Degrees Mean Negative Diopters Explained | Generated by AI
Question: In the context of “a 150–200 degree reduction in glasses prescription,” does “150 degrees” mean +1.50 D or −1.50 D?
Answer:
“150 Degrees” in Prescription Context = −1.50 D (Minus/Negative)
In the context of the sentence you quoted — reducing glasses for myopia (nearsightedness) — “150 degrees” refers to −1.50 diopters (D), meaning the glasses are made weaker than the full prescription by 1.50 D.
Understanding the “Degree” Terminology
In many Asian countries (Taiwan, China, Hong Kong), prescription strength is commonly expressed in “degrees” (度) rather than diopters directly in everyday conversation. The conversion is simple:
100 degrees = 1.00 diopter (D)
So:
- 150 degrees = 1.50 D
- 200 degrees = 2.00 D
- 500 degrees = 5.00 D
Why “Reduction” Means Minus (−)
A negative value in a prescription indicates myopia (nearsightedness), meaning the person can see well up close but has difficulty seeing far away, while a positive value indicates hyperopia (farsightedness).
The sentence talks about reducing the prescription — i.e., intentionally prescribing glasses that are weaker than the full correction needed. This is sometimes called undercorrection.
Undercorrection refers to the practice of wearing glasses that are deliberately made weaker than the optimal visual correction required. It may also be known as underpowered or lower-power glasses.
So if your full myopia prescription is, say, −6.00 D (600 degrees), a “150-degree reduction” means your glasses are made at −4.50 D (450 degrees) instead — effectively subtracting 1.50 D from the negative prescription.
Is “Reduction” the Same as +1.50 D?
Technically, you are adding +1.50 D to the lens power to make it weaker. However, the sign on the resulting prescription is still negative (e.g., −4.50 D). The “+1.50 D” is the amount of reduction, not the final prescription sign. The person is still wearing minus lenses for myopia — just weaker ones.
Does Undercorrection Help?
This is a debated topic. Research has clearly shown that undercorrection is not beneficial to children with myopia and can actually speed up myopic progression. However, some adults explore undercorrection for near-work comfort or as part of vision improvement approaches. In adults, reducing the strength of a prescription makes some sense for those suffering refractive myopia caused by failure of the near-focusing accommodation system to relax when looking at distance.
Summary Table
| Term | Meaning |
|---|---|
| “150 degree reduction” | Weaken the lens by 1.50 D |
| Sign of the reduction amount | +1.50 D (added to the negative lens) |
| Final lens type | Still negative/minus (for myopia) |
| Example: −6.00 D with 150-degree reduction | Results in −4.50 D glasses |
References:
- Zenni Optical – Understanding Diopters
- All About Vision – What Is a Diopter?
- My Kids Vision – Should Kids’ Glasses Be Undercorrected?
- mivision – Myopia: To Correct, Un-Correct or Under-Correct