# Thread: Glasses Prescription to Contact Lens Presciption

1. ## Glasses Prescription to Contact Lens Presciption

Hi,
I have the following prescription (glasses)

Right : -4.00/-0.50 x 3
Left : -4.75/-1.50 x 154

after i did the vertex conversion, it become
Right: -4.00/-0.25 x 3
Left : -4.50/-1.25 x 154

The customer request that he wanted without any toric (price issue)

May someone help how to do a calculation so that the contact lens is just plain sphere and no cylinder involve?

2. Originally Posted by danielccn
Hi,
I have the following prescription (glasses)

Right : -4.00/-0.50 x 3
Left : -4.75/-1.50 x 154

after i did the vertex conversion, it become
Right: -4.00/-0.25 x 3
Left : -4.50/-1.25 x 154

The customer request that he wanted without any toric (price issue)

May someone help how to do a calculation so that the contact lens is just plain sphere and no cylinder involve?

Not sure what the RX rules are for Asia, as where you are listed as being from, but the only true calculation change should come from the prescribing doctor. Removing the cyl power can be seen as changing the whole RX and unless your country's rules allow a contact dispenser or equivalent Optician to do as such, you are putting the patient and your self into trouble.

Additionally, if price is an issue, there should be a discussion on what other alternative toric options are available. Yes, toric options are expensive, but if that is what the patient truly needs to see correctly by the prescribing doctor, that is what must be ordered.

3. Originally Posted by danielccn
Hi,
I have the following prescription (glasses)

Right : -4.00/-0.50 x 3
Left : -4.75/-1.50 x 154

after i did the vertex conversion, it become
Right: -4.00/-0.25 x 3
Left : -4.50/-1.25 x 154

The customer request that he wanted without any toric (price issue)

May someone help how to do a calculation so that the contact lens is just plain sphere and no cylinder involve?

Your vertex compensation calculation looks incorrect, especially the right eye cylinder cut.

The powers in the principle meridians are

Rt -4.00 and -4.50
Lt -4.75 and -6.25

Converting to FL (F = (1000/D))

Rt 250mm and 222mm
Lt 211mm and 160mm

If the the vertex distance is a typical 13mm, the above becomes...

Rt 263mm and 235mm
Lt 224mm and 173mm

Converting back to diopters (D = 1000/F) the principle meridians become...

Rt -3.80 and -4.26
Lt -4.26 and -5.78

The final Rx adjusted for vertex distance is...

Rt -3.80 -0.46 x 3
Lt -4.46 -1.32 x 154

Hope this helps,

Robert Martellaro

4. If it's under your authority, you could try using a spherical equivalent, going off of Roberts math, that'd be:

(-3.80) + (-0.46/2) = -4.03
(-4.46) + (-1.32/2) = -5.12

(Rounding)

OD -4.00 Sph
OS -5.00 Sph

Dont know how well Spherical equivalents transfer to contacts, but that's the math that I get.

5. ....

6. ....

7. ...

8. Originally Posted by GrahamEye
Not sure what the RX rules are for Asia, as where you are listed as being from, but the only true calculation change should come from the prescribing doctor. Removing the cyl power can be seen as changing the whole RX and unless your country's rules allow a contact dispenser or equivalent Optician to do as such, you are putting the patient and your self into trouble.

Additionally, if price is an issue, there should be a discussion on what other alternative toric options are available. Yes, toric options are expensive, but if that is what the patient truly needs to see correctly by the prescribing doctor, that is what must be ordered.
Thank you for your response... i am taking this case as a case study to understand how to do it the right way. of course the contact lens prescription must use the formula vertex in order to get the final prescription for contact lens. What i want to know after did the vertex conversion... is that the final result that should give to patient with all the neccessary cyl power ?

9. Originally Posted by Robert Martellaro
Your vertex compensation calculation looks incorrect, especially the right eye cylinder cut.

The powers in the principle meridians are

Rt -4.00 and -4.50
Lt -4.75 and -6.25

Converting to FL (F = (1000/D))

Rt 250mm and 222mm
Lt 211mm and 160mm

If the the vertex distance is a typical 13mm, the above becomes...

Rt 263mm and 235mm
Lt 224mm and 173mm

Converting back to diopters (D = 1000/F) the principle meridians become...

Rt -3.80 and -4.26
Lt -4.26 and -5.78

The final Rx adjusted for vertex distance is...

Rt -3.80 -0.46 x 3
Lt -4.46 -1.32 x 154

Hope this helps,

Robert Martellaro

Hi,

Yes i did the calculation wrong on the right side cyl... so using this vertex conversion.. this is the final result that i should give to patient and by rounding up to the nearest one

R -4.00 -0.50 x 180
L -4.50 -1.25 x 150

am i right this is the proper result for a toric lens ?

