The power of optometry is the degree of eyeglasses, which is not the same as the power of contact lenses. If you wear contact lenses for the first time or have just changed your power, you may not know what to choose. In fact, there is a conversion formula. But first you must learn to read your eyeglasses.


What is OD and OS

The first step in understanding the prescription of glasses is to understand the meaning of "OD" and "OS", which are the abbreviations of oculus dexter and oculus sinister, and are Latin words for "right eye" and "left eye".


There may also be a column labeled "OU" in your prescription for glasses. This is the abbreviation of the Latin oculus uterque, which means "eyes".


Although these abbreviated Latin words are often used in prescriptions for glasses, contact lenses and ophthalmic drugs, some doctors and clinics have chosen to modernize their prescriptions for glasses and use RE (right eye) and LE (left eye) instead of OD and OS.


In your glasses prescription, the right eye (OD) information precedes the left eye (OS) information. Ophthalmologists write prescriptions in this way because when they face you, they see that your right eye is on your left hand (first) and your left eye is on your right eye (second).


Other parts of the glasses prescription

Your prescription for glasses also contains other terms and abbreviations. These include:


Sphere (SPH). This is the number of diopters (in diopters (D)) prescribed to correct near-sightedness or far-sightedness. If the number that appears under this heading has a minus sign (-), it means you are nearsighted; if it has a plus sign (+), it means you are farsighted.


Cylinder (CYL). This represents the lens power of astigmatism. If there is nothing in this column, then the astigmatism you need to correct is small or not. The number of astigmatism can be preceded by a minus sign (used to correct for myopic astigmatism) or plus sign (used for hyperopic astigmatism). In the prescription of glasses, astigmatism is always written after the myopia/hyperopia degree.


Axis. This represents the line position (angle) for correcting astigmatism. With a number from 1 to 180 degrees, contact lenses are generally one level every 10 degrees. 90 corresponds to the vertical line of the eye, and 180 corresponds to the horizontal line. If astigmatism is included in the prescription for glasses, it must also include a line position, which is always written after the astigmatism power, starting with "x" when writing.


Addition (Add). This is the degree applied to the bottom of a multifocal lens to correct presbyopia. The number that appears in this part of the prescription is always a "plus sign," even if there is no plus sign in front of it. Usually, it ranges from +0.75 to +3.00 D, and the diopters of both eyes are usually the same.


Sphere conversion

Contact lens power = eyeglasses power ÷ (1-0.012 × eyeglasses power)


Suppose you have 500 degrees of myopia, that is, the degree is -5.00 (the degree of myopia is negative, and the degree of hyperopia is positive, don’t forget to add a decimal point in the formula)

That is (-5.00)÷[1-0.012×(-5.00)]

= -4.71698...


After rounding up, it is -4.72. If you put a contact lens with a degree of 0.25 degree or 0.50 degree, the closest degree is:


-4.75 degrees


Therefore, you should choose -4.75 degree contact lenses.


Cylinder conversion

Contact lenses can slightly correct astigmatism by 0.25-0.50 degrees. If the astigmatism is 0.50 degrees or less, you do not need to wear astigmatism contact lenses. Astigmatism 0.75 degrees or above is recommended to buy astigmatism contact lenses, astigmatism 0.75 degrees can be switched to 0.25 degrees astigmatism contact lenses, 1.00 degrees to 0.50 degrees, 1.25 degrees to 0.75 degrees, and so on.


Axis conversion

Round up to the nearest tens.


Below is the power conversion table, you can find your contact lens power according to your own refraction power or frame glasses power.


The conversion of the power is for reference only, and the final power is subject to the prescription of the registered ophthalmologist. For further inquiries, please feel free to contact us.