Need help with this prism

[MIOPE]

We have this patient, male 65 years old that has been using for more than 4 years this prescription.
R -2.75 -1.50 x 170 3.0 prism base out 20/30
L -3.25 -1.00 x 180 4.0 prism base out. 20/40
Cr-39 Single vision lenses base 4.25.
He went to his ophthalmologist last week because he was having diplopia and he gave him this prescription.
R -3.00 -1.50 x 170 10.0 prism base out
L -3.00 -1.00 x 180 10.0 prism base out.
Note: Here there are no Opticians so we Optometrist sell and manufacture lenses too. In this case i am acting as an Optician in U.S. since the prescription came from a outside source and i only make the eye glasses as the prescription say.

I made the lenses on Poli base 3.25 (needed enought blank thickness) and mount then on the same frame he was using the old prescription. With new prescription he complains of blurred vision at far distance but the diplopia was not present.
I checked again the vision and the prescription of the old glasses and is the same i mentioned as olg glasses prescription.
One question i made myself was, How he can have so bad vision with a 0.25 change on prescription?
The patient did not have time this day i took him to the examine room and did a fast overrefraction while wearing the new glasses and found that a +.075 esf. gave the best vision and there was no diplopia. So according to this fast overrefraction the prescription should be
R -2.25 -1.50 x 170 10.0 prism base out
L -2.25 -1.00 x 180 10.0 prism base out

Then we reassembled the old glasses on the frame and i did was able to do a fast overrefraction while wearing the old lenses and found that on the right eye he does not accept changes so -2.75 -1.50 x 170 is the best prescription. On the left eye he has -3.25 -1.00 x 180 but accepted a +0.25 sometimes and a +0.50 others.
So according to the overrefraction made on the prescrption he has been using for more than 4 years, the prescription should be.
R -2.75 -1.50 x 170 10.0 prism base out
L -2.75 -1.00 x 180 10.0 prism base out

So same patient and i now have 2 different prescriptions.!!!!!!!!!

This morning, i placed one of the lenses that has 10.0 prism on a automatic lensometer and found that it has the prescription ok. see figure 1
The i raised up this lens to see if change in vertex distance had any efect and found that it does not as expected.
Then i lower the lens on the thickest end and found that the spherical power started to increase negatively -3.00,-3.25,-3.5,etc. Actually, when this lenses are on the frame and the patient is wearing them, the frame has a curvature that makes the thicker part of the lens closed to the face than the nasal side of the lens.
See picture 2.

So should i make him new lenses with
R -2.25 -1.50 x 170 10.0 prism base out
L -2.25 -1.00 x 180 10.0 prism base out
prescription because in this case they act as if they where
R -3.00 -1.50 x 170 10.0 prism base out
L -3.00 -1.00 x 180 10.0 prism base out.

I really need fresh ideas on this.
Help is welcome.

[Wes]

In the pics you posted, the difference in power can be accounted for by the fact that the lens is effectively tilted the way its held.
I suspect the blur is coming from the poly. It has a low abbe, and will break light into its component colors with prism. I don't use poly for much and certainly nothing higher than 3D prism.
Try a 7 and 6 D fresnel prism, respectively over the original lenses. If that works, remake in 1.60 or 1.70 if possible.

[eyeguy21]

Yeah, that's my first thought. Axial chromatic abberration comes across as a blurred or smeared effect.

[drk]

Wess says what I think: that's a whole lot of prism to be made in a high index material.

Seriously, 10^ is asking too much. You can't win. This isn't a normal pair of glasses.

I'd remake with a better abbe lens, probably Trivex, and roll and polish that edge.

Don't forget to consider a radically smaller frame, preferably a round one.

[MIOPE]

In the pics you posted, the difference in power can be accounted for by the fact that the lens is effectively tilted the way its held.

Yes, in picture 2 the lens is tilted and the prescription if read higher but this tilt is similar to the one once the lens is mounted and the patient is wearing them.