10. Originally Posted by McAnerin
If it's under your authority, you could try using a spherical equivalent, going off of Roberts math, that'd be:

(-3.80) + (-0.46/2) = -4.03
(-4.46) + (-1.32/2) = -5.12

(Rounding)

OD -4.00 Sph
OS -5.00 Sph

Dont know how well Spherical equivalents transfer to contacts, but that's the math that I get.

Yes that is what i come out with too
R -4.00
L-5.00
after i use the spherical equivalents...

I know that this prescription is not the best but what i want to know... is this the right way to calculate?

11. I'm pretty uncomfortable helping someone out from another side of the world with their patient care.

Is this how contact lenses are prescribed in Asia? From someone who barely knows what they're doing?

Is it equitable for your health care system to depend on our health care system for guidance?

12. In Asia, you can get glasses dispensed to you in an array of odd & curious ways.

13. So maybe it isn't a utilized method, but I thought there was an actual formula for calculating a contact Rx from a glasses Rx? I had heard something to do with the cyl strength and removing or adjusting dependent of if it was over a certain amount. Same kind of process for using the Access lenses.

I found these:

https://coopervision.com/practitione...ion-calculator

https://www.swisslens.ch/experts/toolbox/

which doesn't seem to alter the Rx much at all...

Maybe this isn't worth looking into because I don't work with contacts, but I guess I'm just curious.

14. Originally Posted by danielccn
Yes that is what i come out with too
R -4.00
L-5.00
after i use the spherical equivalents...

I know that this prescription is not the best but what i want to know... is this the right way to calculate?
I agree.
Virtually all contact lens companies have their own custom ‘nomograms’to determine CL parameters, including power, which they have produce after a huge number of trial fits. Such nomogramsusually result in a satisfactory ‘first fit’ close to 75% of the time.
Using ‘Vertex Compensation’ tables available at the back ofany good CL textbook, in OD a power of -4.00 normally requires no vertex power adjustment for soft lenses since vertex compensation at 12mm results in a -3.82,and a very small amount of minus tear lens is created upon wear. And the -4.50 meridian vertexes @12 to -4.27, roundedto -4.25 and giving us a final CL Rx of
OD: -4.00-0.25x003
For OS: -4.75 vertex @12mm results in -4.49, and the -6.25 vertexcompensates down to -5.81.
-4.49 rounds to -4.50, and other meridian of -5.81 roundsdown to -5.75 giving us
OS: CL Rx: –4.50-1.25x154
Since many CL companies start their toric lens stock at 0.75cyl,a toric CL Rx with 0.50 cyl is usually made into a spherical equivalent power.So -4.00 + (-0.25/2) = -4.12sph, or
OD: -4.00 sph in soft lens form due to that slight minustear lens formed upon wear.
The OS: eye should ideally have a toric lens for -1.25 cyl.But if a cheaper spherical equivalent is absolutely desired then final a sphequivalent CL Rx would be -4.50 +(-1.25/2) = -5.12, or -5.00sph in soft lens form.
This OS sph equivalent power will result in somewhat reducedvision in OS but the patient specifically wished this modality, and it may workfor them since other eye’s vision will be quite good. A sph equivalent wish whentoric is indicated is sometimes seen in the use of spherical Halloween lenses.
Some CL companies start their toric cyl lens stock at -0.50cylwhile others at -1.00cyl. In any case, once you trial fit the OS lens, the patientwill indicate whether she can tolerate the sphere equivalent power in left eye. And you might refine or verify the finallens power through the use of flippers.

15. Originally Posted by tmorse
I agree.
Virtually all contact lens companies have their own custom ‘nomograms’to determine CL parameters, including power, which they have produce after a huge number of trial fits. Such nomogramsusually result in a satisfactory ‘first fit’ close to 75% of the time.
Using ‘Vertex Compensation’ tables available at the back ofany good CL textbook, in OD a power of -4.00 normally requires no vertex power adjustment for soft lenses since vertex compensation at 12mm results in a -3.82,and a very small amount of minus tear lens is created upon wear. And the -4.50 meridian vertexes @12 to -4.27, roundedto -4.25 and giving us a final CL Rx of
OD: -4.00-0.25x003
For OS: -4.75 vertex @12mm results in -4.49, and the -6.25 vertexcompensates down to -5.81.
-4.49 rounds to -4.50, and other meridian of -5.81 roundsdown to -5.75 giving us
OS: CL Rx: –4.50-1.25x154
Since many CL companies start their toric lens stock at 0.75cyl,a toric CL Rx with 0.50 cyl is usually made into a spherical equivalent power.So -4.00 + (-0.25/2) = -4.12sph, or
OD: -4.00 sph in soft lens form due to that slight minustear lens formed upon wear.
The OS: eye should ideally have a toric lens for -1.25 cyl.But if a cheaper spherical equivalent is absolutely desired then final a sphequivalent CL Rx would be -4.50 +(-1.25/2) = -5.12, or -5.00sph in soft lens form.
This OS sph equivalent power will result in somewhat reducedvision in OS but the patient specifically wished this modality, and it may workfor them since other eye’s vision will be quite good. A sph equivalent wish whentoric is indicated is sometimes seen in the use of spherical Halloween lenses.
Some CL companies start their toric cyl lens stock at -0.50cylwhile others at -1.00cyl. In any case, once you trial fit the OS lens, the patientwill indicate whether she can tolerate the sphere equivalent power in left eye. And you might refine or verify the finallens power through the use of flippers.

