# How can I determine the magnifying power of my Glasses? **URL:** \ **Category:** Factual Questions\ **Created:** [April 2, 2021, 2:47am UTC](https://boards.straightdope.com/t/how-can-i-determine-the-magnifying-power-of-my-glasses/939199 "2021-04-02T02:47:58Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Ficer67](https://avatars.discourse-cdn.com/v4/letter/f/c57346/32.png) [@Ficer67](https://boards.straightdope.com/u/Ficer67)\ **Post date:** [April 2, 2021, 2:47am UTC](https://boards.straightdope.com/t/how-can-i-determine-the-magnifying-power-of-my-glasses/939199/1 "2021-04-02T02:47:58Z")
The title says it all… I bought a few pairs of glasses over the years, and I don’t know the power of each pair. I wear each pair, and I see, about right, but I want to know what power they are. So, how can I tell? Thanks - Ficer67 ---
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [April 2, 2021, 3:44am UTC](https://boards.straightdope.com/t/how-can-i-determine-the-magnifying-power-of-my-glasses/939199/2 "2021-04-02T03:44:18Z")
If you stop by an optical department, they have a machine that will measure it. Walmart, COSTCO etc. Or are these off the rack reading glasses? If so it’s often marked on the frames temple piece. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [April 2, 2021, 4:43am UTC](https://boards.straightdope.com/t/how-can-i-determine-the-magnifying-power-of-my-glasses/939199/3 "2021-04-02T04:43:49Z")
Quick and dirty method. Assuming your glasses are positive dioptre, which reading glasses are. The power in dioptres is the reciprocal of the focal length in metres. Wonder outside and use your glasses to focus the sun onto something. Measure the distance from your glasses to the surface your are focussing on in metres. Take the reciprocal. Bingo - dioptres. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [April 2, 2021, 11:44am UTC](https://boards.straightdope.com/t/how-can-i-determine-the-magnifying-power-of-my-glasses/939199/4 "2021-04-02T11:44:51Z")
> [@Ficer67](#): > > I bought a few pairs of glasses over the years, and I don’t know the power of each pair. I wear each pair, and I see, about right, but I want to know what power they are. So, how can I tell? You’ve been given correct answers so far. The device that **GaryM** mentions is called – believe it or not – _a lensometer_. Old-style lensometers look like the ones in the Wikipedia article (and I’ve used them for years), and require an operator. Modern ones look like little plastic rectangular boxes and give yo the result automatically [Lensmeter - Wikipedia](https://en.wikipedia.org/wiki/Lensmeter#:~:text=A%20lensmeter%20or%20lensometer%2C%20also,of%20lenses%20in%20spectacle%20frames). You can measure the focal length as **Francis** says if you have a positive lens, but if the focal length is really long this might not be so easy. Here’s a better method – go to a pharmacy with a rack of reading glasses and put your glasses together with one lens of one of the labeled pairs until you find one that cancels out the effect of your glasses. The reason that lens power is measured and reported, rather than focal length, is because lens powers _add_. Putting your 5 diopter lens next to a minus 5 diopter lens results in them cancelling out, so the power of your lens is minus the value of the cancelling lens.\* Opticians used to have big boxes of lenses in “diopter sets” so they could quickly and dirtily measure unknown lenses, or try out different powers for clients \*Not exactly true. The powers very nearly add if placed right next to each other. There’s an exact formula involving the spacing between the lenses, but the distances have to be measured from the principal planes of the lenses, not their surfaces. It’s easier just to say that “lens powers add” ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [April 2, 2021, 2:51pm UTC](https://boards.straightdope.com/t/how-can-i-determine-the-magnifying-power-of-my-glasses/939199/5 "2021-04-02T14:51:14Z")
> [@GaryM](#): > > Or are these off the rack reading glasses? If so it’s often marked on the frames temple piece. The cheap dollar store reading glasses I have have the magnification on the inside of one of the… side thingies. The catch is that it’s written small enough that you might have to be wearing glasses to be able to read it. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [April 2, 2021, 3:28pm UTC](https://boards.straightdope.com/t/how-can-i-determine-the-magnifying-power-of-my-glasses/939199/6 "2021-04-02T15:28:42Z")
> [@Thudlow\_Boink](#): > > The catch is that it’s written small enough that you might have to be wearing glasses to be able to read it. This is one reason they have magnifying glasses ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [April 2, 2021, 7:19pm UTC](https://boards.straightdope.com/t/how-can-i-determine-the-magnifying-power-of-my-glasses/939199/7 "2021-04-02T19:19:25Z")
> [@Thudlow\_Boink](#): > > The catch is that it’s written small enough that you might have to be wearing glasses to be able to read it. That’s OK. The OP says he has several pairs. 😉 My wife has one set of glasses she uses specifically to find her other sets of glasses. Don’t ask; neither you nor I want to know. ---
**Author:** ![Ficer67](https://avatars.discourse-cdn.com/v4/letter/f/c57346/32.png) [@Ficer67](https://boards.straightdope.com/u/Ficer67)\ **Post date:** [April 2, 2021, 11:33pm UTC](https://boards.straightdope.com/t/how-can-i-determine-the-magnifying-power-of-my-glasses/939199/8 "2021-04-02T23:33:31Z")
Thanks for the great advice guys. The power(s) are all printed on the inside of the arm of the glasses, on all of the glasses. I can’t believe I did not see it before. I am going to play with the focal length method when I get the opportunity to. Thanks again - Ficer67