How Do I Calculate Which Magnifier I Need?

Why You Can’t Trust “X” Magnification

Magnifiers often feature an “x” rating (e.g., 2x, 3x, 4x), which suggests the image size is two, three or four times the size of the original object. With handheld magnifiers, this rating is inaccurate and misleading.

While the “x” rating is a very accurate measurement for binoculars and telescopes, it’s unreliable for handheld magnifiers because the method for calculating “x” varies widely and is rarely specified.

Using Diopters Instead of “x”

Instead of “x,” we use diopters (D). A diopter measures the focal length of a lens. The focal length is the distance at which a lens brings light rays into sharp focus (like when a magnifying glass is used to start a fire). To calculate the approximate diopter strength you need, we use three key pieces of information, often relying on M size charts instead of Snellen (which assumes a 20-foot viewing distance).

Calculating Magnifier Diopters

Step 1: Find Your Reading Ability and Distance

You need a chart that uses M size (download a printable one from our site).

1. Smallest Size You Can Read (M): Identify the smallest font size on the M size chart that you can clearly read.
2. Reading Distance (Meters): Measure the distance you are from the chart (in meters) when you successfully read that size. (If you are in the U.S., you will need to convert your measurement to meters, as diopters are a function of meters.)

Step 2: Determine Your Target Size

• Target Size (M): Decide what you want to read. This could be playing cards, large print text or standard newsprint, which is typically 1M.

Step 3: Perform the Calculation

The calculation helps you find the magnification necessary to bring your smallest readable M size down to your target M size at your current reading distance.

The formula is:

Example Calculation:

1. Divide M size by Distance: 4÷0.4 = 10
2. Divide by Target Size: 10÷1 = 10 Diopters

Therefore, you would need a 10 diopter magnifier.

This calculation is an approximation, it’s a guide to find the correct range:

• Under-Correction: If you use a magnifier that is too weak, you won’t be able to read your target material, making the task much harder.
• Over-Correction: If you use a magnifier that is too strong, it will shrink your field of view drastically, limiting you to seeing only a couple of characters at a time.

For best results, once you have your calculated diopter, try magnifiers that are slightly lower and slightly higher than that number (e.g., if you calculated 10D, try an 8D, 10D, and 12D) to find what works best for your specific task and visual comfort.

Charlie Saccarelli

President, Chadwick Optical

As 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.