# Optical Forums > Ophthalmic Optics >  prescribing prism by decentration. how to calculate?

## Win C

Hi

I have the following prescription :

R-3.00/-1.75x63
L-4.50/-2.00x15 
PD63 
Prism 2 baseout R and L

can anyone help me with how much to decentre to get the required 2 prism base out? 

I used the prism calculator from opticampus for calculation and there is vertical prism of about 0.2 to 0.3 base up after decentration presumably from the cylinder.  Do i just ignore the vertical induced prism since they are minimal and about the same in both eyes?

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## HarryChiling

.........

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## Win C

Hi Harry,

thanks! I will try to go thru that thread and see if I can work it out. If not I guess I  have to send the job to the lens and let them work it out. :o

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## Win C

There is actually a very quick and easy way to decentre the above lens (single vision stock) to achieve a 2 BO prism without having to do complex calculations (found this out from the lab). 

Just simply put your lens algining it to the axis you want into the the digital lens meter until you locate the position where it shows 2 base out in the read out and then mark it.

Why didn't I think of that!:o

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## IndianaOD

Decentered prism is not equal to ground in prism IMHO

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## Jacqui

> Decentered prism is not equal to ground in prism IMHO


I agree, but others will argue the point.

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## HarryChiling

.............

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## Jacqui

> Why not?


I wear prism (at read only) and it doesn't seem the same to me. I know in theory it is the same, but not to me. This is why I'll make Franklins for myself rather than decentering an exec.

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## HarryChiling

............

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## Jacqui

Doesn't seem the same. I use prism blocking.

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## chip anderson

I don't ever think I have ever heard an "optical expert" including Russell Stimson and Ralph Drew who felt there was a bit of difference in decentration prism and ground in prism.  At least if one is dealing with spherical lenses.

Time for Der Meister to chime in.

Chip

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## RT

> You know when it's ground in the generator/software is moving the optical center, unless your still using the old prism rings.


Not true.  Having worked on the software for computerized generators, I can absolutely guarantee you that there were separate routines for decentration and prism.

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## Jacqui

What I am trying to say is: prism by decentration may be the same as surfaced prism in theory, but it don't look like it when I wear them.

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## IndianaOD

At primary gaze it may be effectively the same.  What about when you look out of primary gaze?

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## chip anderson

If you turn a Baby Ruth upside down, it still tastes the same.
There is *no* difference.  A diamond is a diamond whether made by tectonic pressure or in a monster hydrolic press.  Prism is just a decentered lens no matter how created.

Chip

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## mshimp

ground in or decentered it's all the same! Only exception would be on weak powers. Not enough power to induce the prism you have to grind the prism in. Standard Prentice rule. Of course we are talking about SV.

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## obxeyeguy

> What I am trying to say is: prism by decentration may be the same as surfaced prism in theory, but it don't look like it when I wear them.


Jacqui, try using a FT-45 and use an inset of say 6 when surfacing.  Decenter lens for edging layout using distance OC, voila, with a +2.50 add, 1  diopter base in seg only.  You can't do that with an exec, as you only really have one OC.

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## RT

> Prism is just a decentered lens no matter how created.


Tell me how far you should decenter a plano power lens to get 2 diopters of prism.

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## gemstone

> I don't ever think I have ever heard an "optical expert" including Russell Stimson and Ralph Drew who felt there was a bit of difference in decentration prism and ground in prism.  
> 
> 
> Chip


 But it's still fun to read the opinions.  I can't believe this is still argued.

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## mshimp

> Tell me how far you should decenter a plano power lens to get 2 diopters of prism.


 

see my post for the exceptions to the rule.

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## Chris Bowers

RT that depend on the lens index

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## Jacqui

> You can't do that with an exec, as you only really have one OC.


HORSE HOCKEY !!! I do it quite regularly with execs and it works out rather nicely. And what's this garbage about only one OC?? Mine always have 2 or 3.

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## Jacqui

> At primary gaze it may be effectively the same.  What about when you look out of primary gaze?


