# Optical Forums > Ophthalmic Optics >  Lens Center & Edge Thickness Calculator

## Spex

Gentlemen,
I am looking for a formula to find center & edge thicknesses so I can do comparisons. The only varibles I know are index of refraction, lens Rx, effective diameter of frame, thinnest edge/center, & front base curve. The end result I would like to find is what would be the center thickness of a plus lens or the thickest edge thickness of a minus lens with different indexes of refraction.

Several formulas or programs that I have found that will do calculations like this require unknown information. For example: One program prompts you to input front & back surface curves. If you only have the Rx & front base curve, the back surface curve will be different on each index of refraction. But if a formula knows the index of refraction, front base curve, & Rx, it should be able to calculate the back surface needed, & therefore calculate the edge or center thicknesses on either plus or minus power lenses.

I hope I have been clear enough so that everyone can understand what it is I am looking for. If not PLEASE, post what it is I have missed so that I can finally get what I am looking for.

Thanks in advance!!

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## eyesguy

Hi Spec:

If you have Excel, Download Darryl's "Optics Lite" . It will do what you need and then some. I could give you easy formulars for minus lenses, but plus lenses will give you fits if your not familiar with the optics.

Good luck,

eyesguy

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## Spex

Thanks for the reply, Eyesguy!

I have the Opticslite Excel app. But I can't get it to give me the info I need. Maybe I am not putting in the correct info. If you are familiar with it maybe you could email me privately & help me figure out what I am doing wrong.

I would appreciate any formula's you could share. I have only been in the optical business a mere 31 years. So I may not have the hang of it yet.

Thanks again!! I look forward to hearing from you!

emailto:blinkr@email.com

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## Judy Canty

Also in the download forum, Darryl has posted an article titled, "Methods for Estimating Lens Thickness," which should give you the information you need.

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## Steve Machol

> _Spex said:_
> I would appreciate any formula's you could share.


Also take a look at the Basic files for the Lab Calculator and Surface Chart Creator programs.  You should be able to extract the thickness formulas from those.

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## Spex

Thanks Judy! I just found that a few minutes ago. I will be checking it out!

Thanks for the suggestion Steve! I just did that & copy & pasted it to notepad & printed it out. I will be putting it into an Excel spreadsheet soon. I will let you know how it works out!!

I really appreciate all of the help I have gotten on this from everyone on the Optiboard. This IS a great messageboard. Thanks for all of your hard work Steve!!

BTW, to those of you on the board I have given Steve the title of Maharishi Steve, Master Optiboard Guru!!

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## John R

> _Spex said:_
> Gentlemen,
> I am looking for a formula to find center & edge thicknesses so I can do comparisons.


Got a online version on my site that will show you how the index affects the curves and a sag chart.


Quick link

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## Spex

Thanks John R!!

Very Nice!!

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## John R

I have just added to the form, if you do a index change it will now show the reduction of edge subs @ 60m/m.

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