I suspect the blur is coming from the poly. It has a low abbe, and will break light into its component colors with prism. I don't use poly for much and certainly nothing higher than 3D prism.

Ok, i will use index 1.6 and a smaller frame as suggested.
The old frame was 52/18 the new is oval shape 48/18.

Now the only thing left is the prescription. This lenses has the prescription send by the ophthalmologist which has a sphere of -3.00 and when the patient wear them, the blurred vision almost disappears if a +0.75 is placed in front of each eye. So it look to me that -3.00 has a effective power of -3.75 when placed in front of the eye since the lens is tilted as it is in the lensometer.
Any comment on this?

[Robert Martellaro]

Yeah, that's my first thought. Axial chromatic abberration comes across as a blurred or smeared effect.

More likely lateral chromatic aberration (prism/abbe or P/v). Axial (power/abbe or F/v) is usually not a concern due to the eye's greater amount ACA (one to two diopters).

In the above Rx, 10^ induces .33^ of LCA on-axis when poly is used, possibly enough to lose two or more lines of acuity. Although Spectralite, Trivex, and 1.60 (in that order) would perform better, cr39 or crown glass will provide the best possible vision. As drk says, a somewhat round 40mm to 42mm eye wide DBL frame is your best bet to keep the ET below about 10mm. A 20^ Fresnel press-on prism should also be on the table.

[Wes]

I wouldn't worry too much about that. The farther away from the prism reference point, prp (what you would normally think of as the OC, where the patient looks through) the more power you'll induce. Measure at the prp and go with it. You might consider a center bevel or a one third, two third bevel so as not to skew the visual axis too much. Cosmesis on these are going to be poor regardless. If you can get trivex, it might be a better, but thicker, solution optically as DRK suggested. Roll, but don't polish to luster. With that thick of an edge, the light it would let into the lens from the edge might drive your pt crazy. Good luck.

[drk]

Good post, RM, although I didn't understand all of it...

Can you get Trivex or 1.6 in extra thick? Would that be necessary?

How about a round, plastic, preppy frame? (Probably is a truck driver type.)

[eyeguy21]

More likely lateral chromatic aberration (prism/abbe or P/v). Axial (power/abbe or F/v) is usually not a concern due to the eye's greater amount ACA (one to two diopters).

In the above Rx, 10^ induces .33^ of LCA on-axis when poly is used, possibly enough to lose two or more lines of acuity. Although Spectralite, Trivex, and 1.60 (in that order) would perform better, cr39 or crown glass will provide the best possible vision. As drk says, a somewhat round 40mm to 42mm eye wide DBL frame is your best bet to keep the ET below about 10mm. A 20^ Fresnel press-on prism should also be on the table.

Interesting. Good info. I've always associated ACA with Blurriness or smudgy effects and LCA with Halos or color fringing but I guess that goes out the window when your talking about this much prism. Spectralite? Love it. Always looks great and with the pair I own the optics are amazing. Do you use it a lot? Around here it's almost an afterthought but I actually think it's a great material. I'd almost go so far as to say I prefer it over 1.67. Mixed feelings about the 20D fresnell but I see your point about leaving it on the table as an option.

[Wes]

Miope, If Robert says try CR-39, do it. He's more knowledgable than I am. I thought it might be too thick, which is why I made the recommendation for higher indices with decent abbe values. I didn't see his post, when I was working on mine.
Again, good luck.

[uncut]

This is an interesting case miope. I would question the need to increase the prism the amount it has been. I have found through experience that increases of 1 or 2 diopters is enough of a change to reduce symptoms and induce cosmesis and behavioural difficulties. I would ask if the prescriber would allow you to introduce this person to a percentage of the prism gradually, while monitoring the result. This would allow you to introduce the minimal amount required, which may be less that the 10 suggested/prescribed.

Introducing the prism in stages would also allow you to re-refract each time.

[braheem24]

If you decide on 1.60 keep in mind you need to ensure you order mr-8 (n=1.595 mr-8 abbe of 41) vs mr-6 (n=1.593 abbe of 36).

