# Diopter to Radius Conversion for NCLE

The value of a contact lens base curve can be expressed in both millimeters of radius and diopters of power. Converting from one to the other is one of the basic skills opticians must master for contact lens fitting. You also need to know it to pass the NCLE. There will be questions on it directly, and questions in which diopter/radius conversion is one of several steps.

NCLE questions could take many different forms:

- Give the diopter value for the following radius value.
- Which of the following is the steepest/flattest base curve? (the answers will be mixed)
- Which of the diopter values is steeper/flatter than 8.3mm? (also mixed answers)
- If we increase/decrease a 8.3mm base curve by 0.15mm what would be the resulting diopter value?
- If the base curve you order is 8.3mm and the curve you get is 8.1, what is the difference in diopters?
- You order a CL with base curve 7.9, but it comes in 0.25D to steep. That is the dioptric value of the received lens?
- Given the following
**K**values, what is would be the base curve if fitted “on**K**” (all the values are in millimeters.) - What would be the intermediate curve of a lens of base curve 8.9mm. (all the values are in diopters.)

You get the idea.

## Conventional “Rules of Thumb” for Radius/Diopter Conversion.

All NCLE prep course and manuals contain a Diopter to Radius conversion chart. No one can possibly *memorize* this chart so there will be a formula, a “**Rule of Thumb**” for you to memorize. It could be worded like this:

**For every change of o.50 in base curve diopters there will be an approximate
change of 0.10mm in radius of curvature.**

This is a rough “rule of thumb.” It gives you an approximate value only. And it all hinges on remembering that:

**7.5mm = 45.00 diopters**

You base all your calculations on that.

So, supposing you get this question:

**If the base curve measures 8.3mm what is the value in diopters?**

The first thing we do here is to figure out the difference between **7.5** and **8.3** and then calculate how many **0.50 diopters **you need to add to, or subtract from, (this is very important!) **45.00D **to get your answer. If you are going UP in radius, you need to go DOWN in diopters. If you are going DOWN in radius, you need to go UP in diopters.

We are going UP in radius here, so we **subtract** from **45.00D**.

**8.3 – 7.5 = 0.8 **which is **0.10 x 8 **or **0.80 x 0.50 = 4.00D**

**45.00D – 4.00D = ** 41.00D

But that is only *approximate*. It is not a perfect answer. The greater the value AWAY from the **7.5mm ** or **45.00D**, the less accurate that “Rule of Thumb” will be.

But there is a much better way!

### Accurate Shortcut for Radius/Diopter Base Curve Conversion.

What if there was ONE number, one constant or value, that would accurately allow you to convert radius to diopters AND diopter to radius in **one single calculation?**

There is such a “magic number:” **337.5**

It is all you need for radius conversion.

Just divide **337.5** by either the radius value OR the diopter value and you will get the answer you are looking for.

**337.5 ÷ 7.5mm = 45.00 diopters
337.5 ÷ 45.00D = 7.5 millimeters**

Going back to our example of **8.3mm** let’s use the magic number.

**337.5 ÷ 8.3 = 40.662**

This is the exact diopter value. You don’t need to remember if you should add or subtract (though you should understand the concept!) There is one less step involved where you can make a careless error (which happens to the best of us!) and your list of answers will include ONE that fits your calculation. **40.662** could round down to **40.50** or it could round up to **41.00**. But which one is more accurate?

**337.5 ÷ 8.2 = 41.15 **over **41.00** but closer to it.

**337.5 ÷ ****8.4 = 40.17 **under **40.50 **but closer to **40.00**

**40.662 **would correspond more to **40.50 **than **41.00**

But if the best answer given is **41.00 **that’s it!

#### Practice Essential for NCLE Preparation.

The more you review the material, the better you will do on the NCLE. And the best way to review is to take practice tests.

At www.passyouropticalboards.com we have thousands of questions in our database, and each NCLE prep test you take will give you a unique selection of contact lens questions. After you complete each test, you can review it over and over, and by pressing the **Learn More** button, you can read explanations of the right and wrong answers for all the questions in the test. All the mathematical steps will be shown.

The more tests you take, the more you will learn!

Good Luck on your NCLE. We want you to ACE it!

My name is Hebert Elias and I’m going to get the NCLE in August what’s a good price you guys could give. Me

If you have already signed up for your NCLE and you want to practice before you take it, check out our practice tests. You can take one test for %22.95 or our 10 test package for $95.00. Each test is different. Answers and explanations are given. We even explain the math.

Can I print these test to have to study away from the computer? And how long do I have access to these questions?