If your eye doctor determines you need corrective lenses he or she will write an eyeglasses prescription for you at the end of your exam. This prescription specifies the lens powers required to correct your vision. A prescription for eyeglasses cannot be used to purchase contact lenses or vice-a-versa. Contact lens prescriptions contain additional information that can only be determined during a contact lens fitting.
At a minimum, your eyeglasses prescription will contain the following information: your name, the prescribing doctor's name, the date of the exam, and the sphere power, cylinder power, cylinder axis, prism power and orientation, and add power of the lenses.
Your prescription may contain the abbreviations OD (oculus dextrus) and OS (oculus sinister) to specify lens values for your right eye (OD) and left eye (OS).
The numbers on your RX form may be written as -1.50 or as -150 they are the same just some Doctors use a shorthand and do not use the decimal point.
Sample Eyeglasses Prescription Form
Sphere Power: The sphere power indicates the lens power prescribed to correct nearsightedness or farsightedness. Eyeglass lenses are normally prescribed and manufactured in one-quarter diopter (0.25 D) increments.
If a sphere power has a minus sign (-) preceding the diopter value, the lens is a concave lens for the correction of nearsightedness. Concave lenses (also called minus lenses) are thicker at the edge of the lens and thinner at the center.
If a sphere power has a plus sign (+) preceding the diopter value, the lens is a convex lens for the correction of farsightedness. Convex lenses (also called plus lenses) are thicker at the center and thinner at the edge. Convex lenses are also used in reading glasses and are added to the bottom portion of bifocal, trifocal, and Progressive lenses to correct Presbyopia.
Cylinder Power: The cylinder power indicates the lens power prescribed to correct astigmatism. If you have astigmatism, your eye is shaped like a football rather than a baseball.
Like sphere power, cylinder power is measured in diopters (D), in 0.25 D steps. Cylinder power is usually specified in minus-power format, but may be written in plus-power format as well.
If you have no astigmatism, the cylinder power and cylinder axis columns of your prescription will be left blank.
Why are some prescriptions written with + cylinder and others in - cylinder? Some ophthalmologists and a few older optometrists write prescriptions in plus cylinder form. Many years ago, refracting instruments and spectacle lenses were ground with the cylinder correction on the convex, front surface and hence the rationale for plus cylinder notation. During the last 20 years or so, most refracting instruments and nearly all spectacle lenses have been ground with the cylinder correction on the concave, rear surface.
You may enter your cylinder power as plus or minus cylinder as written by your Doctor. Be assured your glasses will be fabricated for the proper prescrition.
Cylinder Axis: Most eye doctors use three digits to describe all meridian locations (e.g. "001" instead of "1"; "090" instead of "90").
The range for Cylinder Axis will be 1 to 180. If you do not have a cylinder power you will not have a cylinder axis
Prism: In some cases, your doctor may prescribe prism power to help your eyes work more efficiently as a team an/or stay in proper alignment.
Prism is added to an eyeglass lens prescription only if a problem with eye teaming or eye alignment is detected.
Unlike the diopter units used for sphere power and cylinder power, prism diopters are usually expressed in fractions (i.e.1 1/2) rather than in decimal units.
If prism power is prescribed, a second value called the prism base (or simply, base) is included to indicate the direction (in, out, up or down) of the prism in the lens.
Add Power: If you have presbyopia or other near focusing problems, your doctor may write an eyeglasses prescription that includes an add power. The add power is extra magnifying power added to the lower part of your lenses so you can see near objects more clearly.
Lenses used for add powers are always plus-power lenses, typically within the range of +1.00 to +3.00 D. Add powers are nearly always equal for both eyes.
Lens prescriptions that include an add power can be used for bifocals, trifocals, and progressive ("no-line bifocal") lenses.
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correction in one lens. Unless otherwise specified at ordering the bifocal height will be set 3mm below the center of the lens. The standard bifocal width is 28mm, other sizes are available at additional cost. 
viewing, while the center portion is divided into intermediate and the bottom area for near viewing.
