When you get new glasses, your optician takes several measurements. These aren’t just random numbers, they are the blueprints for how your lenses are placed and shaped to give you the clearest, most comfortable vision.
1. Pupil Measurements: Where Your Vision Centers
These numbers tell us where to put the exact center of your prescription in the lens so you look straight through the clearest spot.
A. Pupillary Distance (PD)
- What it is: The distance between the centers of your two pupils (the black dots in your eyes).
- Why it matters: The horizontal center of the lens must line up perfectly with your PD so your eyes work together without strain. If the PD is wrong, your eyes have to pull inward or outward to focus.
B. OC Height (Optical Center Height)
- What it is: How high or low your pupil sits within the frame when you are looking straight ahead.
- Why it matters: This is arguably more important than your PD, especially if your two eyes have very different prescriptions. If the height is off, it can cause the image to shift up or down, leading to discomfort or double vision. We need to get both the horizontal (PD) and vertical (OC Height) alignment correct.
2. Frame Measurements: Sizing the Lens
These numbers are printed on your frame and help the lab manufacture a lens that fits exactly.
| Measurement | What It Means | Why It Matters for You |
| A Measurement | The horizontal width of the lens. | Used to determine the overall width of the frame. |
| B Measurement | The vertical height of the lens. | Used to determine how tall the lens is. |
| DBL (Distance Between Lenses) | The space between the two lenses, over your nose bridge. | Used with the ‘A’ measurement to calculate how wide the entire frame front is. |
| Effective Diameter (ED) | The largest measurement needed to cut a lens for this frame. | This number heavily influences how thick your finished lens will be, especially for stronger prescriptions. |
The Centering Calculation: The lab takes the size of the frame (Frame PD) and compares it to your PD. That difference tells them exactly how much they need to shift (or de-center) your lens inside the frame so the center of the lens lines up with the center of your eye.
3. Comfort and Fit Measurements
These measurements ensure your glasses don’t cause headaches or fall off your face.
Temple Length
- What it is: The length of the arms (temples) of the glasses, from the frame hinge to the end of the arm.
- Why it matters: The temple must be long enough to curve securely behind your ear. If it’s too short, the glasses sit incorrectly, putting too much weight on your nose and causing a lot of discomfort.
Beware of Online Ordering, Especially Progressives
If you order large frames online, be extremely cautious, especially if you need progressive lenses (lenses with multiple prescriptions).
- The Risk: Progressive lenses must be customized to how the frame actually sits on your face. If a very large frame is designed without the careful measurements of an optician, the built-in reading area might be placed too high or too low, making the lens completely useless for reading or computer work.
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.