That's what I've been talking about. Out of the primary area things are different. I'm a speed reader and use much more area that the average person.

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## optical24/7

Jaq, tell me how you could tell a ground from a decentered prism....(if you didn't make them)?

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## Jacqui

I can't really explain it, but it just seems different. IndianaOD asked if the outer areas were different and I must say yes deffinately. Theory says that it's the same, but I think there is a difference somewhere in the lenses.

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## Darryl Meister

Practically speaking, for spherical-ish surfaces at least, there shouldn't be any difference between prism obtained through decentration compared to prism obtained by surfacing. Of course, this assumes that the edge and center thickness of the lens remain constant.

Prism is produced when the front and back surfaces are at an angle with respect to each other at some point on the lens. Surfacing prism into a lens using a conventional generator and prism rings tips the back surface of the lens relative to the front surface.

The same effect can also be achieved by moving or decentering the back surface relative to the front surface. If you wedge a lever under a log, for instance, raising the lever so that it forms an angle to the ground, the log rolls away from the lever.

Decentering a lens with power works in much the same way. In this case, however, the angle formed by the steeper surface as it rolls forms a steeper angle than the flatter surface, resulting in a relative angle between the two (or prism).

Nevertheless, an illustration of the relationship between the front and back surfaces produced in any of these cases would reveal a similar configuration if they are all compared with the position of either the front or back surface held constant.

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## Wes

> Practically speaking, for spherical-ish surfaces at least, there shouldn't be any difference between prism obtained through decentration compared to prism obtained by surfacing. Of course, this assumes that the edge and center thickness of the lens remains constant.
> 
> Prism is produced when the front and back surfaces are at an angle with respect to each other at some point on the lens. Surfacing prism into a lens using a conventional generator and prism rings tips the back surface of the lens relative to the front surface directly.
> 
> The same effect can also be achieved by moving or decentering the back surface relative to the front surface. If you wedge a lever under a log, for instance, raising the lever so that it forms an angle to the ground, the log rolls away the lever.
> 
> Decentering a lens with power works in much the same way. In this case, however, the angle formed by the steeper surface as it rolls forms a steeper angle than the flatter surface, resulting in a relative angle between the two (or prism).
> 
> Nevertheless, an illustration of the relationship between the front and back surfaces produced in any of these cases would reveal a similar configuration if they are all compared with the position of either the front or back surface held constant.


It's the same thing.

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## HarryChiling

............

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## Darryl Meister

> Yeah but you can't just say that anymore, unless your Der Meister


I'm glad to know that my name carries such stock. But I still provided a rather descriptive answer explaining _why_ they are the same. ;)

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## HarryChiling

.............

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## fagin

> I'm glad to know that my name carries such stock. But I still provided a rather descriptive answer explaining _why_ they are the same. ;)


and very nice it was, too. I believe the problem that a lot of folk have with comprehending prism lies with the comparison of the apparent complexity of surfacing a lens at an angle to produce a prism with the very simple idea of shunting a standard lens about so that its optical centre is moved. If you refer purely to the prismatic _effect_ of a lens, then the method by which this is induced becomes secondary to the desired final result.

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## shanbaum

> Tell me how far you should decenter a plano power lens to get 2 diopters of prism.


All the way.

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## Murray O'Brien

Prism is prism is prism. Don't matter how you get it folks it's all the same! Doesn't matter how you skin the rabbit: same rabbit underneath the skin!

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## Jacqui

> Prism is prism is prism. Don't matter how you get it folks it's all the same!!


Not when I look through it. :D

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## Murray O'Brien

Mmm, nearly 30 years in the business and this is the first time I've ever heard of this subject given consideration. Any dfference you feel must be due to some other issue. Base curves? Thickness? Material? BVD? MRP position? Wrap angle ? Panto? I'm assuming you were comparing specs made to exactly the same R/X.  As a matter of interest what is your R/X and what lenses were you comparing?

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## leestaniforth

hi all,

As many have pointed out, prism is prism etc blah blah, and of course you only surface prism into a lens when power of the lens is not enough to induce said prism and cut out into you frame considering oc's and required blank size of a stock lens.

