Unless you are buying frame only or frames with plano (no Rx lenses), you will need a prescription to place an order for corrective eyeglass lenses. You will also need to have your pupillary distance (PD). You should also know that a contact lens prescription cannot be used to make eyeglasses.
Eye examinations do not necessarily result in a PD measurement but proper manufacturing of your lenses will require an accurate PD value, either one measurement, or two. At the time of your optical examination you can ask the person doing the exam to measure your PD. The best and most accurate PD measurements are when you have two measurements (right-OD and left-OS) since no one is exactly symmetrical on the right and left side of their bridge. A single PD number is a measurement of the distance between the centerline of your pupils and it is measured in millimeters (mm). A measurement with OD and OS PD values is a measurement to each pupil centerline from the center of the bridge (nose) area. This measurement is used to position the optical correction in the lens directly in front of the pupil for each eye. That is why two PD measurements are best. We cannot make your corrective lenses without a PD value. If you so choose we will use an average PD value to manufacture you eyeglasses lenses.
Because there are many variable points of focus in a multifocal lenses – such as bifocal or progressive lenses – the need for an accurate PD is even more important. The PD value helps us adjust the various fields of view in your multifocal lenses to the optimal position when the PD measurements are accurate. There are many ways to obtain your PD if your practitioner is unable or unwilling to provide these measurements so check other resources on the EyeglassUniverse.com site for how you might be able to help yourself. In the end we want you to be satisfied with the quality and accuracy of your lenses and the PD value is crucial to this goal. With your PD you are able to purchase eyeglasses from any source and not just the place where your exam was performed.
We know through tens of thousands of patients that adults have PD values between 50 and 70 mm. Because children grow PD values can range average over a wide area from 36 mm to 56 mm with the lowest values for toddlers and the higher values for teens. We use an average 58 mm PD for women and an average PD value for men of 63 mm. If your PD value is expressed as 63/60, the first number if the PD for distance vision and the second number is called the near PD for reading or near activities. 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 and multifocal lenses.
Sphere - SPH – Sphere or Spherical - Provides the depth of the focal field in the lens.
A minus sphere (sphere value preceded by a minus sign) means you are myopic or near sighted. This means you have better near vision than you have the ability to see distant images. A majority of corrective lenses made correct for nearsightedness. Typically the nearsighted lens is thinnest in the center and thickest at the edges in order to bend the light for optimal correction. As the power of the lens increases the edge thickness gets thicker so if the aesthetic appearance of the lens edge concerns you we suggest selecting frames with smaller lenses, narrower lenses, buying frames with rounder lens openings, and buying higher index plastic lenses to help reduce the edge profile.
A positive sphere (sphere value preceded with a plus sign) means that you are farsighted and the lens is thickest in the center of the lens. In this case the far vision is better than the near vision. In cases of positive sphere values the center of the lens will tend to magnify the eye and some find this unappealing. To help minimize the effect the techniques suggested to reduce edge thickness can work to minimize the center thickness for farsighted lenses. Choose frames with smaller lens openings, narrower widths, and consider buying higher index plastic for your lenses. We don’t suggest semi rimless or rimless frame designs because the lens is thinnest on the edges and these designs may not provide sufficient material where the lens is anchored to the frame. There may be instances where we can provide guidance on the frame selected and your lens requirements if you give us a call or contact us via email.
Cylinder or CYL – When your eye is not exactly spherical the light doesn’t focus exactly where needed for optimal vision. When your eye has this condition we need to add cylinder to correct for the irregular shape of the eye. This eye condition is referred to as an astigmatism. Cylinder describes the amount of correction around an Axis that is required to permit the light entering the eye to focus in the ideal location for perfect vision.
Axis – Axis is the value in degrees from 1-180 that will work best for the application of cylindrical correction to move light entering the eye to the position where the best vision is obtained. If your Rx includes an entry for cylinder it must also have a value entered for Axis on the affected eye or eyes. An entry of “SPH” in the area reserved for cylinder values it simply means that your eye is spherical and requires no cylinder (astigmatism) correction for that eye.
High Power (also extra strength) – At EyeglassUniverse.com we have attempted to price lenses at a final price considering adding the additional charges required for high power prescriptions. Every lens material has a maximum range of powers that it can apply to correct your vision. Generally, the higher power the prescription the higher the index the material must be. In some rare instances after your order is submitted we may need to add a “high power” charge to your order but we will notify you of this prior to processing your purchase and obligating you to complete the transaction with EyeglassUniverse.com.
Near Add or ADD (power) – Add power provides the additional capability for a lens for reading and near or close activities. It represents the strength of the overlay in the lens for reading and near activities. It is expressed as a positive number and may vary in in .25 or .5 increments from zero. It is on your Rx form where ADD is indicated and will usually apply equally to both the OD and OS eyes. If only one value appears it is intended that this value apply the same for each eye. Some doctors may indicate the value with “NV” with a number representing the Add power for your prescription.
In some instances, where the spherical corrections in your prescription are very minor, the ADD power might become the biggest correction in your lens. In cases such as these you might be wise to consider a lined bifocal over a variable focal (progressive) unlined lens design. Progressive lens designs tend to make the area where you need the greatest corrective power the smallest portion of the lens. In a progressive lens the lower quarter and central one third of the lens is where the near vision is optimal. A lined bifocal spreads out the add power across the bottom third (approximately) of the lens giving you a much larger and more acceptable working lens area for near activities.
Segment (SEG) Height – Segment height is the approximate location of the division of the lens into different power zones. Segment heights are produced using standard parameters in our lenses. Bifocal lenses have a SEG set at 2 mm below the horizontal centerline of the lens as measured up from the bottom of the lens. Progressive lenses will vary some depending upon whether you wear your eyeglasses high up on your nose of in the middle. Progressive lenses worn higher up on the nose have a SEG set at the horizontal center line of the lens when measured from the bottom. If you wear them down on your nose we set the SEG at 2 mm above the horizontal center line of the lens as measured up from the bottom of the lens. Accomplishing fine tuning of your vision through the lenses is accomplished by adjusting the nose pads to move the lenses up or down slightly to accomplish the best vision but that won’t be necessary in most instances.
The Law requires licensed Optometrists and Opticians to provide you a complete written prescription with the measured corrective variables as was determined in your eye examination. If you wear multifocal lenses you should always obtain all required prescriptive variables for proper fitting even if ordering single vision lenses.