+13+1 Vertex Powers >Need Help

[sandeepgoodbole]

I have Ziess Foci Meter. We keep the Convex surface up and measure. What would you term this : Front Vertex or Back Vertex ?
These days because of IOLs, people with +10 or above powers are very rare. But ocassionally, they appear.
When we measure Such High Plus Lens in inverted position,
(Convex Point at the bottom), the power is shown reduced by at least half Sph.
While I was cheking a Cr +13.50+1.0 , which I was expecting to show +13+1.0 ,when hold Convex point down position, it showed only +11.50+1.0
I tried to find out from various posts on OB. Then I also tried playing with LABCAL and Optics Lite.
I came to a conclusion : There is a Horizone I must explore.
I need Opto Mathematical Explaination of the trems :

1. Nominal Magnfication
2. Equivalent power
3. Back Vertex Power
4 What are fitting Vertex and Wearing vertex ? How to determine those ?

5. How to estimate accomodation for a User ?

6
i)Why Equivalent Power is termed as FE instead of EP
ii) Why Back Vertex is Short Termed as (FV) why not BV ..?

[Jeff Trail]

sandeepgoodbole said:
I need Opto Mathematical Explaination of the trems :

1. Nominal Magnfication
2. Equivalent power
3. Back Vertex Power
4 What are fitting Vertex and Wearing vertex ? How to determine those ?

5. How to estimate accomodation for a User ?

6
i)Why Equivalent Power is termed as FE instead of EP
ii) Why Back Vertex is Short Termed as (FV) why not BV ..?

1) Do you want the nominal power formula? or the spectacle magnification formula? ..might want to rephrase this one :)

2) do you mean spherical equivalent?

3/4) vertex distance and effective power and "fitting" vertex related to wearing..and toss in a little of number 5 question,
Fitting for vertex compensation or recomputed power, depends on how you were trained for which "label" may apply. But the formula for vertex compensation is
Dc=Dl/(1-dDL)

Dc=compensated power
Dl= original lens power
d=change in vertex distance in meters

You would really need to use this formula for any high powered RX that will be worn at a vertex OTHER than the refracted vertex. Which BTW an OD should really supply on the script for extreme RX's to allow the dispenser to refigure compensated power.
Back and front vertex power, is the "nominal power" (sum of the two surfaces) "back vertex power is the power measured at the back of the ophthalmic lens, is what the wearer sees..this is what is normally measured in the focimeter (lemsometer) ..the formula for back vertex power is Db=D1/1-(t/n)D1+D2
D1=front surface power
D2=back surface power
t=thickness between the front and back vertices of the lens (in meters)
n=index of refraction for the lens material
The back vertex of the lens is the point where the axis of the lens intersects the back surface of the lens. The "focal length" based on this definition of BVP is the distance from the BV of the lens to the secondary focal point..
The front vertx power (neutralizing power) is the power found when the ray is incident first on the back side of the lens. The front VP is often used in the contact end of optics and is marked as FVP.. to measure the FVP place the lens front twoards the lens stop instead of back towards the lens stop..the formula for front vertex power is
DF+D2/1-(t/n)D2+D1
Now the thickness of the lens has a direct effect on the power so you have to make adjustments in vertex, the equivalent power (your number 2) of the lens is determined by this formula D=D1+D2-[t/n]D1D2
The "equivalent power is actually the way to locate the principal planes for a thick lens. The equivalent power formula provides the postion of the pricipal planes with respect to the FIRST and SECOND focal points ..the formula for a "single" thick lens is Deq=D1+D2-t/nD1D2 .. and f=ns/Deq using this definition of "f" you can locate the principal planes with respect tolens vertices..H1 & H2 are the principal planes..NOW (sheesh) from here we can determine image size and placement (thick lens)..the biggest problem in thick lens is it is NOT diviated BUT is displaced..Now you have to allow for these things as well as using the vertex from eye to back surface and how that is compounded by the distance (vertex) of thickness of the actual lens and material..knowing the refractive vertex is helpful as well when trying to find the accomadative power to duplicate into a worn power..simple hmmm? ...you digest all this and than ask away..your questions were out of "order" in how you would formulate the answer so I kinda jumped around and put them back into order.. I'm sure you will have questions, but right now my fingers are to tired to keep going :) I think I hit the high parts and maybe after a rest and you have time to go over it and have questions we can continue...or maybe Darryl or Pete,Chip, Harry, JRS anyone? or another would like to jump in and fill in any spots I missed..

