# How to measure you diopters with your differentials on?

Been measuring by the naked eye but when I measure with my glasses on & then add the glasses diopters I’ve been getting slightly different results.

If I’m wearing -3.5 glasses & my edge of blur is 42cm (-2.5) does this mean I’m -6 diopters?

With my glasses off my edge of blur is 18cm which is -5.5.

Am wondering if I’m measuring wrong or if I need to calculate differently

That math works to me.

The point of measuring with diffs is recognising how hard it is to get an accurate measurement with the naked eye, so you’re around that range and that sounds fine. You can use other metrics like how your distance vision is changing to determine whether to reduce or not as well.

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@NottNott haha, I just watched your video on understanding diopters

Thanks for that btw!

Am wondering though, since -.5 diopters is such a big difference, does this mean I’m measuring with my naked eye wrong?

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At lower diopters, it can be hard to see the edge of blur precisely so you can a bad measurement. At higher diopters, you can usually see the edge of blur really well, but the small cm measurement inaccuracy leads to a big change.

I think 25-45cm measurement like you’re doing now is a good compromise between these two attributes of cm measurement. Personally I’d go with the diff measurement, particularly because it gives you a worse result too in this case. Conservatism when reducing is good

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1/6 is 16.77 cm (call it 17 cm)
1/5.5 18.18 cm (call it 18 cm)

So you’re somewhere around -5.75 D…

I would bet that it changes more than 1 cm throughout the day depending on your mood, what you ate, how tired you are, how good the lighting is, etc.

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