HI tmorse,

Thank you so much for the detailed explanation. All those you mentioned, i also come out with the same... so end up right is -4.00 and left is -5.00
which rounded up.
I also understand this by right is not the proper way, the proper way is to give the patient whatever toric they have in order to achieved the best vision for them. Sometimes is Asia, patient requested that way although we already told them that is not the proper way. So that is the best we can do for them in proper calculated conversion and rounded up.

I here want to thank to all the people that helped me on this issue except those who left some RACIST remark !!!!!
I know what i am doing , i just need to check what other people will do if they are in my shoe, which end up my calculation is same as people who contributed.
Experience and skill comes along with time and practice.... Really appreciate all the help and guidance that you guys gave..
I want to thank all those especially the following....a Big Thank You !!!!

GrahamEye
Robert Martellaro
McAnerin
Quince
tmorse

16. Originally Posted by Quince
So maybe it isn't a utilized method, but I thought there was an actual formula for calculating a contact Rx from a glasses Rx? I had heard something to do with the cyl strength and removing or adjusting dependent of if it was over a certain amount. Same kind of process for using the Access lenses.

I found these:

https://coopervision.com/practitione...ion-calculator

https://www.swisslens.ch/experts/toolbox/

which doesn't seem to alter the Rx much at all...

Maybe this isn't worth looking into because I don't work with contacts, but I guess I'm just curious.
Hi,

Yes i did use the coopervision calculation . The thing is patient wanted without any cyl power. so I have to do a spherical equivalent to get to the nearest sphere power without any cyl. The vertex calculation is to convert glasses power to contact lens power... which the patient suppose to use that result from the coopervision calculation. I went a step further because of patient requested that no cyl because of his requirement which we already advice not to...but he insisted. So from that result , i did a spherical equivalent and went and round up the result. which you can see from tmorse detailed calculation.(Thank you tmorse).

Please someone do comment if i mentioned anything that is incorrect. Thanks in advance. :)

17. What you asked is so basic that an optometry student could do it in their first year.

I think your country needs doctors.

Fitting contact lenses (off a spectacle Rx, no less) but having to ask such basic questions means you know nothing about contacts, let alone their physiological properties.

Your health care system is inferior.

I don't think we should participate in helping you provide inferior care any more than we should participate in helping do-it-yourself consumers.

I think well meaning Optiboard opticians should examine this point, for future reference.

To wit: if some optometrist posts questions about cataract extraction technique for some mission trip to Africa, should I chime in and give him my "opinion" on how to do it? Or, just stay out of that mess?

18. What drk said.

19. [QUOTE=danielccn;541655]I understand this by right is not the proper way, the proper way is to give the patient whatever toric they have in order to achieved the best vision for them. QUOTE]

This statement is not always correct, and I should have explained a little better.
For soft lenses, we need to look at the contact lens Rx and its Sphere/Cylinder ratio in -cyl form. Here we had-4.00-0.25x003 for a 16:1 sph/cyl ratio. The higher the sph/cyl ratio of CL Rx, the better a spherical equivalent power will work. And a CL Rx of up to -1.00cyl can be tried successfully as a sph/equivalent if the sph/cyl ratio is 3:1 or higher. Thus a -2.00-1.00 x 180 has a sph/cyl ratio of only 2:1 and a toric lens is indicated. But a -2.00-0.50x180 has a 4:1 sph/cyl ration and a sph/eqiv of -2.25 will probably be well accepted.

A Cl Rx: -5.00-1.00 x180 has a 5:1 sph/cyl ratio and so a -5.50 Sph equivalent will probably do just fine. But it also depends on the intended use of the CL lenses. An accountant looking a small figures all day would benefit from a -5.00-1.00x180 toric, while most regular CL wearers would be quite happy with the cheaper -5.50 sph equivalent power, especially if finances are an issue.

