Diopter Review Time!

Right now you should have lots of data to help you make sense of your actual starting point for your eyesight.

Here’s what you need:

  1. Three days of centimeter averages for the left and right eye.
  2. Your Snellen results with your regular glasses.
  3. Your current prescription strength (of your regular glasses).
  4. Your average comfortable ergonomic distance from your screen.

That’s quite a lot of information!

No wonder really, that most people are happy to just go to the optometrist. You are asked no questions, have to make no changes, learn nothing and just get an “instant fix” instead.

Not to worry though, this too, will get pretty easy before long.

What you have learned by now is something very important about glasses.  Which is…?  Do you know?

Glasses move the focal plane in your eyes.

In every day terms: glasses move how far you can see clearly.

Say for example your prescription right now is a -5.00.

With that -5.00 you can see clearly as far as you want.  No blur; things just get smaller.  If you go down to a -4.00, the distance you can see clearly reduces.  It’s like a “bubble” of clear vision.  If you go down to -3.50, you might see clearly just as far as your computer monitor.

That’s the thing you learned with the centimeter; more diopters, more distance.

The optometrist just does it the easy way and cranks things all the way to 11.  The wheels won’t fall off till way later, long after you walked out of there with seemingly perfect vision – and a hefty credit card receipt.

And you pay for that extra distance the optometrist adds.  In particular, any time you wear more distance than you need, you’re at risk of hyperopic defocus which stimulates axial elongation of the eyeball and means higher myopia.

Or again, in every day terms: if you wear more prescription than you need, you might also be increasing your myopia.

So what we want to do is only wear what we actually need.

One extra consideration here is a basic rule: the less prescription complexity, the better. We’ll cover that more later, but it basically means you don’t want to own a dozen pairs of glasses of varying prescriptions and switch between them all.

For now, and for most people in general, two is all we need.

One for close-up and one for distance.

Here’s how this fits in with our prescription review.  And this is one of the more involved topics, so don’t worry; we won’t dig this deep very often in these sessions.

The questions for today are two:

1.  Is your distance prescription appropriate?

That’s an easy one to answer.  Which lines did you read on the Snellen chart?  20/20?  Then you probably haven’t been to the optometrist in a while.  20/15?  20/13?  “Better”?

Note that down somewhere you won’t ever lose it.  The date, your prescription strength and your Snellen results.

You want this information when you look back, to gauge your improvements.

As a general rule, any more than an “easy” 20/20 in a fluorescent lit environment is going to be an over-prescription.  You get over-prescribed to make sure you can see well at night, even after a long day at the office.  Remember though, you are paying for every extra bit of distance (diopter) whenever you wear it but don’t need it.

We have to have multiple prescriptions to lessen the impact of bad focal plane stimulus.

For now, we’re going to keep your distance prescription.  Once we cover all the close-up topics, well be back for it.  We measure it now because in a few weeks your eyes will have changed (for the better).  We want to record your “before” numbers now, so you can appropriately appreciate your improvements.

Second question:

What should be your close-up prescription?

Here’s where it gets tricky.  The question is, in simple terms: how many diopters do you need to see to your recorded average viewing distance?

The easiest way to assess this would be at the optometrist.

But that’s probably out of the question.  Just imagine their faces when you ask them to test how many diopters you need to see clearly to 60cm.

The next easiest way is to use a test lens kit.  You can buy these online for around $120 and they’re nice to have but you don’t need one right now. We’ll talk about them a bit more later. If you have high myopia or a lot of astigmatism and are curious about being able to test different scenarios, a test lens kit is “nice to have”.  Not absolutely necessary, just handy.

The third easiest way (you did hire the guru, the guru has options for you) is to go to a convenience store that sells plus lenses (reading glasses).

Here’s what you do there:

Take your regular glasses.  Find a book or newspaper at the convenience store and put it about the distance you are from your screen; this would preferably be on a shelf at eye level. Yes, you might look slightly silly – just enjoy it.  If anybody asks, just say “I have a guru” (and then look at them mysteriously, it always works).

