Learn more About 'Single Vision' & 'Multi-Focal' Prescriptions
There are two basic types of prescriptions. One is termed Single Vision (SV), which targets one depth of viewing only (distance, mid-range [often termed "computer glasses"], or reading). This lens has no lines to demarcate viewing areas, since it's only focusing on one distance throughout the entire lens (unlike the Progressive or lined tri-focal, which include three ranges, or a lined bi-focal, with two).
A single vision (SV) prescription may also the appropriate correction for people typically younger than approximately 40 years of age. The eye muscles automatically compensate for the distances in the viewing field, so prescriptions for younger people don't generally include provision for adjustment by an Add number.
When the muscles begin to lose elasticity, the eye's "auto focus" (like many cameras) is no longer working well and that loss of function is compensated for by a MF lens in the eyeglasses, or an Add-adjusted single vision lens.
The other basic type of prescription is Multi-focal (MF), which means that the lens has more than one viewing depth, enabling both distant and nearer vision with clarity, using only one pair of glasses. A MF lens may be a Progressive, a Lined Bi-Focal, or a Lined Tri-focal. If the multi-focal lens has no lines, it's called a Progressive, period. It is not a no-line bifocal, an example of industry misinformation.
All the essential information for any of the three MF correction styles is contained in the same prescription format, characterized by the presence of an "Add" (sometimes designated as "NV") number, but different Optical professionals write in different styles. A single vision prescription is always written without a separated "Add" number! If you see an Add designation, it's a MF prescription.
Multi-focal Prescription Example 1:
Oftentimes, the "+" sign in front of the Add number may be missing, as Add (by definition) is always positive. It's usually written by quarters of diopter, so +2.5 actually means +2.50. Some optometrists, rather than writing the same number on both lines, will either write it between the lines, or add "OU," which in Latin means Oculi Uterque, the same for both eyes. The example PD of 68 indicates that 68 is the distance PD; if you're converting the use to either of the other two SV uses, the PD is reduced by 3mm for reading, and may be adjusted proportionally for mid-range, depending on the depth of the monitor. The closer the format, the smaller the PD, but the range is seldom greater than 3mm.
The PD (the distance in millimeters from the center of one pupil, to the center of the other pupil) must be provided, as it's impossible to prepare a corrective lens without knowing the Pupillary Distance, which is used to locate the optical center in the lens. It is measured from your eyes, and if not the spatial center of the lens in your glasses.
Look at your prescription before you leave your Doctor's office. If your PD hasn't been recorded, ask that it be measured for you, so that you know your prescription is professionally complete and accurate. It's a very simple process, which may be done with any millimeter ruler – not hi-tech! But it's important that it be accurate.
Multi-focal Prescription Example 2:
With this format, you'll notice that the Add has taken on a different designation, now called NV, which means Add. It's still exactly the same thing - just an increase in strength.
Add is the number necessary to convert the distance use to near use, by combining it with the Sph number.
This type of prescription pad is very complete, as it includes all of the information possibly necessary to produce any pair of glasses for the person for whom it was written. You are also entitled to all of this information when paying for an eyeglass prescription from a licensed professional.
* Prism/Base - These fields are usually left empty, as they are not found in most people's prescriptions. Prism refers to a focal image displacement caused by a muscular imbalance, sometimes characterized by squinting or a"lazy eye," which "wanders." The images created by each eye cannot be fused, naturally. The strength needed to offset this is called "prism diopter," and the base refers to the direction of the displacement. The correction might be noted as .25 bi (1/4 diopter, base in). If you find designations for Prism/Base, please include a note under "Extra Information" when ordering from our website.
Multi-focal Prescription Example 3:
Example 3 is very generic, and missing the PD – so eyeglasses cannot be ordered until the PD is known.
In all three of the foregoing examples, the wearer is near sighted, indicated by the negative symbol in front of the Sphere. If the Sph number were positive, it would indicate far sightedness.
