Low Vision Aids

These are devices such as magnifiers and telescopes which may help patients with poor vision to perform specific tasks such as reading or television viewing.

Low vision aids may be recommended when a patient’s vision is poor and cannot be further improved by spectacles or contact lenses.


These are devices that can help a patient see in more detail for specific visual tasks such as reading or sewing.

 A whole range of different types are available, from conventional hand-held ‘magnifying glass’ type magnifiers, to specialist magnifiers mounted on stands or incorporating illumination systems.


In a myopic eye the light entering does not form a sharp point on the retina but falls short, causing a blur circle to form on the retina, in theory the eyeball or globe is too long.

The eye can be described as Myopic and this condition is also known as Short-sightedness. Minus powered lenses are prescribed to correct Myopia.


An involuntary movement of the eye which may be pathological in cause.

Nystagmus usually appears as minute side-to-side movements of the eye and an inability to maintain steady fixation.

Optical Centre

The optical centre of a spectacle lens is the point on the lens where the true prescription is measured.

The optical centre of the lens is placed in the spectacles to be in front of the pupil by measuring a patient’s Pupillary Distance (PD).

Plano Lenses

‘Plain’ lenses that do not incorporate a prescription are described as Plano lenses.

They may be used when somebody who does not require spectacles wishes to wear glasses for fashion reasons. Sunglasses without prescription lenses come with tinted plano lenses as standard.

Polarising Filter

A polarising filter may be found in some Plano sunglass lenses and can often be requested in Prescription sunglass lenses.

A Polarising filter works by stopping light reflected off horizontal surfaces (such as water, snow, wet roads, metallic car surfaces,) passing through the spectacle lens. This can greatly cut down glare and dramatically improve the clarity of vision in spectacle lenses. Polarising filters are of great for those who like to ski, sail or fish.


This is the condition we all develop where our ability to focus from a distant to a near object, becomes more difficult.

Initially a person with Presbyopia  (a Presbyope) will move reading material further away from the eye to see it clearly. To read comfortably be a Presbyope will be prescribed a Reading Add in their spectacle prescription. This can be made as a pair of single vision reading spectacles or more commonly incorporated into a Varifocal, Enhanced Reader or a Bifocal pair of spectacle lenses.


Prism may be prescribed for a patient to correct double vision, to help their eyes work together to form a single image in the brain..

A prism is usually prescribed due to a muscle imbalance in the eyes. Prisms are often split between the eyes in the spectacles to be equally weighted and appear cosmetically more aesthetic.

Pupillary Distance – PD

This is the measurement taken of the distance between a patients pupils, to allow the Optical centre of the spectacle lens to sit in front of the pupil, when the patient is looking straight forward in the Primary position of gaze.

This measurement can be taken Binocularly or Monocualrly.

Quarter Wave Optical Thickness (QWOT)

The optimum thickness of an anti-reflection coating to reduce reflection, this is a quarter of a wavelength of light.

Reading addition

This is the element of the spectacle prescription that may be given when extra help is required to see at a near (or reading) distance.

This is usually prescribed with the onset of presbyopia but can occasionally be prescribed for children with Juvenile Stress Myopia. The reading addition is ‘added’ to the distance correction to give the reading correction, which can be then made up as reading spectacles. Reading spectacles allow the wearer only to see at a near distance and not beyond that into the distance. It can be incorporated into bifocal or varifocal spectacle lenses also to allow more distances to be seen clearly without removing the spectacles.


The light-sensitive lining of the inside of the eye. The retina is composed of specially adapted cells which react to light entering the eye and send signals to the brain.

It is possible for the retina to become torn or detached; this will affect its ability to detect light. In many cases retinal detachment can be treated but this must be done urgently. Brosgill Opticians offers the Optomap Retinal Exam in our Street Lane branch; this advanced technology allows a thorough examination of the retina and has the ability to detect sight-threatening problems, which may not be apparent using conventional examination methods.


Also known as Myopia, this is when the eye has too much focussing power and light is brought to a focus before it reaches the retina.