But.. did anybody actually answer the question .. how?:hammer::finger:, of course prentice rule, but what if the powers are not along 90 or 180 i.e 


lee

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## Murray O'Brien

Hello Lee, 
I don't understand your question. Could you expand on it a bit? If you're talking about the first post in this thread then I think that was answered early on. And it really doesn't matter if the cyl axes are oblique, the method of marking up prism in a vertometer remains the same.

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## harry a saake

Jacqui, your just a classic overthinker, because you know there different, you have that mindset, and your looking for it, and telling yourself there is something different

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## Jacqui

I've had others tell me that thsy see it too, Harry. What I think is happening is that as we move from the OC to the edges we pick up some distortion and this is why we're seeing a difference. Yes, it may be minor, but it's still there. On a surfaced prism we're seeing out of the area that has the least distortion since it's made for the use. This distortion will happen with anyone's lenses, yours, mine, Essilor's or el cheap-o imports.

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## leestaniforth

> Hi
> 
> I have the following prescription :
> 
> R-3.00/-1.75x63
> L-4.50/-2.00x15 
> PD63 
> Prism 2 baseout R and L
> 
> ...



Sorry, i'll try again.

The above R Lens RX has an axis of 63. If the axis was 
180 you would -3.00 to place into p=c/f, however as the axis is 63.. what power do you have to work with?:cheers:

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## bob_f_aboc

> Sorry, i'll try again.
> 
> The above R Lens RX has an axis of 63. If the axis was 
> 180 you would -3.00 to place into p=c/f, however as the axis is 63.. what power do you have to work with?:cheers:


The power at the 180 is ~-4.44 and the power at the 90 is ~-3.32. This is obtained by using the Oblique Meridian Formula.

DT=DS+DC(sinα)^2
Power in desired meridian = Sphere Power + (Cylinder Power * (sine of difference between prescribed meridian and desired meridian)^2)

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## MIOPE

So, let see if i understood what has been said. On a single vision lens the prism can be achived by descentration if the power and  size of the uncut lens is enought. 
The power of the lens we are talking about is the power at 180 for in or out prism and 90 for up and down prism. 
On low power lenseses the prism produced by descentration may not be enought and i will need to grind it.
I should not try to produce a prism by descentration on aspheric lenses.
Since bifocals has been mentioned, on non single vision lenses the prescribed prism should be grinded.

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## Jacqui

> So, let see if i understood what has been said. On a single vision lens the prism can be achived by descentration if the power and  size of the uncut lens is enought. 
> The power of the lens we are talking about is the power at 180 for in or out prism and 90 for up and down prism. 
> On low power lenseses the prism produced by descentration may not be enought and i will need to grind it.
> I should not try to produce a prism by descentration on aspheric lenses.
> Since bifocals has been mentioned, on non single vision lenses the prescribed prism should be grinded.


Yes, you are quite right. :D Although I still think all prism should be surfaced for the best performance.

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## Luis davila

hi, you have three ways to do it:
First Ask for the lab to surface the prism 
second you can dot up with the lensometer just putting the center in the second circle depnds if it is BO or BI and R or L.
The last one is usin this formula to find how much milimiters to descenter.
d=sph+cil(senaxis)2/Prism

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## YrahG

> Although I still think all prism should be surfaced for the best performance.


I really don't understand why this keeps being mentioned, there is no evidence to support that other than anecdotal.  More than a few professionals have mentioned that there is no difference and yet it keeps being brought up.  On the off chance someone might use this resource as the end all be all on the subject it would be nice to see this myth dropped.

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## Jacqui

It's not a myth, try wearing a prismatic lens for 45+ years and then you will know.

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## Mukesh

I do it with this application I have for our lab. Image attached.






> Hi
> 
> I have the following prescription :
> 
> R-3.00/-1.75x63
> L-4.50/-2.00x15 
> PD63 
> Prism 2 baseout R and L
> 
> ...

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