In an effort to make our customers and those who are looking for information on prescription glasses or sunglasses increasingly aware, with this article we will provide simple notions regarding diopters. This term is certainly not new to those who have already worn glasses for a long time, although not everyone knows its meaning in detail. Those who are approaching the subject for the first time will have some more tools to avoid feeling lost when they hear about it.
We will explain what is meant by this term, its meaning, the types of diopters and how the calculation is carried out to determine how many we have and – consequently – how many we need in order to correct our eyesight.
What are diopters?
Let’s start with the basics, starting from the concept that the word “diopter” represents in ophthalmology and optics: it is the unit of measurement of the optical refractive power of the lens, expressed in “m−1″. It therefore indicates how much power of the lens is needed in order to reach the value that establishes what is conventionally normal vision, i.e. 10/10.
It expresses the ability to change the direction of the rays of a beam of light and this can happen in two ways: divergent or convergent. It can be a positive or negative value, based on a calculation that is performed to obtain as a result the reciprocal of the focal length expressed in meters, according to this formula:
How many diopters do we have?
The human eye, in normal and relaxed conditions, has a dioptric power that tends to range from approximately 59 to 62 diopters, measured in “(+/- 0.5)”, which corresponds to the focal length of 16mm. During an eye exam, the eye’s ability to focus images is therefore verified. When it – which is in itself a dioptric system – is no longer able to “capture” light and converge it correctly towards the retina, then it means that there is a so-called “refractive defect“, that is, we may be in the presence of:
- astigmatism,
- myopia,
- hyperopia.
The lenses will therefore correct the present defect, being able to count on a wide range of customization. In fact, the lens can be characterized by a positive (+) or negative (-) value, based on the necessary correction.
To simplify, when we talk about diopters +, we must modify a hyperopia using convex lenses. When instead they talk to us about the need for diopters -, then we must correct a myopia with concave lenses. In the case of astigmatism, we finally talk about toric diopters.
In any case, the aim is always the same: to provide a tool to achieve as much as possible a definable optimal vision, that is 10/10 (ten tenths).
Types of Diopters
The healthcare professional, following a thorough eye examination, will be able to recommend the best optical system to compensate for deficiencies and correct defects. They can do this thanks to different types of lenses, to be chosen based on what emerged from the consultation.
If you are wondering how to interpret an eye prescription, at the end of the visit, the doctor will fill out a “sheet” with the drawing of two semicircles, on which the diopters, tenths, distances etc. will be indicated.
It is thanks to this prescription that the optician will be able to compose the glasses, paying attention to the type of lenses indicated in the prescription. These can in fact have different characteristics:
Spherical
Spherical lenses are those used in cases of myopia (and are characterised by the – sign) or hyperopia (with the + sign). They are indicated on the prescription sheet in the “Sphere” column, with the acronym “SF” or “SPH”.
Cylindrical
The cylindrcial ones have been studied and created to correct astigmatism. In this case, on the prescription we will note the filling in of the “Cylinder” column, with the acronym “CIL” or “CYL”.
Prismatic
This type of lens is indicated in very specific cases: to simplify, it is prescribed when vision is doubled. It mostly corrects phoria and strabismus or is recommended to patients who are waiting for surgery, in certain situations that require it. Since these are very particular subjective situations, prismatic lenses are also difficult to create in some cases.
How are diopters calculated?
We must imagine diopters as a long scale: each step is worth 0.25. Conventionally, it has been established that each lens has the power of 1 diopter when it allows parallel rays to converge at a distance of 1 meter.
So, to continue with the example, if you try a 2 diopter lens, it means that the rays will converge at a distance of 50 cm and so on. We can conclude that the dioptric power of a lens is the opposite of the distance at which it can focus parallel rays.
To measure how many diopters a pair of glasses we already own has, the optician or eye doctor will use a focalometer.