Difference in RX OU

[Tallboy]

Magnification is caused/induced by power, material, index, thickness, vertex along with lens size and decentration ( the last two both relate to thickness). Asphericity potentially could or could not influence magnification by thickness*. ( Atoric, not this Rx.)

To control or reduce magnification as much as possible, you need to address all of the above. The two variables that will mean the most are thickness and vertex on any Rx. Controlling thickness starts with lens size and PD ( As small and round/oval as possible and FPD as close as possible to patient's PD). Material (index) will make a difference because higher index's uses (require) flatter curves reducing the thickness (and vertex). (Be aware that not all hi-indexes have great abbe values. With high powers like the above, you need to start thinking of LCA's and their affect on their vision). Using a frame that sits as close as possible to their eye reduces the vertex distance.

* As far a aspheric/atoric helping. Depends. If you're talking using corrective curve deviation that we traditionally think of with aspheric/atorics, no. A +/-.50 cyl would have an extremely small amount of "curve deviation" for the cyl. The smaller the lens, the less curve deviation or asphericity is required. This is a 49 eye size so little deviation is induced. I don't think just asphericity ( or atoricity) in this small of eye size would reduce thickness much.

Now then, if you were to do a radical aspheric design like IceTech ( I don't work for them) makes then you could reduce the thickness even more. ( by radical asphericity I mean a radical curve deviation to remove lens material, not a corrective curve deviation. Think SuperModular lenses (ok, I'm old and remember those..)

I'm sure someone will add or correct me on the above.

This is why I read this board.

[Darryl Meister]

Image size is calculated using the corneal vertex distance... A side benefit is reduced dynamic magnification, now there are many discrete solutions for an isogonal lens system... subsequently correct the dynamic magnification (ie prismatic effect) while maintaining an isogonal correcting system... Digital surfacing corrects any spherical aberration...

Peter, it is still not clear to me from this response whether you are attempting to fully eliminate dynamic magnification, which would require extreme lens shapes and a reversal in aniseikonia, or to simply reduce it by eliminating traditional aniseikonia (or stationary magnification).

As you known, in order to eliminate differences in retinal image size (i.e., aniseikonia), the stop distance for each chief ray must be measured to the entrance pupil, whereas to eliminate differences in spectacle-induced anisophoria (i.e., dynamic magnification), the stop distance must be measured to the center of rotation.

Consequently, since the center of rotation distance is nearly twice as long as the entrance pupil distance, you really cannot eliminate both, simultaneously, unless you introduce (not correct) a significant amount of spherical aberration to either increase or decrease the magnification gradients as needed.

Of course, using spherical aberration to accomplish this would also produce a great deal of geometric distortion and blur away from the center of the lens, assuming that this approach is even feasible or acceptable to the wearer. (I'd have to work out the math to determine how much distortion would actually be involved.)

I'm sure we've discussed this before, so maybe you have clarified this apparent discrepancy in another thread. If you seeking to eliminate aniseikonia, you will certainly reduce (but not eliminate) dynamic magnification, which I have no doubt would still result in some visual improvement for the wearer.

Best regards,
Darryl

[MakeOptics]

My reply never showed up interesting.

Here is the gist:

1. Right eye thicker and left lens thinner, if you can't go thinner on the left then start making the right thicker.
2. Vertex disparity, move the left lens closer to the eye to reduce mag and the right eye further to increase mag. You cn do this with some bevel tricks move the left lens bevel towards the front and the right lens bevel towards the back this should buy you anywhere from 0.5mm to 1mm of disparity between the lenses.
3. Aspherics and Spherical, use an aspheric curve for the left and a spherical curve for the right. The spherical lens will have greater mag and be on a higher base if you use a lower index material then the left.
4. Which brings us to #4 different index materials, use a lower index for the right then the left this will allow for equalizing the thickness of the lenses and also allow you to use a higher base for the lower Rx power creating the additional mag needed.

Don't be rigid in the magnification, you can either increase on mag in left, decrease the right or a little of both. Have fun, good luck and post pictures.

