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 Shortsightedness (Myopia)
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For a shortsighted person, close objects are clear, but distant objects are blurred and difficult to distinguish.
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Either the optics of the eye are too strong or the eye’s axial length is too long.
When the eyeball is "too strong", the image will be focused at a point in front(short) of the retina, thus the name.
The higher the degree of myopia, the shorter the distance at which the person can see clearly.
Myopia can be corrected using a concave lens (negative lens) that is thinner in the centre and thicker at the edge. This negative lens will focus the image back onto the retina so that it becomes clear and sharp.
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Hyperopia (Long-sightedness)
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Longsighted people see better at a distance but may find it difficult to focus on near object. Although distance objects are properly focused, the extra "accommodation" required to focus on near objects may cause eyestrain, headaches, or blur.
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Either the optics of the eye are too weak or the eye’s axial length is too short.
When the eyeball is “too weak”, the image will be focused at a point behind (too long) the retina, thus the name
Hyperopia can be corrected using a convex lens (positive lens) that is thicker in the centre and thinner at the edge. This positive lens will focus the image forward onto the retina so that it becomes clear and sharp.
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Astigmatism
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People with astigmatism may have blurred near and distant vision. They do not perceive the contrasts between horizontal, vertical and diagonal lines in the same way. Astigmatism can be a visual error on its own or is accompanied by myopia, hyperopia and presbyopia.
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Astigmatism is usually caused by an irregularly-shaped cornea that is slightly oval, that is more curved in one direction than another. This will result in the eye having different powers in two different positions perpendicular to each other.
Astigmatism can be corrected using a cylindrical lens whose curvature offsets the corneal irregularity.
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Presbyopia
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Presbyopia is a natural process that, as we age, will affect everyone in the population older than 40, regardless of whether or not they have always had normal vision or have vision anomalies such as myopia, hyperopia and/or astigmatism. The timing as to when the symptoms of presbyopes will surface will vary between individuals. The general trend is that the onset for emmetropes (people with perfect eyesight) and hyperopes have an earlier onset of symptoms around the age of 39-42 whereas for myopes, the symptoms are noticeable at a later stage around the age of 45.
Presbyopia is a person's increasing inability to maintain near objects, such as fine prints in focus. People with presbyopia tent to hold reading materials further away to be able to focus on what they want to see.
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The crystalline lens grows throughout our life becoming larger and less able to change shape. If the lens can't change shape it can't focus (accommodate) onto near objects.
Although all lenses for presbyopia correct near vision, they have different characteristics. There are two main types of lenses used to correct presbyopia:
 Progressive Addition Lenses
Bifocals
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Progressive Addition Lenses
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Progressive Addition Lenses (PALs) deliver a gradual change of focus from distance to near. The continuous progressive surface increases power of the lens from the distance portion to the near portion by changing the curvature of the lens surface.
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As a result progressive lenses deliver clear vision at any distance. And because the lens has no visible lines, the wearers make natural, seamless movements to the correct vision area for distance, intermediate or near vision.
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Advantages of Using Progressive Lenses
 Better vision than bifocals/trifocals – PALs simulate a person’s normal vision
A complete range of working distances for any task
Attractive – look like a pair of ordinary lenses
Convenient – only one pair of glasses is needed
No image jump
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Bifocals
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Bifocals has two regions on the lens; distant and near vision. However, this older type of lens has a visible line that creates an abrupt transition or jump in the transition from distance to near vision and vice versa.
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Round Segment
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Flat-Top
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Curved Top
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Executive
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