Secondly, If optics is the main concern, I would split the prism based on BVA assuming he's OD dominant I would do...

OD 20/30 8BO
OS 20/40 12BO

[Wes]

Both good points. And this is how the best glasses get made...

[Robert Martellaro]

although I didn't understand all of it...

That would be my fault. Try http://www.opticampus.com/cecourse.p...ic_aberration/

Can you get Trivex or 1.6 in extra thick?

The flatter curves should be available in the 10mm to 15mm range.

Would that be necessary?

I would think at least 10mm

LCA with Halos or color fringing

Some will see the color- I only see the blur. It might depend on the individuals's eyes, maybe on the lighting conditions and/or the object contrast.

Spectralite? Love it. Always looks great and with the pair I own the optics are amazing. Do you use it a lot?

More than half of what I dispensed before Trivex hit the market. It'll probably all be gone in a year or less. I used Finalite a lot also- 1.60 index, low 40's Abbe and a specific gravity in the low 1.20's.

I would question the need to increase the prism the amount it has been.

I'd rather see a script like this from an optometrist or orthoptist.

Miope, If Robert says try CR-39, do it. He's more knowledgable than I am.

Don't believe it, Miope, at least the more knowledgeable part. Please note wss2020's ABOM designation.

There are many "right" answers here, depending on the client's priorities and sensitivities. Good luck sorting that out with your client!

[MIOPE]

Following the advices on this forum, i found a frame 42/22 which i reshape to 40.8/22. Made the lenses of CR-39 on base 4.25. The prescription i used was a little lower than the one prescribed because the tilt of the lens increased the minus power of the lens. so instead of -3.00 sphere i used -2.62. On monday i will see how it works.
Robert Martellaro thanks for the link to opticampus. It made more clear certain concepts.

[MIOPE]

Today it delivers the lenses to the patient.
The Ophthalmologist RX was.
R -3.00 -1.50 x 170 10.0 prism base out
L -3.00 -1.00 x 180 10.0 prism base out.
And since i already made the glasses with this prescription and a +0.75 was needed infront of this lenses to improve vision. This new lenses are Cr-39 and the RX grinded is
R -2.37 -1.50 x 170 10.0 prism base out 20/25
L -2.37 -1.00 x 180 10.0 prism base out. 20/30
No diplopia and good vision while sit but when tried to walk he felt dizzy. He will try them at home for a longer time and will see what happends.

What i did learned on this case is that the same lens shows two different prescriptions depending on how it sits on the lensometer. See on first picture the lens is tilted and in second picture the lens sits freely on the lensometer. The prescription that should match the one prescribed by the ophthalmologist is the one while the lens is tilted acording to the curvature of the frame (picture3) and that is why i had to redo this job and grind a weaker lens (about .62 weaker).
Note: Picture 1 and 2 are used here just to show the two positions of the lens on the lensometer and how it affects the reading. The prescription of the new lens is 0.75 weaker than the one shown on this pictures.
Does anyone knows a formula to calculate this change on prescrition on a prismatic lens?

[HarryChiling]

The following paper will cover everything your looking for, except how to compute the tilt value to use.

http://www.fbmn.h-da.de/~blendowske/...S_jan_2002.pdf

The amount of tilt will depend on the amount of prism and the type of bevel you use. Here is a step by step play through a good average for tilt.

Step 1 - Compute the thickness of the nasal and the temporal sides. Factor in prism when necessary.
Step 2 - Use your bevel ratio times the thickness difference to compute the amount of tilt due to bevel placement.
Step 3 - Use that bevel thickness difference divided by the A size to compute the tangent of the tilt
Step 4 - Use this tilt value as a faceform tilt in blendowske's formula's to compute the compensated power.

You can also use Darryl's calculator in the files directory to do the computing for you although I believe that you'll have to figure out the tilt bevel value yourself.

