Tuesday, November 16, 2010


Before you buy or order an eye glasses,you must understanding your prescription.

To place an order, you will need a prescription for corrective eyeglass lenses and your pupillary distance (PD). A contact lens prescription cannot be used to make eyeglasses.
When you have your eyes examined, request the PD (pupillary distance) from the person who does your exam.  The pupillary distance is the distance in mm between the centers of your pupils.  Corrective lenses can't be made without a PD.  It’s used to position the optical center of the lens so that you will be able to easily see through the lens, and to center the correction in front of the pupil of your eye in the eyeglasses.  If a pair of frames cannot accept your PD, you can't simply change your PD simply to make it work because you will not be able to see through a lens produced with an incorrect Pupillary Distance.
The correct PD is particularly important for a multi-focal lens (bifocal and progressive), as there are multiple viewing fields which are adjusted on the basis of the PD supplied.
The PD is often left off the prescription, although it’s usually recorded in your file.  Having your PD empowers you to purchase eyeglasses from any supplier, not just with the place where you got your exam.  It is your right to have your PD - you paid for it with your prescription.
PDs may be indicated in a variety of ways.  You will often be given a single number – this should be your distance vision PD and is what you will need for eyeglass orders on our site.  Most adults have a PD between 53 and 70, and children will be between 41-55.  You might be given two numbers such as 63/60. This would mean that you use 63 as your PD for distance vision. The smaller number would be used only when converting the prescription to single vision reading glasses. You may be given fractional PD measurements, right side and left side, and split PDs may be written as 30.5/31, for example.  Simply add the two numbers together to get your distance vision PD.
SPH - Spherical - Provides the depth of the focal field in the lens.
A negative sign in front of the number indicates that you're nearsighted, which is the case with the majority of people who wear corrective lens. In nearsightedness, the thinnest part of the lens is the center, and the wider the lens becomes, the thicker it will be at the outer edge. If you're concerned about the outer edge thickness of the lens, you would be advised to consider narrower lens widths.
With a postiive sign preceding the Spherical number, you're farsighted, and the thickest part of the lens is in the center. The size of the lens is also important, because the larger the the lens, the thinner the edge will be. This may eliminate the consideration of half rim frames because the lens may become too thin to provide for the groove, to carry the filament, to help keep the lens in the frame.
Cyl and Axis - The two designations will be included if you have an astigmatism, an irregularity in the shape of the eye, which causes shape distortion in the viewed image. Cyl will describe the amount of correction you need to overcome this, and Axis is the angle at which the correction is placed. The Axis will be recorded as a degree reading between 1 and 180 degrees.
If you find the designation "Sph" written in the Cylinder area of the prescription, it simply means that the eyeball is spherical, and there is no correction for astigmatism needed in that eye.
Extra Strength If you are ordering a single vision pair of glasses, and the prescription strength is beyond the corrective capacity of a standard lens, an extra strengh charge per pair of glasses will apply depending on the lens type and strength of correction. When you enter the prescription values in any of the four data entry fields (OD Sph, OD Cyl, OS Sph or OS Cyl), and an extra strength charge is warranted.
NV-Add (or simply Add)- This is the reading portion of a lens in a multi-focal prescription, and this number indicates the strength of the overlay on the lens, for reading. It is the same positive number for both eyes, and it may be written only once on your prescription. The ADD can be found in the ADD sectionof the prescription, and is often annotated "NV," as in "near vision." It may also be written as a large, positive number, across both the OD and OS lines of your prescription.
Please be advised that if your spherical corrections are very minor, making your "Add" the strongest corrective part of the prescription, you may find that the design of a lined bi-focal is more suitable to your need, rather than a progressive (unlined) lens, as well as far more comfortable to wear. The reason for this is that your area of greatest need, is minimized in the design of a progressive lens (approximately the lower quarter and central third of the lens, 13-16mm in corridor width), while the lined bi-focal provides a larger, clearly delineated portion of the lens, for reading and other close work.
If you’ve had your eyes examined by a licensed Optometrist or Optician, they are required by law to provide you a complete written prescription. If you wear a multi-focal prescription, it’s in your best interest to ask for the basic, underlying multi-focal prescription (even if you simply want a single vision lens for a different purpose).
Multi-focal prescriptions may be modified for single vision use, or even a different multi-focal use, by working with the Sph and Add numbers. If there is a correction for astigmatism in one eye, or both eyes, those designations will not change, regardless of the depth of the focal field.
Single vision distance only –Enter values for Sph, Cyl and Axis for both eyes, but leave off the Add number. Use the distance PD value.
Single vision reading glasses – Combine the Add and Sph numbers for each eye. The result creates the new Sph values for each eye.  In the case of a Positive Add and a negative Sph, net the difference, and the new number becomes the Sph. If your Sph is already positive, you simply combine the positive Sph and the Add number, using it for the new Sph. The Add data entry field will be blank, by virtue of it having been combined with the Sph number.  Enter Cyl and Axis values for both eyes- these values are not changed in any way. For single vision reading only, narrow the distance vision PD by 3mm and enter that as the single-vision reading PD value.
Single vision computer glasses – Split the Add number in half, and combine that split number with the Sph. Discard the remaining Add, and leave the Add data entry field blank. Enter the Cyl and Axis values for both eyes. These values are not changed in any way. Use the distance PD, unless your monitor is virtually in front of your face, in which case, you may wish to narrow the PD a millimeter or two, for the sharpest focus.
Multi-focal Computer/Desktop glasses – To create this modification, split the Add number in half (if there is no equal division of the Add number, you’ll need to decide if you want your focus closer or further, and the quarter diopter difference will shift the depth of the focal plane, accordingly) and combine half of the Add with the Sph. The balance of the Add will stay in that data entry field.  Use your distance PD, unless your monitor is extremely close (less than 18”), in which case, you may wish to narrow the PD by a millimeter or two. Finally, enter the Cyl and Axis values for both eyes. These values are not changed in any way.
Prism Corrections - We do not prepare prism corrections.
Glasses Without Correction in the Lens - It is quite possible to order eyeglasses with no prescription in the lens. You simply leave the prescription data entry fields defaulted to zero, but you will be required to enter a PD, to get the computer to process the order. Use 63, as it is a median number for an adult. It’s a good idea to include a note at the end of the order, in the Customer Comments section, indicating that this is an order for a pair of glasses with no correction in the lens. A pair of glasses ordered in this manner will have an optical lens, and may be worn.


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