20. [QUOTE=tmorse;541705]
Originally Posted by danielccn
I understand this by right is not the proper way, the proper way is to give the patient whatever toric they have in order to achieved the best vision for them. QUOTE]

This statement is not always correct, and I should have explained a little better.
For soft lenses, we need to look at the contact lens Rx and its Sphere/Cylinder ratio in -cyl form. Here we had-4.00-0.25x003 for a 16:1 sph/cyl ratio. The higher the sph/cyl ratio of CL Rx, the better a spherical equivalent power will work. And a CL Rx of up to -1.00cyl can be tried successfully as a sph/equivalent if the sph/cyl ratio is 3:1 or higher. Thus a -2.00-1.00 x 180 has a sph/cyl ratio of only 2:1 and a toric lens is indicated. But a -2.00-0.50x180 has a 4:1 sph/cyl ration and a sph/eqiv of -2.25 will probably be well accepted.

A Cl Rx: -5.00-1.00 x180 has a 5:1 sph/cyl ratio and so a -5.50 Sph equivalent will probably do just fine. But it also depends on the intended use of the CL lenses. An accountant looking a small figures all day would benefit from a -5.00-1.00x180 toric, while most regular CL wearers would be quite happy with the cheaper -5.50 sph equivalent power, especially if finances are an issue.

Thank You tmorse, now i know how to justifiy the spherical equivalent's "QUALITY" using the ratio... this is great !!!.. I learn so much from you..thank you so much for the tips.... really help me to do a better job.... and also yes.. i know the job of the patient also a factor. That is why history taking of the patient is also very important part. ( read a clinical procedure book).
Thanks to u , i am able to do a computer program using all this calculation and also now i can include a calculation base on the RATIO, which instantly give me an "Quality" of the spherical equivalent. ( I was in related IT field before) and now i have created a program using Filemaker Pro for Patient's record, POS, Inventory, and calculation for glasses to contact lenses) vertex conversion, retinascopy conversion...etc....

Thank you

21. Originally Posted by rbaker
What drk said.

Does Not Matter ...Life goes on :)

22. Originally Posted by drk
What you asked is so basic that an optometry student could do it in their first year.

I think your country needs doctors.

Fitting contact lenses (off a spectacle Rx, no less) but having to ask such basic questions means you know nothing about contacts, let alone their physiological properties.

Your health care system is inferior.

I don't think we should participate in helping you provide inferior care any more than we should participate in helping do-it-yourself consumers.

I think well meaning Optiboard opticians should examine this point, for future reference.

To wit: if some optometrist posts questions about cataract extraction technique for some mission trip to Africa, should I chime in and give him my "opinion" on how to do it? Or, just stay out of that mess?

Why did you edit your initial RACIST post , why not let everyone see what you wrote and deleted it and replace with this post....
Ya right, optometrist can do all this. but i am not Optometrist....i just taking someone's prescription and learn from it and i did not give that prescription to patient. So say what u want... leave your initial RACIST post before in here and let other determine if you are RACIST or not and not related to this optometry thingy. If you don't help is ok , but DON't act RACIST !!!

is funny u edited your message and don't let other see what u wrote before....if u are sure about what u wrote before... u would not have deleted and trying to hide those RACIST comment......

Luckily there are kind hearted people willing to pass on their knowledge and skills to help...and Africa? you mentioned Asia people should not come here and ask for guidance... what plan to do in Africa? well... "go depend on your Africa healthcare system"..that i guess what you would said.....

23. Let me clear a bit , from my description i NEVER said that i am optometrist and also my status clearly state that is OptiBoard Apprentice.

So which part of ENGLISH , doesn't understand? Below is a google defination which i get, maybe Google got it wrong entirely.

As a dispensing optician you will dispense and fit spectacles and other optical aids, to both adults and children, WORKING FROM THE PRESCRIPTIONS WRITTEN BY OPTOMETRISTS AND OPHTALMOLOGIESTS.. You'll advise patients on various types of lenses and spectacle frames, including style, weight and colour.

apprentice

noun1.a person who works for another in order to learn a trade.

Forum Discussion ...hmmm.. what is a discussion ? anything discussed here must be 100% correct? must be know it all ...?

so if someone is not optometrist, he/she cannot ask this SIMPLE contact lens fitting? so a student coming here to learn, maybe should ask he/she to go back to his/her academic system to get help ?? It is sad to see this world has this kind of SELFISH and ARROGANT people who really does exist. I pity those who work with this people and pity those not the same "RACE" as him, because YOU WILL NOT GET THE HELP BUT YOU WILL GET RACIST REMARK!!!!

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