Vision and EyeCare FAQ
Section 1: Optics of the Eye and General Information

1.1: What is Myopia ?

Myopia is often referred to as "short-sightedness" or "near-sighted". An eye is myopic when the "far point"; a point at which light from an object is focussed on the retina, is located at a finite distance in front of the eye. Myopia can be due to either an eye which is too long relative to the optical power of the eye (axial myopia), or because the optical power of the eye is too high relative to the length of the standard eye (refractive myopia). The focus is correctly adjusted with a "minus" power lens, or concave lens.

WWW resources:

URL: http://www.aao.org/ - American Academy of Ophthalmology.
URL: http://www.ozarksol.com/medsource/ - contains nice pictures explaining the focal point for each condition
URL: http://www.west.net/~eyecare/myopia.html - details and pictures to explain myopia.
URL: http://www.execpc.com/~warren/optweb/errors.html - John Warren OD home page
URL: http://www.web-xpress.com/vhsc/eyedz.html - informational brochures on eye conditions



1.2 What is Hyperopia ?

Hyperopia is often referred to as "long-sightedness" or "far-sighted". An eye is hyperopic when the far point is at a virtual point behind the eye. Generally the hyperopic eye is too short with respect to the refractive state of the standard eye (ie an emmetropic eye or eye requiring no optical correction) or because the optical power of the eye is too low relative to the length of the standard eye. The focus is correctly adjusted using a "plus" lens power or convex lens.

WWW resources:

URL: http://www.aao.org/ - American Academy of Ophthalmology.
URL: http://www.ozarksol.com/medsource - contains nice pictures explaining hyperopia and showing the focal point of image formation in this condition
URL: http://www.west.net/~eyecare/hyperopia.html - details and pictures to explain the condition.
URL: http://www.execpc.com/~warren/optweb/errors.html



1.3 What is Emmetropia ?

Emmetropia is just another name for an eye that has no optical defects and a precise image is formed on the retina.



1.4 What is Astigmatism ?

An astigmatic eye generally has two different meridians, at 90degrees to each other, which cause images to focus in different planes for each meridian. The meridians can each be either myopic, hyperopic or emmetropic. The correction for astigmatism is a lens power at a particular direction of orientation [ see section 4.1 ] Astigmatism causes images to be out of focus no matter what the distance. It is possible for an astigmatic eye to minimise the blur by accommodating, or focusing to bring the "circle of least confusion" onto the retina.

URL: http://www.west.net/~eyecare/astigmatism.html - details and pictures to explain the condition.
URL: http://www.web-xpress.com/vhsc/astigm.html - diagrams and questions and answers about the causes, symptoms of astigmatism.



1.5 What is Presbyopia ?

Presbyopia describes the condition whereby the amplitude of accommodation, or ability to focus on objects at near, decreases with increasing age. It is corrected by a different prescription for reading, which is additive to the normal spectacle correction used for distance vision. Some recent research indicates that presbyopia may be caused by structural changes in the tendons and elastic fibres of the posterior ciliary body. The age related increase in fibrillar material could cause decreased compliance of the posterior insertion of the ciliary muscle. For more details see Tamm E., Lutjen-Drecoll E., Jungkunz W., & Rohen J.W., "Posterior attachment of ciliary muscle in young, accommodating old presbyopic monkeys." Invest. Ophthal & Vis. Sci. 1991 Apr;32(5):1678-92 URL: http://www.west.net/~eyecare/presbyopia.html



1.6 How is Visual Acuity Measured

Visual acuity is the measure of the sensitivity of the visual system. It is expressed in Snellen notation, expressed as a fraction, where the numerator indicates the test distance and the denominator denotes the distance at which the letter read by the patient subtends 5 minutes of arc. Normal vision is expressed as 20/20 (or 6/6 in countries where metric measurements are used). An acuity of 20/60 means that the patient was tested at 20feet but could only see letters that a person with normal vision could read at 60feet. Other WWW references: URL: http://www.west.net/~eyecare/visual_acuity.html



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© Grant Sayer , email: grants@research.canon.oz.au
 

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