# Optical Forums > General Optics and Eyecare Discussion Forum >  Prism Confusion  Please Help!

## hvu

Prism Confusion – Please Help!   
  When verifying the prescribed Prism, I’m confused with plus and minus lens. I hope that someone can help me out on this. Here is the example.

  EXAMPLE:
  A right lens prescription reads: +3.00 -1.25 x 135, 2^ Base Out
  How much the lens be centered before it is spotted?

  SOLUTION:
  To verify this prescription, the lens must be placed in the lensmeter and moved until the illuminated target is located 2^ units to the left (temporal) of the crosshair origin (Target center location always corresponds to prism base direction. This is true whether the lens is plus or minus in power)

  Here is my problem:
  Based on my understanding, the right lens will be placed on the lensmeter as normal (with the back side of the lens against the lensmeter stop, the power being measured is called the back vertex power). And this is a plus lens, so that its prism bases are base to base at the center (OC). Therefore, I need to move the lens to the left 2^ units to find the right spot for this case. Am I right or wrong on this?


  But if the example above changes to minus lens like: -3.00 -1.25 x 135, 2^ Base Out for the right eye too. I think the lens will moves to the right instead so that it will have the Base Out?

  I’m very confused. Can anyone explant it to me with a picture for plus and minus lens for BO, BI, BU, BD.

  Thanks,
  HVU.

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## gemstone

> Prism Confusion – Please Help! 
> When verifying the prescribed Prism, I’m confused with plus and minus lens. I hope that someone can help me out on this. Here is the example.
> 
> EXAMPLE:
> A right lens prescription reads: +3.00 -1.25 x 135, 2^ Base Out
> How much the lens be centered before it is spotted?
> 
> SOLUTION:
> To verify this prescription, the lens must be placed in the lensmeter and moved until the illuminated target is located 2^ units to the left (temporal) of the crosshair origin (Target center location always corresponds to prism base direction. This is true whether the lens is plus or minus in power)
> ...


You are not confused. You are correct. You just have to move the minus power lens to the right to move the oc target to the left for base out.

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## Fezz

Try to remember that when viewing lenses thru a lensometer:

1.) A plus lens "center" will follow whatever direction you move the lens.
2.) A minus lens "center" will move in the opposite direction.

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## hvu

Thanks gemstone for a quick respond. That is exactly what I thought, but when I read the solution in the book it said “Target center location always corresponds to prism base direction. This is true whether the lens is plus or minus in power”. I just don’t understand what it means exactly. Can you help me on this?

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## gemstone

That is correct.  You move the minus lens to the right, the target moves to the left for base out , right lens.  I need to edit my first response.

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## optical24/7

> Thanks gemstone for a quick respond. That is exactly what I thought, but when I read the solution in the book it said Target center location always corresponds to prism base direction. This is true whether the lens is plus or minus in power. I just dont understand what it means exactly. Can you help me on this?


It means that when you are spotting prescribed prism, you move the OC in your lensometer in the direction prescribed i.e. in, out, up, down, for the respective lens OD or OS to the prescribed amount, then spot the lens and check to see if it matches the PD. It doesn't matter if it's plus or minus, the OC needs to be spotted in the direction that's prescribed.

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## hvu

> It means that when you are spotting prescribed prism, you move the OC in your lensometer in the direction prescribed i.e. in, out, up, down, for the respective lens OD or OS to the prescribed amount, then spot the lens and check to see if it matches the PD. It doesn't matter if it's plus or minus, the OC needs to be spotted in the direction that's prescribed.




Can you give me more details? I really don't get it.
Here is what I think:
1. For plus lens, the OC will move in the same direction as the base direction. For example, when a plus lens 2^ Base Out; it will move the lens' OC to the left of the lensometer to gains Base Out for the right lens.
2. But, if this is a minus lens, then I have to move the OC of this minus lens  o the other direction if compared to the plus lens, move to right size of the lensometer to gain the Base Out. At this point, the OC also moved to the right which is opposite to the Base Out of the prism.

That is what I really do not get it. But I totally understand what Fezz said about plus and minus lens moving.

Please give me more details.

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## Fezz

> 2. But, if this is a minus lens, then I have to move the OC of this minus lens o the other direction if compared to the plus lens, move to right size of the lensometer to gain the Base Out. At this point, the *OC also moved to the right* which is opposite to the Base Out of the prism.


 
No, the minus oc moved to the* LEFT!*

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## lensgrinder

When you are talking about the lensmeter the mires will always be located toward the temporal side for BO prism this will be to the left for a right eye and to the right for a left eye, regardless of the Rx.

So, if you move a plus right eye OC to the left in the lensmeter it will create BO prism, if you move a minus right eye OC to the left in the lensmeter it will creat BI.  

The attachment that looks like lensmeter mires, if this was a right eye this would be BO prism, if it were a left eye it would be BI prism.  The next attachment is BI prism for a plus lens and BO prism for a minus lens.  The last attachment is the opposite, BO for plus and BI for minus.

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## hvu

> When you are talking about the lensmeter the mires will always be located toward the temporal side for BO prism this will be to the left for a right eye and to the right for a left eye, regardless of the Rx.
> 
> So, if you move a plus right eye OC to the left in the lensmeter it will create BO prism, if you move a minus right eye OC to the left in the lensmeter it will creat BI.  
> 
> The attachment that looks like lensmeter mires, if this was a right eye this would be BO prism, if it were a left eye it would be BI prism.  The next attachment is BI prism for a plus lens and BO prism for a minus lens.  The last attachment is the opposite, BO for plus and BI for minus.


I think that I understand these! But in the book, it says "target center location always corresponds to prism base direction. This is true whether the lens is  plus or minus in power" What does it mean? What is the "target center" referred to?

Thanks!

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## lensgrinder

What book are you referring to?
I would assume by what you say that it is the center of the mires.  Since it says corresponds.  Look at the attached image.

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## HarryChiling

Maybe the reference is to the fact that the target mires in the lensmeter are always going to go in the same direction for BO, BI, BU, or BD no matter the power, however the lens as you know depending upon the power is going to have to be decentered against motion or with motion.  I hope that explanation helps.

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## Laurie

Hello,

I think that "target center" is being interchanged with "optical center"...

"Target center", is the center of the miers...in the middle where the cylinder lines and sphere line cross....in the lensometer.

This will be viewed in whatever the prescribed prism base is...

BU = Up
BD = Dn
BI = to the right for OD, to the left for OS
BO = to the left for OD, to the right for OS

(imagine the patient looking back at you, from the lensometer, and picture in, and out, for OD and OS)  (looking AT, not looking THROUGH) the lenses.

For DECENTRATION  to produce prescribed prism, the optical center is decentered in the SAME direction for PLUS lenses, and OPPOSITE for MINUS.  This is because, for a plus lens, the base is at the OC.  For a minus lens, the apex is at the OC, and the base is at the opposite end.

: )

hope this helps,

Laurie

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