Have you ever looked at that slip of paper from your eye doctor and wondered if you were looking at a secret code? Between terms like “Sphere,” “Cylinder,” and “Axis,” it can feel like you need a degree in physics just to understand how you see.
1. The Sphere (SPH)
The Sphere is the primary power of your prescription. Think of it exactly like its name suggests: a ball. In an optical sense, a spherical lens has a specific “focal length.”
When light passes through a spherical lens, it is bent to focus on a single point. The “depth” of the curve on that sphere determines whether that focus point is shorter or longer. The goal of your prescription is to ensure that light focuses perfectly on your retina at the back of your eye.
2. The Cylinder (CYL) and Axis
While a sphere is like a ball, a Cylinder is like a can. Instead of focusing light to a single point, a cylinder focuses light to a line.
This is used to correct astigmatism, which is when your eye isn’t perfectly round (like a basketball) but is shaped more like an oval (like a football).
- Cylinder: The amount of power needed to correct that “oval” shape.
- Axis: A number between 1 and 180 that tells the lab the exact angle at which to hold that cylinder to offset the error in your eye.
3. Prism and Base
Prism is a fascinating component used to help your two eyes work together as a team. If you’ve ever closed one eye and then the other and noticed the image “jumps,” your brain is usually doing the hard work to fuse those two images into one.
When the brain struggles to do this—causing headaches, double vision, or discomfort—we add Prism.
- Relieving Prism: Helps the eyes align so the brain doesn’t have to work so hard.
- Yoked Prism: Often used after a brain injury or concussion. Instead of bringing two images together, it shifts the entire visual world in one direction to help align a mismatch between where your eyes think you are and where your body thinks you are.
4. The “Add” Power
As we hit our 40s, we often lose our “auto-zoom” function—a condition called presbyopia. The Add Power is essentially a manual zoom.
- It is a number added to your distance sphere to help you see up close (traditionally set for about 16 inches).
- For example, if your distance prescription is +1.00 and your Add is +3.00, your total power for reading would be +4.00.
While traditional bifocals give you two distinct zones, Progressive Lenses offer a smooth transition from distance to near, allowing you to find the perfect focus for intermediate distances, like your computer screen.
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.