Understanding diopters – what you should know

When you have your eyes checked and the optician gives you a prescription, it does not contain a list of medications to buy or procedures to undergo, as you would expect from other doctors. Instead, an optical prescription consists of a bunch of numbers that indicate the focusing strength of the glasses that you need. 

These numbers are called diopters. Basically, the higher the number, the stronger the prescription. For some people, that is the only explanation they need. However, if you are interested in learning more about diopters and what they say about your vision, we will explain it further in the following sections.

What is diopter?

A diopter is a unit of measure that tells the focusing strength of the lenses in a pair of eyeglasses or contacts. It is the number that the eye doctor gives you in your prescription so that you can have the correct glasses made. So how do they arrive at this number? Basically, it is the inverse or reciprocal of the focal length. But what is the focal length?

The focal length is the distance from the center of the lens to the point where the object becomes focused. If the object comes into focus at 1 meter, then that is the focal length. For a focal length of 1 meter, the lens would have a focusing strength of 1 diopter.

What if the focal length of a lens is 1/2 meter? This would indicate an optical strength of 2 diopters. Furthermore, if the focal length is 1/3 meter, the diopter strength would be 3. As you can see, a shorter focal length indicates a higher diopter number.

What do the numbers mean in diopter chart?

A diopter chart is designed to identify the strength of reading glasses that you need, or if you need any at all. The chart is basically a printout of rows of text in different sizes, with the smallest text on top and gradually getting bigger as you move down the page. You must read each line without wearing any glasses or contact lenses. If the first line is not clear, move on to the next, and so on until you reach a line that is perfectly clear.

Now, each line has a number next to it, which indicates the correct dioptric power of the reading glasses you need in order to see clearly. The further down you need to go in the chart, the higher the reading strength you need in your glasses.

Many eyeglasses stores have diopter charts that you can use for buying over-the-counter reading glasses or buying it at Lensmart. This is fine for most cases but if you want the most accurate prescription, it is better to go to your eye doctor for a more thorough exam.

Eye prescription terms related to diopters

Going to the eye doctor can be a bit overwhelming when you are not familiar with the terms that you see in your eye prescription. Now that you have a full understanding of what diopters are, it will be easy to interpret your prescription. First of all, it indicates the specific vision problem you may have, which can be any of the following.

Nearsightedness or Myopia – Nearsightedness is when you see near objects clearly and faraway objects are more blurry.

Farsightedness or Hyperopia – The opposite of nearsightedness, this is where faraway objects are clear and nearby objects are not.

Astigmatism – This condition occurs when there is an irregular curvature in the eye, causing blurry vision.

Presbyopia – This normally occurs during the mid-40s and is generally indicated by the inability to see text clearly within the normal reading range.

SPH – This refers to the amount of spherical correction you need for your vision problem, and can range from 0.0 to +/-20.0. The number is negative for myopia correction, and positive for hyperopia.

CYL – In general, this number ranges from +/-0.25 to +/-4.00 and only applies to people with astigmatism.

Axis – This number indicates the angle for the correct placement of astigmatism correction in the lens.

Eye care tips

Vision normally deteriorates due to age, lifestyle, or genetics. Oftentimes, there’s nothing we can really do to stop it. However, we can minimize the damage that our eyes can sustain with some simple practices, like not smoking, wearing sunglasses when outdoors, and refraining from rubbing at all times.

It’s also important to not strain your eyes at work. Take frequent breaks when working in front of the computer, and blink consciously to keep your eyes lubricated. Finally, if you have to wear eyeglasses or contact lenses, give them proper care to avoid scratching and damage, which could further aggravate your vision. 

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