When and how much to reduce normalized?

So I’m currently at L:-3.25 and R:-2.75
CMs measurments:
L:33cm R:38cm

should i reduce?

a more general question,
what are the methods or ways after which you can determine by how much you can reduce (it a 0.25 reduction from my 20/20 prescription?)

Please help, it tends to make me very anxious as it is a little confusing.

If your cm measurements are accurate, it would seem that you are ready for a reduction. How well can you read the 6m Snellen chart? If you can see the 20/20 line clearly under most conditions, this will also confirm that you are ready for a reduction. Check your eyes separately on the Snellen to make sure they see equally well. Judging from your cm measurements this is not likely to be the case, but it is better to make sure.

The reduction recommended by EM is usually only 0.25, unless you see the 20/15 or even 20/10 line clearly. In that case you could try a bigger reduction. Don’t be anxious about it. If you have reduced too much you can always go back up a little. Glasses for this level of myopia are not that expensive, and too big a reduction will be waiting for you when you are ready for the next reduction. Good luck with your decision.


Okay so if for example I can see 20/20 w -3, so I should wear -2.75 as my normalised? did I interpret it correctly?

Correct - if you can read 20/20 with a lens it is time to take of 0.25D for your new normalised.

As Ursa says, the eye chart is the best source of truth when it comes to determining when it is time to reduce your normalized correction by 0.25D. However, I will add a bit of additional detail to Ursa’s post which may come in handy when doing reductions in the future…

When reducing, keep in mind that vision fluctuates up and down a lot. So it is always good to make sure that your vision is stable before doing a reduction to ensure you don’t reduce too soon. You want to re-measure under the same conditions each day for like a week to confirm that you are seeing consistent results (i.e, 20/20 every day) and that you don’t have any double vision before doing a reduction. If you can check off both of those, you’re good to go.


When I usually measure using the centimetre calculator, it shows that I’ve the correct prescription, but on the snellen it’s usually a 20/25 or 20/30.
I would stick to the snellen chart as I find it more reliable and keep the calculator just as a means of checking on my minute improvements.

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I see cm measurements as a good way to track progress and look for changes. But, in theory, if you can measure accurately, consistently, and account for vertex distance, you should be able to get pretty close to the correct diopter values for your vision. Of course, this assumes you don’t have astigmatism, or you account for cylinder when doing cm measurements.

Also keep in mind that if your CM measurements are consistently underreporting the diopters under the same light levels then there is a good chance that you may not be accounting for vertex distance. If you’re at -4D or above, this can add -0.25 diopters or more to your measurements.

What is vertex distance? Never heard of this

Vertex distance (aka back vertex distance) is the distance between the back surface of a corrective lens (glasses or contact lenses), and the front of the cornea. So basically the distance between your eyes and your glasses. The further away the glasses are, the further forward the focal point of the lens moves, reducing the effective power of the lens.

Most glasses sit about 12-14mm from the eye, so you need to compensate for that when you measure with the naked eye in order to get a proper glasses correction. This becomes more significant the higher the amount of correction required by the eye. Particularly over -4D spherical equivalent refraction.

Wikipedia has a good article on the subject (Vertex distance - Wikipedia). It explains the topic and provides the formula for calculating vertex distance compensation as well. It isn’t hard to calculate (I just built it into the spreadsheet I use for tracking my own cm) but you can also use one of many online calculators to do the adjustment for you. For example: https://bvdcalculator.garethcooper.com

Alright mate! even though I’m under -4, I’ll still read more about this.

If you are struggling to find your way around it might help you to know we just released a chatbot to answer soooooooo many questions. Lots of work put in, should prove very useful. It is programmed to either answer your questions out right or point you to very complete answers within the many available resources, and in most cases, both. Go to facebook.com/endmyopia click send message and get started. Every path ends with a return to start option so you can track down another path or type “help” (without the quotes) at any time to go back to start. Best Wishes