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Optical Terms Explained
What is Accommodation?
Accommodation/accommodate in physiological terms means the automatic or voluntary adjustment of the thickness of the lens of the eye for distance or near vision. Around the mid forties the eye becomes unable to accommodate between distance and reading due to the reduced elasticity of the crystalline lens in your eye. This is called presbyopia.
What is Ametropia?
Ammetropia is an eye abnormality, such as nearsightedness, farsightedness, or astigmatism, resulting from the faulty refractive ability of the eye.
What is astigmatism?
Astigmatism is a very common condition and is nothing to be overly concerned about. It is uneven curvature of one of the eyes optical surfaces, usually the front surface of the cornea, which results in distortion of vision at all distances. Unlike a spherical surface it is incapable of bringing all the rays of light to form a well focused image. In layman's terms the front of the eye is not perfectly spherical like a football, but more rugby ball or egg shaped for example.
Can astigmatism be corrected?
As with other refractive errors, in all but the most severe cases astigmatism can be at least partially corrected with spectacles or contact lenses. If on your prescription you have any numbers under CYLINDER or AXIS this is an indication that you have astigmatism and the prescription you have has been prescribed to correct it.
What is Emmetropic?
Emmetropic is an eye with normal vision, the crystalline lens focuses the rays of light onto the retina. The image is reproduced onto the retina back to front. The retina's photoreceptors transform this image into nerve impulsions which are transmitted to the brain by the optical nerve. This latter transforms the information received from both eyes and produces a unified visual impression. The image we perceive consciously is only produced in the brain, when it reaches the cerebellum's visual cortex. Signals from the eyes have other connections with other places in the brain, for instance for our sense of balance.
After the age of about forty, the emmetropic eye becomes longsighted: Distant vision is normal and only near vision decreases. See presbyopia.
What is long-sightedness, HYPERMETROPIA?
Hypermetropia or Hyperopia results from light rays not being fully focused by the time they reach the retina, due to the eyeball being too flat or too short to suit its optical system. Instead of a clear image forming on the retina as in the ‘perfect' eye, the virtual point of focus is beyond the eye. If the deficiency is not too great the person can exert an effort of accommodation to view distant objects with clear vision. Accommodation is the optical term used to describe the eye's ability to focus at different distances. Long-sightedness results in difficulty seeing objects at close distances. Your prescription will be positive (+), will be thicker in the centre of the lens than the edges and your lenses will magnify objects. When you hold your spectacles at arms length images will be blurry through the lenses.
What is short-sightedness, MYOPIA ?
Myopia is the reverse condition. The eye is too long for the optical system. It results in trouble seeing objects at a distance, only near objects can be seen distinctly. Short-sightedness occurs when the cornea is overly-rounded or the eyeball bulges out too far. This misshapenness causes rays of light to bend too much as they enter the eye and therefore intersect (become focused) short of reaching the retina. Your prescription will be negative (-), will be thinner in the centre of the lens and thicker at the edges and will minify objects. When you hold your spectacles at arms length images will seem smaller, sharp and clear through the lenses.
Presbyopia is the inability to focus on objects up close due to the reduced elasticity of the crystalline lens in your eye. It is a biological process that can not be controlled and routinely occurs around the early or mid forties. Even with perfect distance vision, with or without glasses, a time is reached when reading or close work becomes difficult.
What is Fixation Disparity?
Fixation Disparity is the result of incorrect muscle co-ordination between the two eyes. Prisms are incorporated into the lenses to achieve the correct balance for comfortable binocular vision.
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