The Optics of the Eye
The physical and optical properties of the eye are very similar to those of a camera.
The corneaThe cornea is the clear, transparent 'front window' of the eye through which light enters the eye. It handles about two-thirds of the focusing power of the eye and is critical for good vision. See Info on Eyes – Anatomy. (the clear ‘front window' of the eye) and the natural lensThe lens fulfils the same role as the lens in a camera. It handles about one-third of the focusing power of the eye and is critical for good vision.See Info on Eyes – Anatomy. combine into a complex ocular lens system, to fulfil the role of the lens in a camera. They bundle parallel (distant) light rays that enter the eye and focus an image on the retinaThe retina is the receptor of light at the back of the eye. It fulfils the same function as the film in a film camera or the image sensor in a digital camera. The retina translates the images into electrical signals that are sent via the optic nerve to the visual cortex in the brain, where it is interpreted as the images we see. See Info on Eyes – Anatomy. at the back of the eye. The retina generates an electrical signal from this image, which is then transmitted via the optic nerveThe optic nerve transmits the visual signals from the retina to the visual cortex in the brain, where it is interpreted as the images we see. See Info on Eyes – Anatomy. to the brain, where it is interpreted and translated into the images we see.
The cornea and lens collectively determine the focal power of the eye, of which the cornea is responsible for about two-thirds, and the lens for about one-third of the total power. So, as you may now appreciate, the cornea plays an extremely important role in ensuring good vision. Any defect of the cornea, or any distortion introduced by the cornea, affects vision in a negative way.
Laser vision correctionLaser vision correction is a technique that has the ability to permanently correct refractive errors of the cornea by means of a laser treatment. Several types of laser vision correction treatments are available. See Laser Vision – Introduction to Laser Vision Correction. systems are designed to reshape the cornea of an otherwise healthy eye, so that it can fulfill its focusing role accurately and without distortion.
In the illustration below, light enters a normal seeing eye from the left. The cornea, together with the natural lens of the eye, work in perfect harmony to focus images on the retina at the back of the eye. These images are clear, sharp and in focus.
This illustration shows how light enters the eye through the cornea and lens, how it is focused on the retina at the back of the eye and what the image looks like.
Common Refractive Eye Problems
The 4 most common refractive eye problems are briefly discussed below, with examples of their impact on your vision.
Distant and intermediate objects are seen as blurred and out of focus, whereas close-up objects are sharp and in focus, as demonstrated in the example below. To experience clear vision, these individuals have to wear glasses or contact lenses. The near-sighted eye is usually longer than a normal eye, and its cornea may also be steeper. Therefore, when light passes through the cornea and lens, it is focused in front of the retina. This will make distant images appear blurred. There are several refractive surgery solutionsAny condition that affects the focus of the eye, falls under the generic description of a refractive error. Refractive surgical solutions, therefore, include all corrective surgical measures to restore vision to normal, such as a refractive laser treatment, a cornea transplant and an intra-ocular lens implant. available to correct nearly all levels of nearsightedness.
Farsighted individuals typically develop problems reading up close before the age of 40. The farsighted eye is usually slightly shorter than a normal eye and may have a flatter cornea. Thus, the light from distant objects focuses behind the retina, unless the natural lens can compensate fully. Near objects require even greater focusing power to be seen clearly and therefore, blur more easily. Close-up and intermediate objects are therefore blurred and out-of-focus, while distant objects are in focus, but not necessarily as sharp as they should be, as demonstrated in the example below. LasikLasik is one of several types of laser treatments to permanently correct refractive errors of the eye. See Laser Vision – Introduction to Laser Treatment., refractive lens exchangeA refractive lens exchange is when the natural lens in the eye is replaced by one of several types of artificial lenses. See Other Options – Refractive Lens Exchange. and Intraocular Contact lenses are a few of the surgical options available to correct farsightedness.
Asymmetric steepening of the cornea or natural lens causes light to be focused unevenly (the images focus on multiple points in front of and/or behind the retina), which is the main optical problem in astigmatismAstigmatism is a condition caused by a cornea that has more than one curvature, similar to a rugby ball. The same image, therefore, focuses on multiple points in front of, on and/or behind the cornea, creating multiple and ghost images. Astigmatism may also be found in association with either myopia or hyperopia, which complicates the condition substantially. See Info on Eyes, Optics and Refractive Errors - Astigmatism.. To individuals with uncorrected astigmatism, objects at all distances are blurred and/or appear as multiple or ghost images. Astigmatism can accompany any form of refractive error and is very common. The ideal treatment should therefore address the primary problem, as well as the perhaps not-so-obvious secondary problem in one pass to fully correct vision. Astigmatism can be corrected with glasses, contact lenses, corneal relaxing incisions, laser vision correction, and special implantable intraocular lenses.
presbyopiaPresbyopia is an age-related condition. As we age, the natural lenses in our eyes gradually become inelastic and lose their ability to focus on nearby objects and fine print. From the age of 40 it becomes increasingly noticeable and people often jokingly complain that their 'arms are getting too short'. See Info on Eyes – Optics and Refractive Errors - Presbyopia. is an eye condition related to ageing and causes in previously normal sighted individuals, a progressive decline in the ability of the eye to focus on objects at near. This is part of the normal physiology of the eye over time, and nobody will eventually escape this condition.
The eye's ability to focus on nearby objects declines progressively throughout life. For example, a child with normal vision can focus on objects as close as 5 cm. At the age of 25, this distance declines to about 10 cm and at the age of 60 it declines further to between one and 2 meters. And, of course, the poorer the light, the worse the effect of this phenomenon.
Although presbyopia is not completely understood, it is thought that some of the factors playing a role include a loss of elasticity in the natural lens, stretching of the delicate ligaments attaching the lens to the surrounding muscle (zonulesZonules are small ligament attachments between the natural lens and the cilliary body that keep the lens in position and transmit the force from the cilliary body to the natural lens, thereby changing its shape to enable changes in focus.), and a reduction in the power of the muscles that control the lens (cilliary body). This culminates in an inability to adjust the shape of the natural lens during accommodationAccommodation is the physiological process by which the eye changes focus. A ring of muscle around the lens, called the ciliary body, changes the shape of the lens, which allows the eye to focus at different distances. or close vision effort, which is required to focus on objects at near.
The first signs of presbyopia are eyestrain, difficulty seeing in dim light and a lack of ability to focus on small objects and/or fine print. These symptoms are usually first noticed between the ages of 40 and 50 and people jokingly complain that their 'arms are getting too short'.
Besides glasses, presbyopia can be dealt with in a number of ways. The options include: monovisionMonovision is a term used to describe the situation where one eye is corrected by means of contact lenses or laser for near vision, while the other is corrected for distance vision in presbyopic individuals. It enables the patient to be less dependent on spectacles or contact lenses and applies to near and distance vision. See Near Vision – Laser Blended Monovision. and multifocal contact lenses, Monovision laser vision correction, and new presbyopia correcting implantable intraocular lenses.