Single-vision distance glasses are used for seeing distances roughly two feet away or more. Single-vision intermediate (computer) glasses are used for seeing distances between one-and-a-half and two feet away, basically how far away a person usually is from a computer screen. Single-vision near (reading) glasses are used for seeing objects six inches to one-and-a-half feet away, roughly where you would hold a book you are reading.
For single-vision distance glasses, all that is needed are the plus or minus numbers in the Sphere (abbreviated SPH), Cylinder (CYL) and AXIS sections of a prescription. Ignore the plus number (it will be the same for both eyes) in the NV-ADD (Near-Vision reading ADDition) section of your prescription till you are ordering intermediate and reading glasses.
For single-vision reading glasses, it’s a little more complicated, although it just requires a bit of grade-school arithmetic. Take the plus number in the NV-ADD section of your prescription and add it to the numbers in the SPH section. For example, if you have -1.00 in your SPHs, and +2.00 in the NV-ADD, when you add these two numbers together, you end up with +1.00 in the SPH section. That gives you near single-vision.
Remember, if you are ordering single-vision reading glasses, you will need to lower your distance vision pupillary distance (PD). Your PD is the measurement, in millimeters, between the middle of one pupil to the center of the other.
If you have a single PD number, which would be one number between 50 and 75, subtract three. For example, let’s say your distance vision PD is 63. Lower this by three, to 60, for single-vision reading glasses.
However, you may have a dual PD, which is the measurement from the center of each pupil to the center of the bridge of your nose. In this case, you would have two numbers, each somewhere between the mid-20s to the mid-30s. These two numbers should add up to the single PD number. For example, if you had a dual PD of 31.5 in the right eye and 31.5 in the left, your PD would add up to 63. Therefore, to get your near-vision dual PD for single-vision reading glasses, you would lower each eye’s PD by one-and-a-half, since combining two of that number adds up to three. That would be 60, just as it is if you have a single PD.
For single-vision intermediate (computer) glasses, if you take your NV-ADD number and divide it in half (+2.00 divided by 2=+1.00), then add that to your -1.00 SPHs (+1.00 + -1.00=0.00), you get your intermediate (computer) single-vision. You don’t have to change the PD at all, but you could lower it by one or two millimeters, if you have a high PD (over 66).
If you want to use a pair of bifocals to reconfigure your prescription to get computer vision in the top part of your lens and reading vision in the bottom, do the same thing you would do for single vision computer glasses.
Divide the NV-ADD in half and add this half number to the SPHs. But this time, leave the remainder in the NV-ADD section. That’s how you would get computer vision in the top part of the lens and reading vision on the bottom.
It’s as simple as that.