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Spectacular Spectacles

Prescription Guidelines

Cary TownOptical AboutUs

Eyeglass lenses correct refractive errors within the eyes. When choosing lenses keep in mind that the higher the refractive index, the thinner the lens. Why? The index of refraction of the lens material determines the light-bending ability (lens power) of eyeglass lenses and is the ratio of the speed of light when it travels through air to the speed of light when it passes through the lens material. The speed of light is reduced when it is refracted as it passes through a lens material. Lenses that bend light more efficiently have a higher index of refraction and are therefore thinner. The lens power that you need to ensure high-quality vision is indicated on the eyeglass prescription from your eye doctor. The lens power to correct refractive errors in the eyes is measured in diopters (D). If you are nearsighted the value is negative (thinner in the center and thicker at the edge) and farsighted it is positive (thicker in the center and thinner at the edge).

If you want thinner and lighter lenses that are attractive and comfortable choose high-index eyeglass lenses. High-index lenses (lens material with a refractive index that's higher than that of glass i.e. 1.523) are particularly suggested if you have a strong eyeglass prescription for astigmatism, farsightedness or nearsightedness. The stronger the prescription for nearsightedness the thicker the lens edges as mentioned before. The edges of the lens are most visible, and when thick, can detract from the look of your eyewear. With trendy frames made of plastic and metal, the rim is thinner than the lens and with popular rimless frames the lens edges are completely exposed.

Some of our lenses come standard with UV400 protection which is the highest level of protection your lens can have since it blocks 100% of all harmful light from the sun (These lenses block light rays with wavelengths up to 400 nanometers, so your eyes are shielded from even the tiniest UV rays.)

Spectacle Prescription Information

Common terms used on prescriptions have the follow meaning:

  • Sphere (SPH) - The sphere refers to the amount and type of prescription you require. Short-sight is corrected by negative (-) sphere values, long sight by positive (+). If the sphere value for one eye is negative, then the other is usually also negative and vice versa. If you only see one number for each eye on your prescription, it's the "SPHERE" power. It is common for the doctor to leave out the decimal points. A number like -25 or +175 is the same as -0.25 or +1.75, respectively.
  • Cylinder (CYL) - The cylinder is only relevant if you have an astigmatism (or visual distortion). This value determines the strength of the astigmatism correction. If the cylinder value for one eye is negative, then the other is usually also negative and vice versa. Prescriptions can be written in + cylinder construction (ophthalmologists write it this way) or - cylinder construction (the rest of the optical world writes in).
  • Axis (AXI) - The axis value is the angle of the astigmatism correction. If you have a cylinder reading on your prescription then you should have an axis reading. An axis of 5 (or 05) is the same as 005; 90 are the same as 090.
  • Reading Addition (ADD) - The addition value is the amount of additional correction to add to the sphere value for reading or intermediate glasses. If you only see one ADD power, it's understood that you have the same ADD for both eyes.
  • Pupilary Distance (PD) - Your Pupilary distance (PD) is the distance, in millimeters, from one pupil to the other. A correct PD will ensure that the lens optical center (the point at which light enters the lens undistorted) is placed directly at your pupil. PD measurements are often taken individually for each eye.

Eyeglass prescriptions frequently contain a number of different abbreviations and terms:

  • DV is an abbreviation for distance vision. This specifies the part of the prescription designed primarily to improve far vision. In a bifocal lens, this generally indicates what is to be placed in the top segment.
  • NV is an abbreviation for near vision. This may represent a single-vision lens prescription to improve near work, or the reading portion of a bifocal lens. Some prescription forms use ADD in place of NVwith a single box to indicate the additional refractive power to be added to the spherical of each eye.
  • OD is an abbreviation for oculus dexter, Latin for right eye from the patient's point of view. Oculus means eye.
  • OS is an abbreviation for oculus sinister, Latin for left eye from the patient's point of view.
  • OU is an abbreviation for oculi uterque, Latin for both eyes.
  • A spherical correction corrects refractive error of the eye with a single convergent or divergent refractive power in all meridians.
  • A cylindrical correction corrects astigmatic refractive error of the eye by adding or subtracting power cylindrically in a meridian specified by the prescribed axis.
  • The axis indicates the angle in degrees of one of two major meridians the prescribed cylindrical power is in. Which major meridian is referenced is indicated by the cylindrical correction being in plus or minus notation. The axis is measured on an imaginary semicircle with a horizontal baseline that starts with zero degrees in the 3 o'clock (or east) direction, and increases to 180 degrees in a counter-clockwise direction.