[MakeOptics]

Mr Meister and Dr Shaw, you guys are missing the forest for the trees, vision is not the issue here. The issue is cosmetics, the child has the appearance of one large eye and one small eye. If you made the entire world blurry but fixed this cosmetic issue, she may cry and thank you like you took all her little cares away.

Make her happy.

[Darryl Meister]

Mr Meister and Dr Shaw, you guys are missing the forest for the trees, vision is not the issue here. The issue is cosmetics

I'm actually discussing with Dr. Shaw a slightly different topic associated with magnification at this point as a result of some earlier discussion in this thread. I actually covered the cosmetic point that you are referring to back in post number 19:

"If I recall, Dr. Shaw is attempting to compensate for dynamic anseikonia. This would not eliminate magnification differences between her right and left lenses. In fact, it could potentially make them just as bad, but reversed, so that her right eye looks bigger than her left, which would not really address the patient's chief complaint regarding cosmetics."

Best regards,
Darryl

ps,
And people generally only call me "Mr. Meister" if they're trying to sell me something. ;-)

[Darryl Meister]

Magnification is caused/induced by power, material, index, thickness, vertex along with lens size and decentration ( the last two both relate to thickness).

On a related note, I have Magnification Tables in MS Excel available for download that show the percent change in magnification as a function of lens shape and prescription parameters. These tables are easy enough to generate, but allow for a quick review of the changes in base curve and thickness necessary to produce a given change in magnification.

Although, for this particular patient's complaint, you would actually be more interested in the magnification produced by the lenses for the observer, not the wearer...

( by radical asphericity I mean a radical curve deviation to remove lens material, not a corrective curve deviation. Think SuperModular lenses

Yes, those continuous surface cataract lens designs, including the Super Modular and original AO Fulvue, would essentially taper off into a carrier-curve like periphery, which of course was not useful for clear vision. But some modern aspheric and atoric lens designs for low- and moderate-prescription powers may utilize a polynomial design based upon a merit function with a volume or thickness term that allows the design to maintain optics in the central region of the lens, but then to shift to a reduction in thickness in the periphery of the lens. This effect is more noticeable in minus powers though.

Best regards,
Darryl

[manoj_verma]

Children grow up.

Right ....but in that case ,PD should be more then earlier ,not Less then earlier

[MakeOptics]

I'm actually discussing with Dr. Shaw a slightly different topic associated with magnification at this point as a result of some earlier discussion in this thread. I actually covered the cosmetic point that you are referring to back in post number 19:

"If I recall, Dr. Shaw is attempting to compensate for dynamic anseikonia. This would not eliminate magnification differences between her right and left lenses. In fact, it could potentially make them just as bad, but reversed, so that her right eye looks bigger than her left, which would not really address the patient's chief complaint regarding cosmetics."

Best regards,
Darryl

ps,
And people generally only call me "Mr. Meister" if they're trying to sell me something. ;-)

I say Mr Meister because you are a smart individual that has earned a level of respect so I extended it. Looking back yes you did cover the cosmetics. I just wonder if the side discussion subtracts from teh thread rather than adds content to it. It seems more like a Dr Shaw issue. Oh and I'm always trying to sell something so Mr Meister it is. ; )

[Darryl Meister]

I just wonder if the side discussion subtracts from teh thread rather than adds content to it.

I don't know that an educational discussion regarding the calculation of magnification could really ever subtract from this or any thread even loosely associated with the subject.

Besides, if the last 20 or 30 replies haven't already satisfactorily addressed the original question, I don't know that we could really do much harm at this point with that slight digression...

Best regards,
Darryl

[Robert Martellaro]

Although, for this particular patient's complaint, you would actually be more interested in the magnification produced by the lenses for the observer, not the wearer...

Good point! It's the vertex distance, not the stop distance. Regardless, it will primarily be modifications to the vertex distance and thickness that will make the eye size look more similar, cosmetically.

[MakeOptics]

Besides, if the last 20 or 30 replies haven't already satisfactorily addressed the original question, I don't know that we could really do much harm at this point with that slight digression...

