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How Reading Glasses Work

Reading lenses are rated in diopters, a formula opticians use. Manufacturers of reading glasses often contribute to consumer confusion by referring to magnification power and diopter strength interchangeably. They are not the same thing. But it’s easier for people to understand magnification than the science of lenses.

The Basics of Reading Glasses

Do they magnify?

A common misconception about reading glasses is that they magnify small print. Actually, they make it easier to read small print by providing the correct diopter strength which usually begins at +0.75 all the way up to +4.00 (more on that later). As typically worn, that will crystalize near focus for reading and other close work tasks so that your eyes have an easier time to focus. Reading glasses will not discernibly enlarge the size of text or near objects compared with when they are removed, though the crystalizing effect can often feel that way.

Diopters? What’s that?

Reading lenses are rated in diopters, a formula opticians use. Manufacturers of reading glasses often contribute to consumer confusion by referring to magnification power and diopter strength interchangeably. They are not the same thing. But it’s easier for people to understand magnification than the science of lenses.

A slightly more technical explanation. With every power option, or “diopter”, there is a sweet spot range where close vision is brought into sharp focus. This changes from person to person. If the diopter is too weak for your individual needs, you will need to move the material farther away from your face to see it clearly. If the diopter is too strong, that range of sweet spot will be closer than you prefer. The average person reads material between 14 - 18 inches away from their face. If you would like to restore a comfortable reading range that is within that, find a diopter that sharpens focus in this range. In contrast, if you are working at your desk and your computer monitor is 24 inches away, you will need a weaker power than you would for reading.

Range Of Focus

example of too shoot a diopter

Too Strong a Diopter
forces you to hold reading material uncomfortably close.
example of too weak a diopter

Too Weak a Diopter
forces you to hold reading material farther away than may be practical.
example of an appropriate diopter

An Appropriate Diopter
strength provides a comfortable reading range.

How to Determine Reading Glasses Strength

Wondering what strength of reading glasses do you need? The best answer to this question is easy–have an eye doctor determine the strength you need. But, we know that a lot of people will want to try to approximate what they need on their own. And since you’ll probably search the internet and find all manner of advice, charts, and rules of thumb (some good, some not so good), we’ll include the two most common methods here to save you the hassle. Simply do the following:

Reading Glasses Eye Chart

example of our new Strenght Guide Eye Chart

The Eye Chart Method

  1. Download the Eye Chart PDF and print it out.
  2. Be sure to remove any corrective lenses or glasses.
  3. Hold the paper approximately 14 inches away from your face.
  4. Read the lines from top to bottom. The first line you have difficulty reading is an indicator of the strength that will probably serve you best.

The Power Straddle Assortment Method

It is important to keep in mind that Reading test charts aren’t always precise. That’s why ReadingGlasses.com suggests pairing the Eye Chart Method result with a selection of additional test powers to create a Power Straddle Assortment. Here’s how it works:

Straddle Method Step 1

1. Find
Select the frames you want, and the strength you think will work best. It may be a power you targeted from the eye chart test, or from trying a pair that belongs to someone you know.
Straddle Method Step 2

2. Straddle
Order that strength AND two additional reading powers in the identical frames. Order a pair one power level down in magnification and a pair one power level up. Yes, you’ll be returning two of the three you receive.
Straddle Method Step 3

3. Choose
Experiment with the three powers you selected and see which works best for you under normal reading conditions. It often comes down to two powers that are hard to distinguish between. We find most people will select the weakest power.

Finally, return the glasses in the strengths you don’t want within 30 days for a full refund, by printing a prepaid return label. Shipping both ways is always free on RG.com, so that makes it easy.

advice for those who have been wearing reading glasses for a while

Are your trusty glasses just not working as well as they once did?

Well, that usually means you need to move up in power. The first thing you should do is check the power of the glasses you currently have.

strenght on temple arm diagram

If they are non-prescription ready-made reading glasses, you can often find the strength marked on the inside of one of the temple arms.

Use this strength as a starting point, since you suspect that you need a stronger power than what you currently have. So, the only question is, “How much stronger?”

As mentioned earlier, the best way to determine what you need is, of course, to have an eye doctor perform an eye exam. But, if you are fairly certain that you just need a slightly higher strength, ordering glasses +0.25 or +0.50 diopters stronger than what you have may do the trick. Of course, you can use the Straddle Power method mentioned earlier. The free returns both ways make it super easy.

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