Are “Degrees” and “D” the Same Thing?
Your prescription slip says -3.00D, but the person fitting your glasses tells you “that’s 300 degrees of myopia.” Degrees, D — which one’s actually correct?
The answer: both are correct — they’re just different units. It’s exactly like “1 meter = 100 centimeters”: the same length, just expressed with different units.
- D (Diopter) is the internationally standard unit, used on prescription slips and lens packaging everywhere.
- “Degrees” is the everyday, colloquial way Taiwanese speakers describe it.
The conversion is one simple line: degrees = D × 100.
So 1.00D = 100 degrees, 3.00D = 300 degrees, 0.25D = 25 degrees. Conversely, if someone says “500 degrees of myopia,” in formal units that’s -5.00D.
What Does D Actually Measure? (The Plain-Language Version)
Diopter (D) measures “how strongly a lens bends light,” defined as the reciprocal of its focal length (in meters).
Breaking that down: a lens brings parallel light rays to a single focal point, and however far that point is from the lens is called the “focal length.”
- A lens with a focal length of 1 meter → power = 1 ÷ 1 = 1.00D
- A lens with a focal length of 0.5 meters → power = 1 ÷ 0.5 = 2.00D
- A lens with a focal length of 0.25 meters → power = 1 ÷ 0.25 = 4.00D
See the pattern? The shorter the focal length, the more sharply the lens bends light to a focus, and the higher the diopter number. So the higher the prescription, the more the lens design typically has to account for.
What Do the Plus and Minus Signs Mean?
The plus or minus sign in front of your prescription isn’t about “good or bad” — it indicates the shape of the lens and what it’s correcting for:
| Sign | Lens Type | Corresponds To | How Light Bends |
|---|---|---|---|
| Minus (−) | Concave lens (thin in the middle, thick at the edges) | Myopia | Diverges light |
| Plus (+) | Convex lens (thick in the middle, thin at the edges) | Hyperopia, presbyopia | Converges light |
So -2.50D on a prescription means 250 degrees of myopia, while +1.75D means 175 degrees of hyperopia or presbyopia. The minus sign doesn’t mean “more severe” — it’s simply a different direction.
Reading Your Prescription: SPH, CYL, AXIS, ADD
A prescription slip has more than one number on it. Here’s a full reference for the most common fields, all measured in D:
| Abbreviation | Full Term | Meaning |
|---|---|---|
| SPH | Sphere | The myopia or hyperopia power (± sign) |
| CYL | Cylinder | The astigmatism power |
| AXIS | Axis | The direction of the astigmatism, 0–180 degrees |
| ADD | Addition | The extra plus power added for near vision (used for presbyopia/progressive lenses) |
| R / OD | — | Right eye — the row on the slip representing the right eye |
| L / OS | — | Left eye — the row on the slip representing the left eye |
Here’s something that trips up a lot of people: AXIS is measured in “degrees of angle,” not diopters. It describes the direction of the astigmatism (for example, 90 or 180), on a scale of 0 to 180 — a completely different thing from diopter power. Some people see AXIS 180 and assume they have “180 degrees of astigmatism,” when it’s actually a direction, not a magnitude.
A Quick Conversion Reference
| Diopter (D) | Taiwanese “Degrees” | Common Description |
|---|---|---|
| 0.25D | 25 degrees | Mild |
| 1.00D | 100 degrees | — |
| 3.00D | 300 degrees | Common range for moderate myopia |
| 5.00D | 500 degrees | — |
| 6.00D and above | 600+ degrees | Generally classified as high myopia |
Interpreting your power and finalizing your actual prescription should be based on the complete data an optometrist personally measures — no single number tells the whole story.
Common Misconceptions / FAQ
Q1: 100 degrees = 1.00D — can I just divide my own degree number by 100? Yes, the conversion really is that direct: degrees ÷ 100 = D, and D × 100 = degrees. So 350 degrees is 3.50D. Just note that the astigmatism “axis” doesn’t follow this conversion — it’s an angle, not a diopter power.
Q2: Why do prescription increments always come in multiples of 25 (25, 50, 75, 100)? Because lens manufacturing and refraction typically use 0.25D (that is, 25 degrees) as the smallest increment. You’ll rarely see a prescription like “137 degrees of myopia” — it’ll usually land on 125 or 150 instead, for exactly this reason.
Q3: Is it normal for the two eyes to have different prescriptions? Extremely common — very few people have identical power in both eyes. Your prescription slip lists the left and right eyes separately (R/OD, L/OS). When the difference between the two eyes is significant, the lens choice may need extra consideration during fitting, and an optometrist will evaluate that.
Q4: Is the power on my prescription slip exactly what I get fitted with? Not always exactly the same. The refraction measures your eye’s refractive status, but the final prescription also factors in binocular coordination, visual habits, and frame position, with fine adjustments made accordingly. So there can be small differences between “the refraction data” and “the final fitted power” — that’s a normal part of professional judgment.
Understanding your prescription power is the starting point for understanding your own eyes. If you’d like to go deeper into what a complete vision exam actually measures, and how that string of numbers on your prescription becomes a genuinely well-fitted pair of glasses, feel free to bring your prescription slip in and chat with us — both our Xinzhuang and Banqiao stores welcome the conversation.