Sunday, August 17, 2014

Diopter

Let’s start with a standard unit of measurement on eyeglass prescriptions, called a diopter.

Understanding your eyeglass prescription takes a bit of work, but it can be interesting stuff. Many people are intrigued about what the magic formula their eye doctor has come up with actually says about their eyes.

So you’ve been handed a piece of paper by your optometrist or ophthalmologist with your eyeglass or contact lens prescription. Perhaps you’ve been given these pieces of paper for many years. It’s full of mysterious abbreviations, terms and numbers, and you’re curious…. Just what does it all mean?

Your prescription is all about correcting refractive error – an eye condition that affects millions of us all over the world. (Read Refractive Error Revealed to understand more about how refractive error works.)

Refractive error causes blur, and our glasses or contact lenses correct for this blur based on our own particular type of refractive error. (This is why if you try on someone else’s glasses, you usually see blur. What corrects another person’s refractive error is not what you need, so it makes your vision worse.)

Nearsighted people typically experience blur when looking at objects that are far away. Farsighted people typically see things well when far away, but encounter blur when looking at things close up.
 

What is a Diopter ?

A diopter can be a negative number (which indicates nearsightedness and a lens that minimizes things). Or it can be a positive number (which indicates farsightedness and a lens that magnifies).



But what exactly is a diopter? A diopter simply indicates how powerful a lens is in order to properly focus light on a person’s retina, and it is defined as being “the inverse of a person’s focal length in metres.” Focal length is the distance you need to see an object clearly for a fine-detail task, for example to read a book.

The optical power of the human eye is about 40 diopters. The eye of a normal young person can adjust an additional 20 diopters. By age 25 this accommodation (the ability to alter focus)is usually reduced to about 10 diopters and by age 50 to a mere 1 diopter. It is this diminishing capacity for adjustment, called presbyopia, that warrants reading glasses.

Reading glasses compensate for the loss of natural adjustment. Optical power is additive so the correction is straightforward. An optometrist prescribes a lens that increases the magnifying power of the eye, usually in steps of a quarter-diopter over a range of 1 to 3 diopters. A quarter-diopter is a large enough step that most people can self-prescribe their own reading glasses by simple trial and error.
 
The optical power of a lens with a focal length of 1 meter (about 39 inches) is said to be 1 diopter. Because the formula is based on the reciprocal of the focal length, a 2 diopter lens is not 2 meters but 1/2 meter, a 3 diopter lens is 1/3 meter and so forth. This is important because magnification increases as the focal length gets shorter, which is why a prescription for a higher diopter correction means you need more magnification.

So, a nearsighted person who needs a –1.00 diopter lens can see objects at one metre clearly, but anything farther is blurred. Someone with a –2.00 diopter measurement requires a lens that is twice as powerful – so they can only see objects up to a 1/2 metre away clearly. A –3.00 lens would mean the person can only see a distance of up to 1/3 of a metre clearly, and so on. Most nearsighted people are in the range of -1.50 to -7.00 diopters, which is considered mild to moderate.
 
On the other hand, a farsighted person who needs a +1.00 diopter lens can see objects at one metre clearly, but anything closer than that is blurred. A +2.00 lens indicates someone can see things at 1/2 metre and beyond clearly, but nothing closer.  
The level of magnification in the lenses of reading glasses is called diopter strength. A very weak magnification would be found in reading glasses with a diopter strength of +.75 or +1.00. Off-the-shelf reading glasses sold in drug stores or elsewhere on the Internet are often offered in diopter strengths that start with 1.50, and with only a few options for stronger lenses.

Each pair of our glasses is custom fitted by a certified optician.Generally, the eye care industry classifies reading glasses lenses in diopter strengths that increase by a factor of .25 (e.g., +.75; +1.00: +1.25; +1.50; +1.75; +2.00; +2.25; +2.50; +2.75; +3.00 etc.).

Presbyopia is progressive, and therefore, the strength of magnification in your reading glasses will probably have to be increased every year or two. Presbyopia sometimes affects your left eye and your right eye differently. Therefore, you may need, for example, a +1.50 in your right eye and a +1.75 in your left eye.

This is normal, although many people can overcome their Presbyopia perfectly by using glasses that have the same diopter strength in each eye. Nevertheless, if you fall in the category of people who need differing lens strengths for each eye, you could be doing yourself a grave dis-service if you bought a cheap pair of one-size-fits-all drug store glasses that had the same diopter strength in both lenses. Those are also the kind of stock reading glasses sold on most of the other Internet sites.

We believe we offer you an extremely valuable option in that we can easily accommodate you if you need a different lens for each eye.

Moreover, depending on what else is going on with your body, you may find that your need for stronger and weaker lens strengths can fluctuate slightly during any given period. In fact, you may need a stronger lens strength to eliminate the blur when you read your newspaper in the morning than what you would need to read the same newspaper in mid-day. That is why some experienced users of reading glasses have more than one pair, each with slightly different diopter strengths. According to these experienced users, whether you would find greater comfort with more than one pair is something best determined through trial and error.

Reading glasses are available without a prescription, and are intended as an aid if you have blurred vision at close distances. It is your right to self prescribe your own glasses, and if you do so, you will not be violating any law or regulation. In fact millions of people just like you have determined for themselves what strength glasses they needed, and then bought them without ever consulting a doctor or other licensed professional. The easiest way to find out the diopter strength you need is to visit a drug store, department store or other retail outlet that sells cheap reading glasses, and try on a few different strengths.

If you have no idea where to start, consider using this average age formula: If you are under 40 years old, you probably should start with a +1.25 strength. Between ages 40 and 45, you probably need +1.75. At ages 45 to 50, you probably need +2.00. After age 50, you probably need +2.25.

While this is based on averages, and you should try to find the precise lens that helps you, the nice thing about buying FOCUSERS is that if you make a mistake, you can quickly and at no risk to you, rectify it under our RETURN POLICY and NO EXTRA COST EXCHANGE POLICY.

In addition to the methods mentioned, you can always visit an optometrist or ophthalmologist to have your eyes refracted.