Minus powered lenses are needed to bring the focussed light onto the retina and form a clear image. Some patients with myopia may be able to read clearly without spectacles, but spectacles will be needed to correct the vision for seeing clearly in the distance for tasks such as driving.

Short-sightedness (myopia)

Short-sighted people will see near objects more clearly than they see far objects. This is because light from distant objects is being focussed in front of the retina. It is unusual to be born short-sighted. Most cases of short sight develop in childhood or teenage years.

The first sign may be having to peer at the white board at school or move closer to see things better. Glasses or contact lenses may only be needed for distance vision or they may be required all the time, depending on the degree of short sight. Lenses to correct short sight will be concave, or thicker at the edge.

Silicone Hydrogel Contact Lenses

Silicone hydrogel contact lenses offer both comfort and excellent oxygen permeability.

Allowing as much oxygen as possible to reach the eyes is important for the health of the eyes. A recent advance in silicone hydrogel lenses is the introduction of the first daily disposable silicone hydrogel lens, Acuvue 1-Day TruEye, combining comfort and convenience with superior oxygen transmission properties. Some silicone hydrogel lenses can be worn on an extended or continuous wear basis meaning they do not need to be removed every night.

Snellen letter chart

One of the eye charts that is used during the eye examination to measure a patient’s visual acuity.

Snellen charts have a large letter at the top, and progressively smaller letters on the lines below.

Supra/Semi-rimless frame

A supra frame is a spectacle frame in which the lenses are not completely surrounded by a rim.

A supra frame usually has a metal or acetate rim around the top of the lens only, with the lower part of the lens being supported by a clear nylon cord which appears almost invisible.

Thin lenses

Certain higher spectacle prescriptions when made up can result in lenses which may be thick, heavy and unappealing cosmetically.

Lenses for short-sightedness are thicker at the edges and those for long-sightedness are thicker in the middle. It is possible to have the lenses made of higher-index materials which are better at directing the light and hence do not need to be as thick. High-index materials are available in indexes of 1.67, 1.7, 1.74, 1.8 and 1.9. The higher the index the thinner the lens.


A metal element that is used in the manufacture of spectacle frames.

The advantages for Titanium that make it desirable for spectacle frames are its lightness, great strength and that it is a hypoallergenic material, meaning people who suffer from Nickel allergies can wear it comfortably. Many metal spectacle frames contain Nickel. Beta-titanium is a metal alloy containing titanium that is extremely flexible and robust, this material does however also contain Nickel


We can use a Corneal Topographer to ‘map’ the surface of the Cornea allowing us to see exactly how it changes between steepness and flatness.

This then lets us order contact lenses with much greater accuracy, this is particularly useful for patients wearing Gas Permeable lenses including Ortho K lenses. The topographer can also be used to give a better picture of what might be happening with the cornea is certain conditions such as Kerataconus.

Ultraviolet light

Light which is beyond the range of light visible to humans; ultraviolet or UV light is present in sunlight and can cause damage to the eyes, including cataracts and degenerations of the tissues at the front of the eye.

Sunglasses provide an excellent level of UV protection and help to prevent such damage to the eye. All sunglasses sold comply with British Standards BS EN 1836:2005 for sun protection.


Spectacle lenses incorporating a correction for distance vision, intermediate vision and for near vision.

Varifocals are suitable for presbyopic patients who require a reading addition. Varifocals allow the wearer to see any distance clearly without removing their spectacles although there is a small amount of head movement required to ‘access’ the correct area of the lens. Fitting varifocals requires precise measurements to be taken by a Dispensing Optician within the practice.


A measure of how clearly we can see and perceive detail when vision is not corrected with a prescription.

Visual Acuity

A measure of how clearly we can see and perceive detail when vision is corrected with a prescription.

Visual acuity is usually measured by seeing how much the patient can read on an eye chart. Even patients with high prescriptions can usually achieve good levels of acuity with the help of spectacles or contact lenses.

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