Your eyeglass prescription contains several numbers and terms that are critical for crafting lenses that give you clear and comfortable vision. Here is a breakdown of what each element of your eyeglasss prescription means and how it works.
1. Core Power: Sphere, Cylinder, and Axis
These three elements correct your eye’s fundamental ability to focus light onto the retina, which is necessary for a clear image.
Sphere (SPH)
- What it is: The main correctional power of the lens. It’s like a ball (sphere), meaning it has the same curvature and focal length all the way around.
- Function: A spherical lens focuses all light rays to a single point on your retina. The strength of the curve determines the length of the focal point, fixing issues like nearsightedness or farsightedness.
Cylinder (CYL)
- What it is: An additional lens shape, like a sliced can (cylinder), that has curvature only in one direction.
- Function: Cylinders are used to correct astigmatism, a common condition where the eye focuses light to a line instead of a single point (because the cornea is shaped more like a football than a baseball).
Axis
- What it is: A number between 1 and 180 degrees.
- Function: It tells the lab the exact angle at which the cylindrical lens must be placed to correct the astigmatism in your eye.
2. Binocular Function: Prism and Base
Prism is used to help your two eyes work together as a team.
Prism
- What it is: A wedge-shaped element added to the lens.
- Purpose:
- Bringing Images Together: Often, your two eyes don’t naturally point at the exact same spot, forcing your brain to do extra work to fuse the two images. Prism corrects this eye alignment, preventing double vision, headaches, or discomfort.
- Yoked Prism: After a brain injury or concussion, the visual system and body positioning may be mismatched. Yoked prism shifts your entire field of vision one way (e.g., left or right) to help realign what your eyes perceive with what your body senses, improving balance and spatial awareness.
Base
- What it is: The direction (e.g., “base up,” “base in,” “base out”) in which the thickest part of the prism is placed.
- Function: This tells the lab which direction to shift the light to achieve the corrective effect.
3. Reading Power: Add Power
Add power is an essential element for correcting age-related changes to near vision.
Add Power (ADD)
- What it is: A positive number (e.g., +1.50, +2.00) that is added to your distance prescription.
- Function: Around age 40, your eye’s natural auto-zoom function (called accommodation) begins to decline. The Add Power provides the necessary manual zoom for close-up tasks.
How it Works: If your distance sphere is $+1.00$ and your Add Power is $+3.00$, your reading prescription is $+4.00$.
- Progressive Lenses: If you find that your distance is clear, your near is clear, but the space in between (like a computer screen) is blurry, progressive lenses are often recommended. They smoothly transition the power from the distance prescription at the top to the full Add Power at the bottom, allowing you to focus clearly at any distance.
Charlie Saccarelli
President, Chadwick OpticalAs President of Chadwick Optical, Charlie Saccarelli is the driving force behind the company’s mission to help every patient left behind by the current health care system. Under his leadership, Chadwick has grown from a simple optical lab into a trusted resource for practitioners around the world looking for ways to help the patients that “can’t be helped.” He is a master optician, a father, a bit of a nerd, and a passionate patient advocate who has lectured worldwide on all things optical.