Jeff"boy what a complicated post" Trail

[chip anderson]

Jeff did a very good job. However I have a few questions. Is the patient bilaterally aphakic or only one eye operated on.

If the latter you will not be able to balance image size and scotoma's, distortions etc unless you use a contact lens. If you use a contact you will still have some image size difference to which the patient will usually accept, if not sometimes you have to add a little minus to the contact to ballance the image even though this may not give maximum distance acuity.

Hense you must frost the lens (or occlude) on the eye that has the poorest acutity, whether this is the operated or the non-operated eye until the phakic eye is operated on.

Another point, you do check the distance power useing the convex side of the lens toward the observer. However to detemine the add (which is always +2.50 on a healthy aphakic eye) you must check the front distance vertex power then check the front vertex power of the add and subtract.

Hope this doesn't add too much confusion to the issue.

Chip

[Jeff Trail]

Chip,

Good points.. and I don't think anything can "complicate" low vision more so than it already is (theoretically) The major problem I usually see is that "low vision" has become the forgotten part of our field and most people who came up through the "chain" end are usually lost when dealing with low vision ..I have over the last decade or so even seen the OD's and MD's (newer guys/ rookies etc.) losing a lot in this area on how to properly do low vision correction..
I do some low vision refracting on the side for some MD's and even the use of prism seems to be fading away into the mist of "old time" optics ..I was at an OD office this week who even has signs claiming "low vision specialist" ..he had a great selection of magnifyers and some "press on prism" as the end all answer... oh and charged this crazy amount of money for refracting and dispensing a "HUGE" add I did for him..a 4.50!!
Sad but true I think we guys/gals who still do low vision work are a rare breed nowadays .. to bad a LOT of money out there for people who know their stuff in correcting low vision and complicated visual problems...

Anyone else want to toss in a few cents worth on something me and Chip might have missed? .. I have a couple more formula's I can toss in there once my fingers recover and the smoke clears from over taxing my brain :)

Jeff "low vision is great experience" Trail

[Darryl Meister]

1. Nominal Magnfication

You might have to clarify the context of this term. In low vision aids, for instance, nominal magnification might refer to the power of the vision aid divided by 4, or Magnification = Power / 4. (This relates to something called the least distance of most distinct vision or 1/4th a meter, but I'll spare you the details.)

2. Equivalent power

Jeff gave a good explanation of equivalent power above. However, you probably won't run across this one in practice, so I wouldn't concern yourself too much with it. To simply Jeff's explanation, the equivalent power of a thick lens or a series of thin lenses is the single focal power that will replace the series of lenses or lens surfaces and produce the same magnification.

3. Back Vertex Power

This is the power of the lens as measured from the back surface. For instance, if you position your lens with the back (concave) surface against the lens stop of your focimeter, the reading you get is a back vertex power reading. Similarly, when you measure the lens with the front (convex) surface against the lens stop, the reading you get is a front vertex power reading.

Spectacle lenses are almost always described by the back vertex power, and when you look at a lab slip or lens package, the power on it is the back vertex power. Generally, the back vertex power reading will produce more plus power (or less minus power) than the front vertex power reading. Front vertex power measurements -- with the front surface against the lens stop -- are only used when measuring add power, since the segment is on the front surface.

4 What are fitting Vertex and Wearing vertex ? How to determine those ?

These terms seem to mean the same thing to me, since they both seem to describe the vertex distance of the lens on an actual wearer. When a refractionist determines a patient's prescription, the distance between the phoroptor and the cornea is referred to as the refracted vertex distance. This distance is usually around 13.5 mm. The distance between the actual spectacle lens (or frame aperture) and the cornea is referred to as the fitted vertex distance. Generally, the fitted vertex distance will vary slightly from the refracted vertex distance, which means that some compensation may be necessary in higher prescriptions.