Now take a pair of +1.50 reading glasses and put them over your regular glasses.  Note, if you wear contact lenses this is a whole lot easier (though you have to account for the fact that contact lens prescriptions are not the same as glasses – see here).

Can you still read the page, at your average comfortable computer-screen-equivalent distance, with the +1.50?

There should be just the beginning of blur at the far end of the comfortable distance.

Say you are usually 50-65cm from your computer.  You’ll want to start seeing blur with the reading glasses over your regular glasses at around 60cm.  That’s how to tell you aren’t wearing more prescription distance (diopters) than you need.

Too much blur?  Go down to a +1.25.  No blur yet?  Go up to a +1.75.

Rinse, repeat, till you find the right distance.

Subtract the plus numbers from your prescription strength of your current distance glasses and you’ve got your close-up prescription numbers.

Fancy and yet so simple, right?

Caveat:  since this is your first time, your current glasses might not be the right prescription for you.  That’s the questionable part with all of this.  The rule is that you want your close-up (we call it “differential”) and your distance prescription to have the same relative numbers. Meaning, no difference in left vs. right eye.

Example:  If distance is left eye -3.50 and right eye -3.75, you want your close up to be equally lower in both eyes.  That way you are not freaking out your visual cortex by a change in one eye that’s different from the other.

I know, it’s a lot to take in.  Maybe print this one out or read through it again and write some notes.  Remember, this is one of the more challenging sessions!

Baby steps.  The first prescription reduction is just going to get you acclimated.

The next one will be a whole lot easier.

If you don’t know your current distance prescription strength you can take your glasses to any optometrist and they will measure them for you in a matter of a few minutes. If they hassle you, just make up something about not being sure if they are your old glasses or the right ones..

But there’s more… (Sorry!  I’m trying to make it easy.)

You usually don’t want to have astigmatism correction for your close-up lenses.  If your astigmatism is less than -1.00, it’s usually worth eliminating completely.  Substitute half a diopter in spherical (just regular myopia diopter) for a full diopter of astigmatism.


Your astigmatism is cyl 1.00 axis 175 and your spherical is -3.50.  You saw best with a +1.00 reading glass using my experiment above.

Differential prescription (your new close-up glasses) would then be -3.50 minus 1 diopters, equaling -2.50.  But you also eliminate the astigmatism, so you’ll add half of that 1.00 diopters of cylinder; -2.50 + 0.50 = -3.00.

Make sense?

Really, this is the most complicated out of all the sessions I have for you.  It’s just one hump. Figure this out and you’re all good to go.

No convenience store nearby?

The next simplest option is to take your centimeter, convert into diopters and subtract 1.50 diopters. That’s also a decent starting point, though it doesn’t take into account what you wear for distance right now. And remember: you don’t want your close-up and distance prescriptions to be different from each other in any way other than equal reductions in both eyes and possibly eliminating astigmatism.

Best place to start is the convenience store with plus lenses.  Once you have this figured out every other session we’ll do over the whole of the next eight weeks will be much, much easier.  I promise.

Oh and also, why is Jake not budget-cheap-rehab?

We’ve got support!

I recommend starting with the convenience store adventure, it’ll really be helpful.  If you’re at all unsure after a bit of exploring, just start a new thread in the forum here.  Let me know your centimeter averages, your Snellen results and your current prescriptions.  Include all the data so I can help you make a good assessment.

I told you, you’re not on your own.  😉

Cheers, and do let me know how you get on even if you don’t have questions!  Also watch this Youtube video explaining the diopter bubble (and why you don’t want to just stop wearing glasses).

Important Note:  Remember, if you have presbyopia, drop me a line in the forum with current correction numbers, close-up habits, and centimeter results.  We may need to tweak things for you!

– Jake

Session:  Audio Track

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Session:  Video Stream

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