[MIOPE]

Just used the Lens Tilt & Wrap Compensation found on Opticampus and it works. Just have to figure out the angle between both surfaces on a 10.0 D. prism and also take in account the curvature of the frame . I thig that it will work.

Thanks.

[Robert Martellaro]

Does anyone knows a formula to calculate this change on prescrition on a prismatic lens?

The new eyeglasses look very good, and appropriate. I prefer the look of the standard edge (no roll or polish).

The "view from the top" (photo #3) shows very little dihedral/wrap angle/tilt. I would fill the script as written.

[One science]

We have this patient, male 65 years old that has been using for more than 4 years this prescription.
R -2.75 -1.50 x 170 3.0 prism base out 20/30
L -3.25 -1.00 x 180 4.0 prism base out. 20/40
Cr-39 Single vision lenses base 4.25.
He went to his ophthalmologist last week because he was having diplopia and he gave him this prescription.
R -3.00 -1.50 x 170 10.0 prism base out
L -3.00 -1.00 x 180 10.0 prism base out.
Note: Here there are no Opticians so we Optometrist sell and manufacture lenses too. In this case i am acting as an Optician in U.S. since the prescription came from a outside source and i only make the eye glasses as the prescription say.

I made the lenses on Poli base 3.25 (needed enought blank thickness) and mount then on the same frame he was using the old prescription. With new prescription he complains of blurred vision at far distance but the diplopia was not present.
I checked again the vision and the prescription of the old glasses and is the same i mentioned as olg glasses prescription.
One question i made myself was, How he can have so bad vision with a 0.25 change on prescription?
The patient did not have time this day i took him to the examine room and did a fast overrefraction while wearing the new glasses and found that a +.075 esf. gave the best vision and there was no diplopia. So according to this fast overrefraction the prescription should be
R -2.25 -1.50 x 170 10.0 prism base out
L -2.25 -1.00 x 180 10.0 prism base out

Then we reassembled the old glasses on the frame and i did was able to do a fast overrefraction while wearing the old lenses and found that on the right eye he does not accept changes so -2.75 -1.50 x 170 is the best prescription. On the left eye he has -3.25 -1.00 x 180 but accepted a +0.25 sometimes and a +0.50 others.
So according to the overrefraction made on the prescrption he has been using for more than 4 years, the prescription should be.
R -2.75 -1.50 x 170 10.0 prism base out
L -2.75 -1.00 x 180 10.0 prism base out

So same patient and i now have 2 different prescriptions.!!!!!!!!!

This morning, i placed one of the lenses that has 10.0 prism on a automatic lensometer and found that it has the prescription ok. see figure 1
The i raised up this lens to see if change in vertex distance had any efect and found that it does not as expected.
Then i lower the lens on the thickest end and found that the spherical power started to increase negatively -3.00,-3.25,-3.5,etc. Actually, when this lenses are on the frame and the patient is wearing them, the frame has a curvature that makes the thicker part of the lens closed to the face than the nasal side of the lens.
See picture 2.

So should i make him new lenses with
R -2.25 -1.50 x 170 10.0 prism base out
L -2.25 -1.00 x 180 10.0 prism base out
prescription because in this case they act as if they where
R -3.00 -1.50 x 170 10.0 prism base out
L -3.00 -1.00 x 180 10.0 prism base out.

I really need fresh ideas on this.
Help is welcome.

I think he only isn't used to these lenses. Maybe you should wait couple of weeks until you make any movements.

[One science]

The following paper will cover everything your looking for, except how to compute the tilt value to use.

http://www.fbmn.h-da.de/~blendowske/...S_jan_2002.pdf

The amount of tilt will depend on the amount of prism and the type of bevel you use. Here is a step by step play through a good average for tilt.

Step 1 - Compute the thickness of the nasal and the temporal sides. Factor in prism when necessary.
Step 2 - Use your bevel ratio times the thickness difference to compute the amount of tilt due to bevel placement.
Step 3 - Use that bevel thickness difference divided by the A size to compute the tangent of the tilt
Step 4 - Use this tilt value as a faceform tilt in blendowske's formula's to compute the compensated power.

