Basic information on frame and lens terminology.
Definitions Table 
Originally Posted / Sourced by All about Vision 





Stands for Pupillary Distance.  This is the distance between the center of the pupil of the right eye and the center of the pupil of the left eye.  The PD measurement Eyelation takes is for the DISTANCE PD.  The PD for NEAR vision is usually smaller (usually about 2 – 3mm difference) because as you read or look at something close up, your eyes converge and come in close at the corner of your eyes. Distance is measured in mm. 


Right Eye (Latin abbreviation for oculus dexter)


Left Eye (Latin abbreviation for oculus sinster)


Both Eyes (Latin abbreviation for oculus uterque)

SEG Ht. 

Stands for Segment Height.  

Sphere (SPH)

This indicates the amount of lens power, measured in diopters, prescribed to correct nearsightedness or farsightedness. If the number appearing under this heading has a minus sign (–), you are nearsighted; if the number has a plus sign (+) or is not preceded by a plus sign or a minus sign, you are farsighted.

Cylinder (CYL)

The amount of lens power for astigmatism. If nothing appears in this column, either there is no astigmatism, or astigmatism is so slight that it is not really necessary to correct it with eyeglass lenses. Cylinder power always follows sphere power in an eyeglass prescription and should be designated with either a or symbol.


Always a number from 0 to 180. The axis is defined with a number from 1 to 180. If an eyeglass prescription includes CYLINDER power, it also must include an axis value, which follows the CYLINDER power and is preceded by an “x” when written freehand.


This is the amount of prismatic power, measured in prism diopters (“p.d.” or a superscript triangle when written freehand), prescribed to compensate for eye alignment problems. Only a small percentage of eyeglass prescriptions include prism.

When present, the amount of prism is indicated in either metric or fractional English units (0.5 or ½, for example), and the direction of the prism is indicated by noting the relative position of its “base” or thickest edge. Four abbreviations are used for prism direction: BU = base up; BD = base down; BI = base in (toward the wearer’s nose); BO = base out (toward the wearer’s ear).


See above for “PRISM”.


This is the added magnifying power applied to the bottom part of multifocal lenses to correct presbyopia. The number appearing in this section of the prescription is always a “plus” power, even if it is not preceded by a plus sign. Generally, it will range from +0.75 to +3.00 D and will be the same power for both eyes.

Vertex Distance

The distance between the back surface of a corrective lens (for example – glasses) and the front of the eye (specifically, the cornea). Some people may be sensitive to this. This can be remedied by adjusting the nose pads.

A measurement

The horizontal width in mm of one of the frame’s lenses. Also known as “Eye” size

B measurement

The vertical width in mm of one of the frame’s lenses

ED measurement

The horizontal width in mm of one of the frame’s lenses. This measurement is more important for the lab to know what size lens blank will be needed to fit the frame after being cut and edged (See photo below.)

DBL measurement

Stands for distance between lenses. Also known as the “Bridge” size


The area of space between your lenses and over the bridge of your nose. The measurement is the distance in mm between the two lenses. It is measured between the two closest points of the two lenses.


The part of the frame that goes from the front of the frame to over your ears. Size is measured along the length of the temple, from one end to the other, including the bend (units in mm)

Frame measurement

The frame measurements are typically written in this format: “EYE – BRIDGE – TEMPLE”.

For example, a frame with the measurement 50-19-145 has an “A” measurement of 50mm, a “DBL” measurement of 19mm and a temple length of 145mm


Unit of measure for the refractive (light-bending) power of a lens; eye care practitioners use it in eyeglass and contact lens prescriptions. A negative number refers to nearsightedness; a positive number, farsightedness. For example, someone with -8.00 diopter lenses is very nearsighted, while someone with +0.75 diopter lenses is only slightly farsighted.


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