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 just to focus.

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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, the image can shift up or down, leading to discomfort or double vision. We have to get both the horizontal (PD) and vertical (OC Height) alignment exactly right.

2. Frame Measurements: Sizing the Lens

These numbers are usually printed on the inside of your frame arm and help the lab manufacture a lens that fits your face 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 actual PD. That difference tells them exactly how much they need to "de-center" your lens inside the frame so the center of the lens lines up with your eye.

3. Comfort and Fit Measurements

These ensure your glasses don't cause headaches or constantly slide down your nose.

• Temple Length: This is the length of the arms from the frame hinge to the end of the arm.
• The "Ear" Factor: 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 significant discomfort.

Beware of Online Ordering for Complex Lenses

If you are looking at large frames online, be extremely cautious—especially if you need progressive lenses.

Progressive lenses must be customized to how the frame actually sits on your face. If a large frame is designed without an optician's hands-on measurements, the built-in reading area might be placed too high or too low, making the lenses completely useless for reading or computer work.

Not sure if your current frames are sized correctly for your prescription?

Send your sizing questions to our "Ask Chadwick" series here.