How can you determine the strength of reading glasses?

I would like to determine the actual numeric strength of each lens in the glasses.


Qick and easy

Hold the glasses in front of a white paper or wall until the sun (or a far away window) is in focus. Measure the distance (in metre) between the glasses and the paper. Divide 1 by this value to obtain the glasses dioptre.

Physics behind this

Reading glasses are similar to a thin looking glass. The same optical rules apply to them:

enter image description here
Source: German Wikipedia

In the picture above we can see the object G distance g, the distance b of a projected image B and the resulting focal length f. By measuring g and b we can calculate the dioptre 1/f of our lens with this formula:

enter image description here

To make things easy we may take a far away object such as the sun (where 1/g approximates 0) to measure the distance b between the lens and the sun's projection on a white paper to have a simplified formula for the dioptre

1/f ≈ 1/b.

  • 2
    Wow. Thanks. I'm going to read that over and over until it's obvious. Dec 6 '16 at 10:01
  • 2
    @RockPaperLizard made a quick and easy paragraph for the lazy among us.
    – Takkat
    Dec 6 '16 at 10:11
  • 2
    My reading glasses always say something like "+1.5" or "+2.0". How does that relate to dioptre?
    – BrettFromLA
    Dec 6 '16 at 17:38
  • 1
    @RockPaperLizard actually that is the definition of a dioptre.
    – Aron
    Dec 7 '16 at 8:18
  • 1
    @Lawrence great! Thank you - so . or , was not by convention but by sheer accident in the first place. Interesting!
    – Takkat
    Apr 11 '18 at 8:20

I THINK I GOT IT - for convex reading glasses. Tape a piece of white paper where the sun shines on it. Hold the glasses (as if someone was standing in front of the white paper looking at you, the sun behind you - at least behind your arm so your shadow does not block the sun from the glasses). Move the glasses until the sun is clearly in focus (this is especially easy if there is something far away from you that can be seen in the suns reflection). Measure the distance between the white paper and the glasses (at the distance where the sun is in focus). Convert measurement to cm. 1 divided by cm number. Move the decimal place right 2 spaces. As an example: distance between paper and lense in glasses is 26 inches. 26 inches equals 66 cm. 1/66=.015151515. Move decimal 2 places to right (or multiply by 100)=1.5 is strength of reading glasses.

  • Is this the same as the answer Takkat posted above more than a year ago, with the x100 used to convert between dioptre and 'degrees'?
    – Lawrence
    Apr 11 '18 at 8:07
  • 2
    I don't see that this adds anything to the existing answer.
    – Chenmunka
    Apr 11 '18 at 13:21

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