Understanding Your Eye Prescription

Understanding Your Prescription

When ordering prescription glasses or prescription sunglasses online you will need to have an up to date prescription and it is also helpful to have an understanding of your prescription so that you will be able to enter the details accurately. This article is designed to give you an understanding of your eye prescription in preparation for ordering online, in particular I refer to ordering prescription sports eyewear, however it will be useful to everyone.

Following a professional eye test with a qualified Optometrist (Ophthalmic Optician), you will be given a written or printed and signed copy of your eye prescription.

Your prescription will look similar to the following samples:

Each row of the prescription refers to a different eye.  R.E or O.D refers to your right eye and L.E or O.S refers to your left eye.  The values highlighted in figure 3 refer to the left Eye.

Your prescription is made of three main figures (for each eye). The sphere power (SPH), the cylinder power (CYL) and the AXIS.

Sphere Power (SPH)

The Sphere or SPH,  is the main value of the prescription and gives the overall power of the prescription.

Sometimes the prescription may only have a SPH value for example:  +1.25 , or -3.75, in this case  the CYL and AXIS parts will be left as 0.00 or DS (see figure 1). 

Myopia
If your prescription starts with a ‘minus’(-) before the first number e.g. -2.75, then it means you are short sighted (myopic).
 
Usually objects will look more blurred in the distance, but more clearly defined when close to you.

Hyperopia (hypermetropia)
If your prescription starts with a ‘plus’ (+), before the first number e.g. +2.75  then it means you are long sighted (hyperopic). Usually objects close to you will look more blurred, but more focused for things which are in the distance.  

If you are under 40 years you may not need to wear prescription eyewear.   Your natural level of focusing (accommodation) may keep images clear with no ill effects.  As they have assessed your eyes, your optician will know if  you require a correction.

Cylinder Power (CYL) and AXIS
The CYL and AXIS values are always supplied as a pair, i.e. if a cylinder is specified it must also have an axis.  They are always written in this order, usually the CYL has a - (minus sign) in front of it. Rarely it will have a + (plus) in front, and it is important that the correct choice is made.

All SPH and CYL powers are specified in (0.25) steps, i.e. the value written after the decimal point will be 0.00, 0.25, 0.50 or 0.75.

The AXIS is an angle between 1 to 180 degrees.  If there is a CYL specified on your prescription, then there will be an accompanying AXIS. 

Astigmatism
If you have a CYL and AXIS after the SPH number, then you have astigmatism. This is quite an ordinary part of an eye sight prescription, and means that the front of the eye (cornea) and/or lens is rubgy ball shaped rather than perfectly spherical.  This distorts the light coming into the eye. 

ADD
If there is an ADD on your prescription, this is the additional power needed for a reading prescription. Ignore this figure if you are ordering distance prescription glasses or prescription sunglasses online as the ADD value will not be required.

Your PD measurment
The Pupillary Distance, or pd, is the measurement of the distance between the pupils. It is an important factor creating prescription eyewear, and enables the optical centre of the lens to be placed accurately in front of the eye.  Often the pupillary distance is not included on a standard prescription, but a value is needed for ordering prescription glasses/prescription sunglasses.  The pupillary distance can be requested from your Optician, who will already have this measurement if they have made spectacles for you in the past (there may be a charge for this).  Alternatively in many cases it is appropriate to use the average pd which for men is 32 for each eye. 

Further information and advice regarding eye prescriptions can be found in the help centre section of www.balanceoptics.com where you can also purchase superb quality sports sunglasses.  I am a qualified Optician, and I am happy to answer questions and provide advice if you would like further explanation of your eye prescription.
 

Varifocals and Cycling

Should I wear Varifocal Prescription Sunglasses when Cycling?

Whether to use varifocal prescription sunglasses for cycling is a question we are commonly asked by cyclists. Our usual response is simply no, although Varifocals are great as everyday lenses, for most sports they are not ideal and here’s why.

With a varifocal lens, the power at the top of the lens is different to that at the bottom of the lens. When looking thought the upper part of the lens you are able to see objects in the distance, whilst the bottom of the lens is designed for reading. This allows you to be able to see at all distances, but means that objects below you for example the ground, and your handlebars when cycling, are magnified, and appear to be closer to you than they actually are. This creates visual distortion, which can lead to incorrect judgement of distances which isn’t ideal for cycling and it could have a negative impact on your performance!

Although you may miss the ability to read as easily, it will be a small price to pay compared to the visual distortion you will experience with varifocals when cycling.

For the best vision in sports, it is important to have a view that is uninterrupted and not distorted. If you have a distance prescription, then consider prescription sunglasses or the use of prescription inserts in your regular sports sunglasses rather than varifocals. Oakley produce excellent prescription cycling sunglasses, as do Rudy Project and Bolle and you can find more information, obtain professional advice and shop at www.balanceoptics.com.

If you have any questions please feel free to ask me. I’m a qualified Optician and happy to give advice.