5. How to estimate accomodation for a User ?

The easiest way is to look at a chart of accommodation versus age. (You'll find one in just about any book on refracting.) You can "guesstimate" it by looking at the prescribed add power. If the wearer has less than a +2.50 add power, subtract the add power from +2.50 and double the difference to determine the minimum accommodation the wearer likely has. For instance, if the wearer has a +2.00 add, take +2.50 - +2.00 = +0.50 x 2 = +1.00 of accommodation. If the wearer has an add power of +2.50 or higher, he/she probably has very little accommodation left.

6. i)Why Equivalent Power is termed as FE instead of EP
ii) Why Back Vertex is Short Termed as (FV) why not BV ..?

The "FE" should really be F subscript E, and the "FV" should really be F subscript V. The letter "F" is generally used to represent focal power in optical equations. The subscript "E" refers to equivalent focal power, while the subscript "V" refers to vertex (back vertex being the most common) focal power. But you could really use whatever variables/letters you want in these equations. The use of "F" is just the most common convention. It doesn't really matter, the answer will still come out the same. ;)

Best regards,
Darryl

[Jeff Trail]

>These terms seem to mean the same thing to me, since they both seem to describe the vertex distance of the lens on an actual wearer. When a refractionist determines a patient's prescription, the distance between the phoroptor and the cornea is referred to as the refracted vertex distance. This distance is usually around 13.5 mm. The distance betweenthe actual spectacle lens (or frame aperture) and the cornea is referred to as the fitted vertex distance. Generally, the fitted vertex distance will vary slightly from the refracted vertex distance, which means that some compensation may be necessary in higher prescriptions.<

Darryl

I think she might be talking about fitting vertex as just that..the vertex compensation for power and lens placement and what she maybe calling "wearing vertex" is the other problems including magnification and thickness of lens and from front surface vertex back..I'm not sure but that is the only thing I can think of...

Jeff "cut me, do I not bleed resin" Trail

[Tim Hunter]

chip anderson said:
Hense you must frost the lens (or occlude) on the eye that has the poorest acutity, whether this is the operated or the non-operated eye until the phakic eye is operated on.

Another point, you do check the distance power useing the convex side of the lens toward the observer. However to detemine the add (which is always +2.50 on a healthy aphakic eye) you must check the front distance vertex power then check the front vertex power of the add and subtract.
Chip

If you do have someone with anisometropic aniseikonia (frequently caused by cataract surgery)with a large power difference between the eyes you could just give a "balance" lens (a lens balanced for weight and thickness with the other eye and not designed to correct the refractive error) rather than occlude/frost. This only works if the vision is reduced sufficiently by the "balance" to induce suppresion. It does however have the advantage of looking cosmetically better.

I wouldn't agree that the ADD is always +2.50 on a healthy aphakic eye. Any aphake or pseudophake will be entirely reliant on their reading ADD for their near focus and if a patient wants to read at 50cm I would give a +2.00 ADD or at 33cm a +3.00 ADD. The power of the ADD in these circumstances is linked to the working distance the patient requires and therefore as the ADD power is the reciprical of the working distance a +2.50 ADD would give a working distance of 40cm which may not be suitable for all healthy aphakes.

[sandeepgoodbole]

[QUOTE]sandeepgoodbole said:
I have Ziess Foci Meter. We keep the Convex surface up and measure. What would you term this : Front Vertex or Back Vertex ?
These days because of IOLs, people with +10 or above powers are very rare. But ocassionally, they appear.
When we measure Such High Plus Lens in inverted position,
(Convex Point at the bottom), the power is shown reduced by at least half Sph.
While I was cheking a Cr +13.50+1.0 , which I was expecting to show +13+1.0 ,when hold Convex point down position, it showed only +11.50+1.0


I asked the Customer to show me his old Spects. I chekcd those bottums Up. I noticed that one of those was showing +11.0+1.0
and other one was +12.+1.0.
Then I explained him that with any of such variations, only difference he can notice in practicle life is different objects would appear more sharp with each spects. The shrapness would depend on the distance of the object and there would be hardly anything that would hamper his normal working.
He understood it. And He is happy. Now, a week is passed almost.
The jist is : as 11.5 is the Centre point of his 2 old Rxs, he can adjust it with almost no trouble.
Understanding the Change is very important.
What do you think folks ?