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The Difference Between a Contact Lens & Spectacle Prescription

Glasses Prescription explained

A prescription is normally written out by an optometrist like this:

R. -1.00/ -1.25 * 175

The first figure that you will see on an optometrists prescription is the SPHERE (abbreviated Sph). This is the main power of the prescription and represents myopia (shortsighted) when prefixed with a minus sign i.e -1.00, and hypermetropia (longsighted) when prefixed with a plus sign i.e +1.00. Often the Sphere power has a DS after it like this: +3.00DS. The "DS" signifies Dioptres Sphere, which is the unit of measurement for spectacle power. 

The next figure you see is the CYLINDER power (abbreviated cyl). This represents the astigmatism in your prescription. Often this figure is shown like this -1.25cyl. Astigmatism is always along a certain angle (axis) from 0 to 180 degrees. i.e -1.00 * 175. Finally the sphere and cylinder power are usually separated by a slash
 ie sphere/cylinder * axis The R. signifies the right eye. (L. for the left)

A prescription for glasses forms the basis for a contact lens fitting. If you look at your prescription it will be in a format similar to one of these;

R. -3.00DS
L.  -3.50DS
This indicates that you are myopia (shortsighted) of moderate degree. Powers can
range from -0.25DS to well in excess of -20.00DS. An average power for the myopic 
population would be about -3.00DS.

R. +3.00DS
L. +3.50DS
This indicates that you are hypermetropia (longsighted) of moderate degree. Powers can range from +0.25DS to well in excess of +20.00DS. An average power for the 
hypermetropic population would be about +3.00DS.

R -2.00/-1.00 * 35
L. -1.25/ -1.50 * 90
This prescription indicates that you are myopic, plus have a small amount of astigmatism
Astigmatism is best described as an irregularity in the shape of the cornea. The cornea
is the clear covering on the front surface of the eye that helps the eye focus. An
astigmatic cornea is often said to be shaped like a football. A normal cornea is round like a basketball. 
The different curvatures on the surface of the cornea cause  the light entering
the eye to focus at different points causing distorted vision. Astigmatism can affect one 
or both eyes, and is sometimes coupled with other visual impairments such as 
nearsightedness and farsightedness (see example below). This example shows a 
prescription that is shortsighted (the -2.00 part in the right eye), with 
astigmatism (the -1.00 part). In addition the final figure (35 in the case of the right eye)
indicates the axis (angle) of the astigmatism. The axis (angle) can lie between 0 and 180 
degrees and indicates the angle of the ovalness of the astigmatism. Low astigmatism
would be below 1.00, moderate 1 to 3, and high above 3.

R. +2.50/ -1.00 * 180
L. +3.25 / -1.25 * 10
This prescription indicates that you are hypermetropic (longsighted), plus have a small
amount of astigmatism

R +2.00/ -1.50 * 170
L. +2.50/ -175 * 180 
Add +2.00
Sometimes the prescription will have an ADD power written after it, in particular if you
are aged 45 or over. This indicates that you are presbyopic and need a additional
power to help you read. You can have presbyopia in addition to any of the
prescription above, or you may have only presbyopia and perfect vision otherwise.


Interchangeable?  

A prescription for glasses is different than a prescription for contact lenses although they can be similar. A contact lens comes in direct contact with the cornea, which is the clear covering over the front surface of the eye. For this reason, there are additional measurements needed for fitting a contact lens and these need to be included in the prescription.

- the curvature of the cornea
- the diameter of the cornea

Base Curve (BC)
The curvature of the cornea is measured to determine the exact curvature (base curve) of the inside surface of the contact lens. This ensures the lens fits the shape of the eye providing optimum vision and comfort.

Diameter (D)
The diameter of the cornea is measured and also necessary to find the optimal contact lens diameter. This measurement is fairly straight forward. A contact lens is usually 2 - 4mm larger than the diameter of the cornea.

The Same Only Different

A contact lens prescription can be the same as a prescription for glasses when:

- the prescription is a lower power, ie: -4.00 to +4.00 and

- there is absolutely no astigmatism present.

If you have any astigmatism or require a lens power greater than +/-4.00, the parameters of the contact lens can change.

- if you have astigmatism under 0.50, a spherical lens should be prescribed.
- astigmatism of 0.50 to 0.75 can be effectively corrected with an aspheric lens.
- astigmatism greater than 1.00 should be managed with a toric lens.

If your prescription power for glasses is greater than +/-4.00, you should see a difference in the power of your contact lens prescription. Generally, every time your glasses prescription increases or decreases by a power of 2, your contact lens prescription will increase by an additional power of +0.25. For every power change of 2 in a glasses prescription, the difference of +0.25 power is needed to compensate for the location of the contact lens. Your glasses are much further from the front surface of the cornea than contact lenses.

It is beyond the scope of this website to discuss all the factors involved in creating a contact lens prescription. It is important, however, that you realize a contact lens is classified as a medical device and caution needs to be taken when ordering and wearing them. Only your eye care provider can determine the success of your contact lens experience and we hope that the information we offer here can help you make informed decisions and ask informed questions.


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