You can also use Darryl's calculator in the files directory to do the computing for you although I believe that you'll have to figure out the tilt bevel value yourself.

Great, thank you for the valuable information; this is the answer to what i wanted to know too.

[MIOPE]

The "view from the top" (photo #3) shows very little dihedral/wrap angle/tilt. I would fill the script as written.

I would think the same since the front surface of the lenses shows no tilt, but the lenses on photo #3 are .62 weaker than the original RX. On the original rx the sphere was -3.00 and this has sphere -2.37.
If you place this frame on the lensometer and hold it so that it will not tilt, (front surface of the lens stays horizontal), the sphere will read -300 but if you let it sit on the back surface of the lens on the lensometer (front surface will look tilted) then they read -2.37.
I used the Lens Tilt & Wrap Compensation Form (at http://www.opticampus.com/tools/tilt.php) on the original presciption (-3.00 sphere ) and tried different tilt values (Facial Wrap) until found that 25 will give -2.35.

Some thing are not clear to me yet.
The important thing is that the patient has good vision with them.

[Robert Martellaro]

If you place this frame on the lensometer and hold it so that it will not tilt, (front surface of the lens stays horizontal), the sphere will read -300 but if you let it sit on the back surface of the lens on the lensometer (front surface will look tilted) then they read -2.37.

I suspect that's more of a problem with lensometers (tilting and prism) than a problem for the wearer.

The important thing is that the patient has good vision with them.

Right. My guess is that you'll have to add back some minus if you refract over the glasses (assuming that the original outside Rx is correct). Let us know what happens.

[MIOPE]

I suspect that's more of a problem with lensometers (tilting and prism) than a problem for the wearer.
Right. My guess is that you'll have to add back some minus if you refract over the glasses (assuming that the original outside Rx is correct). Let us know what happens.

The first time (first round) i made the full prescription with the 10.0 prism on poli. The vision was bad. I overrefracted and a +.75 was accepted. That is why i used the second time a weaker prescription and also overrefracted on them and the result was that the weaker prescription is quite exact. So in this case the eye and the lensometer has coincidence. I measured one of the lens that had full prescription (first round) on a old manual lensometer and if i tilt the lens the prescription changes as it does on the automatic lensometer.
To be honest, in this case i has seen things that i did not expect to see and some of them do not make sense to me. I am learning.
I will report if there are more news.
Thanks for the help i has received on this forum.

[Robert Martellaro]

The first time (first round) i made the full prescription with the 10.0 prism on poli. The vision was bad. I overrefracted and a +.75 was accepted.

I missed that part. So, either the Rx was incorrect, or the "as worn" position needs compensation due to the large prism power.

However, I've seen quite a few strong prismatic Rxs, and not once have I had to modify the lens powers due to prism, although the segment (vertical prism) and fitting points (horizontal) needed compensation.

That is why i used the second time a weaker prescription and also overrefracted on them and the result was that the weaker prescription is quite exact. So in this case the eye and the lensometer has coincidence.

But that could be coincidence. The original Rx may have been incorrect. You'll probably have to perform a regular refraction and see how it compares to the original, and to the overrefraction (over the new cr39 lenses).

I measured one of the lens that had full prescription (first round) on a old manual lensometer and if i tilt the lens the prescription changes as it does on the automatic lensometer.

I believe that's going to be the result with any lensometer- tilting the lens against the lens stop will introduce error if the surface is curved. To verify that the lens is as prescribed, it needs to be flush against the lens stop. Lensometers are simply not capable of measuring powers in the as-worn position.

To be honest, in this case i has seen things that i did not expect to see and some of them do not make sense to me. I am learning.
I will report if there are more news.
Thanks for the help i has received on this forum.

I know the feeling- I'm learning also. We'll probably have both feet in the grave and there'll still be lights going off.