📏 Centimeter calculator

Something that surely should be mentioned in this category, the tried and true centimeter calculator, to be found here:

https://endmyopia.org/focal-calculator/calc.html

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what might be going on? If I measure (using the print out ruler, laminated, looking at 12 point font word “FOCUS”) I get about 20cm. Using the calculator it says ‘-5’ (great I was a lot worse than this for many years). But when I wear -5 glasses I can’t see more than 3rd (20/80) or 4th (20/63) line of chart in distance. Have I misunderstood something? (I thought if the calculator said -5 then -5 glasses would correct - is this not the case); or is some form of active focus happening for close up that doesn’t happen for distance?)

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No I think you understand fine, it’s just that

  1. that is why optos measure at 6m - it’s much more accurate. What is a tiny bit of blur which you might not notice close-up, is magnified as the light beams spread over a longer distance and manifests as worse blur further away
  2. people tend to be lenient on themselves when measuring. Be very strict. Back away from the monitor only until you notice the tiniest bit of blur then go back 0.5cm and measure the distance to there
  3. If you have astigmatism, try to look at lines along your strongest axis so you’re measuring only spherical blur and not directional
  4. cm measurements are done to the eye which means when you convert the distance to blur to diopters it gives you a contact lens prescription not a glasses prescription. This makes a difference with high myopia due to the difference in vertex distance for glasses.
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That’s excellent advice.

Kent

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appreciate it :slight_smile: I noticed this makes a difference for me, so I actually log 2 readings each time I measure cm to blur: weakest and strongest axis but this is only for people with noticeable astigmatism

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At close to -2 in both eyes, I qualify.

I wonder if @jetpad could update the Sight Meter app to have a distance to blur (astigmatism) option? :smile:

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Thanks for the confirmation. Really appreciated. I also use the vision log (with and without glasses and get some results that are also a bit different to each otherand my paper measurements). With glasses tends to be most similar to actual optometrist measures. Perhaps that does have something to do with astigmatism with no glasses… since the glasses do correct for an old version of it, although I really don’t feel I notice it in any way whatsoever.

One more point I forgot to mention is that centimetre measurements are done to the eye which means when you convert the distance to blur to diopters it gives you a contact lens prescription not a glasses prescription. this makes a difference with high myopia

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I know it is a little stupid question but at what point of the face do you put the measuring tape. I usually just touch the outer side of the bone of the eye is that corect?

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I think it doesn’t matter as long as you always use the same point (for tracking progress). cm measurements are not really accurate enough to determine proper prescription (just roughly) anyway.

But you can measure to the cheekbone which seems quite in line with the front of the eye to me

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That is exactly what I was suspecting. Thanks for the clarification.