Updated: Nov 30, 2020
In this post you'll learn...
Where each number goes on your glasses prescription
What the lenses looks like when we change the numbers
How the changes in the lenses alter your sight and perception
How the Optometrist uses the phoropter to measure each of those parts of your glasses prescription.
WHERE ARE THE NUMBERS WRITTEN ON YOUR GLASSES PRESCRIPTION?
First let's start by looking at a written prescription. Inside the box, you'll notice there are two rows of letters marked as "OD" and "OS". OD (latin abbreviation: "Oculus Dexter") = Right Eye. OS (latin abbreviation: "Oculus Sinister") = Left Eye.
Next, look at the first three columns on the prescription which have numbers in them (Sphere, Cylinder, and Axis). These are the numbers that will be modified for your prescription to correct for either nearsightedness, farsightedness, or astigmatism.
I added the PD (pupillary distance) here as well to show you that's where it is written on your prescription. Sometimes the doctor will write the PD measurement down, sometimes not. Either way, you will get it measured when you get your glasses measured.
The distance PD is the distance between each pupil in mm. when you are viewing a distant object. The near PD is the distance between each pupil in mm. when you are viewing a near object about 40 cm. away. The near PD will almost always be smaller than the distance PD because you converge your eyes when viewing closer objects.
Now let's look at what these numbers actually mean for your glasses lenses.
WHAT DO THE NUMBERS REALLY MEAN? WHAT DO THE LENSES LOOK LIKE?
Lens shapes come in three main types. The first two types are called convex and concave which are both spherical.
Convex shape to correct for farsightedness. These are indicated with a (+) sign
Concave shape to correct for nearsightedness. These are indicated with a (-) sign
The following video shows you what the convex (+) and concave (-) lenses look like, and what the viewer sees when looking through them.
3. Cylinder shape to correct for astigmatism. Cylinder takes a (-) sign as well.
A cylinder shaped lens is always going to be designated minus if you got your prescription from an Optometrist. Sometimes, Ophthalmologists will write the prescription out differently so that the cylinder is + power.
Regardless of how it's written, you can think of a cylinder power lens as a can of soda cut lengthwise right down the middle. Steeply curved on one axis and not curved (or less curved) on the opposite axis 90 Deg. away.
Look at this video to show you what the cylinder lens looks like and what the viewer sees when looking through them. You'll also notice what happens when we change or rotate the axis.
HOW DOES THE EYE DOCTOR MEASURE YOUR GLASSES PRESCRIPTION?
Now let's go through how your Optometrist will use machine called a Phoropter to measure the numbers. If you look at the Phoropter, you'll see there's a circular opening on each side for the patient to look through for each eye. The Optometrist will often test one eye at a time and one of those openings might be blocked or closed during your eye exam.
WHAT DO THE DIALS ON THE PHOROPTER DO?
For each of the numbers (sphere, cylinder, and axis), there's actually a corresponding lens wheel that is inside the phoropter that your eye doctor will change throughout the testing.
Changing the sphere power on your glasses prescription:
The large wheel in the right and left sides of the phoropter is the wheel that we use to change the sphere power. If we rotate the sphere downward, we will be adding plus power in 0.25 diopter steps. Adding plus power is equivalent to removing minus power. The opposite effect is achieved if we rotate the wheel upward. During your testing, you should notice changes in clarity as the Optometrist changes the plus or minus wheel.
The phoropter can go up to +18.00 diopters or down to -18.00 diopters depending on your prescription. All in 0.25 diopter steps. That's a lot of lenses in there!
Changing the cylinder power and axis on your glasses prescription:
CYLINDER POWER ON THE PHOROPTER
The silver knob is turned in order to increase or decrease the power of the cylinder to match what you need to correct for your astigmatism. This photo shows the cylinder knob for the left lens. If we rotate the knob clockwise, the power will increase, and counterclockwise, the power will decrease. The units are again, diopters and each click that you hear during your eye exam will represent a 0.25 diopter change in the power. The cylinder knob goes from 0.00 (no cylinder power) to 6.00 diopters of power in 0.25 steps. Again... lot's of lenses in this machine!
On the following two photos, you'll see the cylinder power knob is set to -1.00 diopters and -2.25 diopters.
CYLINDER AXIS ON THE PHOROPTER
There is another knob that sort of surrounds the first cylinder power knob. Rotating this knob changes the axis. Remember from the video above, when we change the axis of the cylinder, it will make things look wider, or taller or warped depending on where the axis is. This knob allows your optometrist to change the axis in 1 degree increments.
See the photos for examples of how to read the axis of your astigmatism on this machine.
I hope you are now educated on how it works! Please share the post and the videos if you've enjoyed. I think anyone training to become an Optician, or a student that is going to start Optometry school could benefit.