CM diopters don’t matchup with current prescription

This is for my 10yrs old son. His prescription says he is OS : -5.0(-0.5 cyl) and OD : -0.4( -0.5 cyl). Cm diopter measurements for the printed text 12 pt is OD:40 cm and OS : 43 cm which doesn’t match to prescription. Can some one help where I am going wrong and which one to be considered as a prescription to start for a differential and normalised?

Lighting used for measurements are indoor ambient light.

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This is just a guess, but it’s probably 2 factors:

  1. the official prescription is over-prescribed, but obviously not by 2 diopters
  2. the cm measurement is unreliable and he is not being strict enough on himself

for 2) try to be REALLY strict, look just at the edge of a letter, a single line and measure cm till the first hint of blur on that line. Is it REALLY 40cm+? Then again, -5 would be around 20cm so I doubt it’s that far off.

vertex distance also does not play such a huge role here but don’t forget you are measureing, with cm to blur, a contact lens prescription. Glasses prescription could be a bit higher

@Lajos - I getting your point and may be you are correct. These may not be strict measurements , so may be a fallacy to them. I will try again on what you suggested.
Again do u think because he is on the higher side of myopia , the cm measurements cannot be confirmed as his actual diopter measurements? And in that case should I just take a final proper test at opto with relaxed ciliary and use that for normalised and differentials?

well obviously, at higher myopia, inaccurate cm measurements make more of a difference, but accurate cm measurements can still be used as a rough guide for his diopter needs.

Since you are not measuring for yourself but basing it on what a kid says, it’s even harder

Lajos, you seem very confident.

and you seem to want to get on my nerves. Go and play somewhere else

like "This is just a guess, " sounds overly confident, geez

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It sounds like he’s over-prescribed. Remember you’re looking for distance to blur and not distance to where he can’t read the letters at all anymore.


Overprescribed or still has a very good range of accommodation not totally forgotten, not totally disabled by wearing glasses for decades. Kids eyes change easier. Lot easier. Both ways.

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definitely possible he’s overprescribed. children have a high range of accomodation. make sure you’re going to an optometrist that prescribes the minimum needed to pass a state driving exam, bonus if that optometrist offers different prescriptions for different distances. Or if you can afford, go to a behavioral optometrist and ask for computer distance glasses for the kid and see what prescription you get.

if you have your son’s pd another option is to order different glasses and test him vs street signs