Buying spectacles can be extremely daunting for a lot of people... What type of frame? Heavy black thick rimmed or almost invisible rimless? Classic or modern? Sporty or elegant? There are so many choices! Then comes the technical stuff... What frame will suit my prescription? Understanding different lens types... Which coatings shall do I need? This list goes on.
Before you choose either an optical frame or a pair of sunglasses to be fitted with prescription lenses, you need to have some understanding of your prescription so that you can choose a suitable frame. Some prescriptions will limit your frame choice. If your sphere power or cylinder power is higher than 2.00 dioptres, you need to avoid really large frames and wrap around sunglasses or your lenses could end up very thick.
An explanation of the different parts of a prescription are:
The sphere power. This may also be abbreviated to “sph”. All prescriptions will include a number for the sphere. This number may begin with a + or a - symbol. If your sphere power has a + symbol, this means that you are hyperopic. This condition may also be referred to as being long sighted or far sighted. If your sphere power has a - symbol, this means that you are myopic. This condition may also be referred to as being short sighted or near sighted. You may also have a sphere power of 0 (zero). This may also be referred to with the word “plano”. Plano = zero. The sphere power is prescribed in increments of 0.25 dioptre steps. A lens that only has a sphere power is the same in all directions which means that it can be turned around (twisted to any axis) and it won’t change the way that it magnifies or corrects. A spherical lens surface is a portion of a sphere (shaped like a ball). The spherical component of a prescription is the main correction. Some prescriptions may also include a cylinder to fine tune the image for the wearer.
The cylinder power. This may also be abbreviated to “cyl”. Most prescriptions will include a cylinder power. A cylinder power is used to correct astigmatism. Astigmatism causes directional blur. You may have a cylinder power combined with a sphere power of zero or it may be combined with either a + or a - powered sphere. If there is a cylinder power on your prescription, the prescriber must supply a number for the axis too or the laboratory won’t be able to make your spectacles. The cylinder power is also prescribed in increments of 0.25 dioptre steps. A cylindrical lens surface is a portion of a cylinder (shaped like a tube).
The axis. This may also be abbreviated to “x”. An axis will be supplied by the prescriber if you have a cylinder in your prescription. This is so the laboratory know which angle the cylindrical part of your prescription needs to be set at in the frame so that it can correct the directional blur in the correct meridian.
The near addition. This may also be called either an “add, near add, reading add” or a “reading addition”. Most people over the age of 50 years will have a near addition on their prescription. This is to correct presbyopia. Presbyopia is a condition which occurs when the crystalline lens which is inside the eye becomes less flexible with age. This condition makes it difficult to focus on close objects. A near addition is the amount of power which is needed to be added to the sphere power so focus on near objects. This power is also measured in 0.25 dioptre increments. To work out your reading prescription, you need to add your sphere power to your near addition. Most near additions are focused so that the wearer can see objects clearly at 40cm from the eye. If the wearer tries to focus further away, the image will be blurred.
The intermediate addition. This may also be known as an “inter add” or “int add”. This will be a lower number than the near addition as the intermediate addition is used to focus on objects which are further away than 40cm. An intermediate prescription may be needed for computer use or for reading music on a stand etc.
Prism. A Prism is used to correct double vision. It does this by moving the image in a particular direction. The direction will be either up, down, in or out. This may be abbreviated to DN for down and either OU or OT for out. Most prescriptions won’t include prism correction.
Here’s an explanation of lens recommendations or dispensing instructions and abbreviations:
SV = single vision
SVD or dist = single vision distance
SVN or near or reading = single vision near
SVI = single vision intermediate
BF = bifocals
TF = trifocals
MF or prog or PAL = multifocals or progressives
EF or Access = extended focus lenses or extended reading lenses (these are blended lenses with more than one focal length which are focused for near and intermediate)
You will probably only be able to order single vision lenses online as the lens laboratory needs specific measurements to make up bifocals or progressives. These measurements need to be taken by an eye care professional with you wearing the frame that you have chosen and cannot be guessed.
Ordering Spectacle Lenses Online
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1 October 2009
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