In a lens prepared to correct near sightedness, the lens will be thicker at the edges, than in the center; the stronger the prescription, the thicker the edge. The eye, behind the lens, may appear to become smaller. This effect may be minimized by either ordering a higher index lens, which compresses the thickness, or, you may order a lens with narrower dimensions, to reduce the edge thickness. Another method of diminishing the appearance of lens thickness may be the choice of a frame which is a full-rim, plastic style. The edge of the frame covers the lens completely.
With far sightedness, the thicker part of the lens is in the center (and eyes appear to be enlarged), but the two possibilities for resolution are the same – a narrower lens, and/or a higher index lens. Since the wider the lens, the thinner the edge becomes with a far sighted correction, you may be deterred from ordering some of the semi-rimless designs, since the lens edge becomes too thin to support the groove which must be ground in the edge of the lens to carry the filament which holds the lens in the frame.
CONVERSION OF MF TO SV:
If you have a multi-focal prescription, and you want a simple and inexpensive pair of glasses for a particular depth-use only, you may convert the MF prescription to SV, as shown in the following 3 examples, for distance vision, for "computer glasses," and for "readers." No single vision application will ever have a separated Add number.
These conversions are all as simple as balancing a checkbook, basic algebraic function, and they apply only to the Spherical and Add designations, which set the depth of the focal point.
When performing use-conversions, the only designations which change are the Sphere and the Add. The Cylinder and Axis are always included as they're written on your prescription, with no modification. They never change, regardless of the changes to the Sphere/Add.
Single Vision Distance Example 4:
Simply leave the Add strength of +2.50 (for up close viewing) out of the lens, and the PD remains the same, as do the cylinder and axis (corrections for corneal aberration, which don't change regardless of focal depth).
Single Vision Mid-Range (Computer) Example 5:
To draw the depth of focus closer, from infinity to about three feet away, simply cut the example Add of +2.50, in half, and apply that half to the Spherical total (-3.75 and +1.25 = -2.50; -3.00 and +1.25 = -1.75). The balance of the Add (+1.25) is discarded. The Cylinder and Axis do not change.
If your monitor is approximately 3 feet from your eyes, or at the end of your arm, you may leave the PD as your distance PD of 68 in this example. If, however, the monitor is much closer, for fine-tuning, you may wish to reduce the PD by 1 or 2 mm. With larger monitors becoming more prevalent, it's likely you'll leave the PD at distance, since you won't be sitting right on top of it, even if you are near sighted…
IF your near sighted Sph were not as strong, say a negative number (-1.00) to begin, for conversion to SV computer distance in this example, using a +2.50 Add, the polarity of the Sph is going to change from negative to positive (-1.00 combined with +1.25 [1/2 of the original Add of +2.50] = +0.25 Sph). Think again, of a checkbook balance, and it becomes less confusing.
Single Vision Reading Glasses Example 6:
With this example, you can readily see that all of the +2.50 Add has been combined with the negative Sph, considerably reducing the number, although it still remains a negative, since it was so strongly nearsighted, to begin with.
If you had -3.75, and you deposited +2.50, you'd still have a (lesser) negative balance of -1.25. And if the OS Sph were -3.00, and you added +2.50, it's still -0.50. The Add of +2.50 has been incorporated to reduce the strength of the divergence of light. The original sample PD of 68mm, becomes 65mm, and the Cyl/Axis numbers, which correct for astigmatism (a corneal aberration which spans all focal depths) do not change.
The PD is reduced by 3mm, because as the focal point comes closer, the eyes begin to converge the image, and consequently, as they turn in, the PD is reduced. If you draw a book from arm's length, to 6" from your face, you can feel the effect of your eyes converging, and become cross-eyed, as the PD narrows.
As above, IF your near sighted Sph were not as strong, say a -1.00, and you wanted to convert to SV reading in this example, using a +2.50 Add, then the polarity of the Sph is going to change from negative to positive (-1.00 combined with +2.50 = +1.50 Sph).