Point made, I enjoy the side discussion. The only reason I posted was to touch on the other side of the coin. The solution seemed to reiterate reducing magnification, reducing thickness, and reducing vertex distance. I thought it was interesting since all these measures can be accomplished with higher costs lens options, when an additional solution is increasing the partner thickness, magnification, and vertex which could lead to slight reductions in cost for the patient.

Since I seem to be belittled for over selling patients and not giving them the proper lens options that they *NEED*, I thought this is one of those interesting scenarios that providing adequate products for the job leads to better cosmetics and lower costs. It's not always going to cost more to make a good looking pair of glasses but it will always costs more to have to make a second pair at another opticians office like in this case.

[Darryl Meister]

It's the vertex distance, not the stop distance.

Just to clarify, magnification formulas are actually based on the distance to the entrance pupil of the eye, which is a few millimeters behind the corneal apex. The entrance pupil is the image of the iris diaphragm, which is the aperture stop of the eye. The center of rotation of the eye is the field stop of the eye, since it limits the field of view. In practice, the difference in magnification as measured from the entrance pupil versus the corneal apex (i.e., vertex distance) is small enough to ignore though.

Point made, I enjoy the side discussion. The only reason I posted was to touch on the other side of the coin. The solution seemed to reiterate reducing magnification, reducing thickness, and reducing vertex distance.

All valid points. And it was certainly not my intention to detract from the conversation. I just like talking about magnification stuff...

Best regards,
Darryl

[Robert Martellaro]

Just to clarify, magnification formulas are actually based on the distance to the entrance pupil of the eye, which is a few millimeters behind the corneal apex. The entrance pupil is the image of the iris diaphragm, which is the aperture stop of the eye. The center of rotation of the eye is the field stop of the eye, since it limits the field of view. In practice, the difference in magnification as measured from the entrance pupil versus the corneal apex (i.e., vertex distance) is small enough to ignore though.

Understood. For spectacle magnification, "h" is the distance from the back vertex of the lens to the entrance pupil. We should use the lens equation and magnification equation to determine the size of the eye behind the lens.

[sharpstick777]

Great post.

Magnification is caused/induced by power, material, index, thickness, vertex along with lens size and decentration ( the last two both relate to thickness). Asphericity potentially could or could not influence magnification by thickness*. ( Atoric, not this Rx.)

To control or reduce magnification as much as possible, you need to address all of the above. The two variables that will mean the most are thickness and vertex on any Rx. Controlling thickness starts with lens size and PD ( As small and round/oval as possible and FPD as close as possible to patient's PD). Material (index) will make a difference because higher index's uses (require) flatter curves reducing the thickness (and vertex). (Be aware that not all hi-indexes have great abbe values. With high powers like the above, you need to start thinking of LCA's and their affect on their vision). Using a frame that sits as close as possible to their eye reduces the vertex distance.

* As far a aspheric/atoric helping. Depends. If you're talking using corrective curve deviation that we traditionally think of with aspheric/atorics, no. A +/-.50 cyl would have an extremely small amount of "curve deviation" for the cyl. The smaller the lens, the less curve deviation or asphericity is required. This is a 49 eye size so little deviation is induced. I don't think just asphericity ( or atoricity) in this small of eye size would reduce thickness much.

Now then, if you were to do a radical aspheric design like IceTech ( I don't work for them) makes then you could reduce the thickness even more. ( by radical asphericity I mean a radical curve deviation to remove lens material, not a corrective curve deviation. Think SuperModular lenses (ok, I'm old and remember those..)

I'm sure someone will add or correct me on the above.

[Now I See]

Thanks again to all...I'm still waiting for them to come in so we can get to work on these glasses! I'm going to start with some of the lower cost ideas, then work my way up. (As a side note, just so you all don't think I'm taking the poor mom for all her money, I'm going to do these at no charge. After all, it's a learning experience for me, too!) If her mom allows, I'll post some pics. Thanks again...I really appreciate your help with this!

[Peter J Shaw OD]

I don't know that an educational discussion regarding the calculation of magnification could really ever subtract from this or any thread even loosely associated with the subject.

Besides, if the last 20 or 30 replies haven't already satisfactorily addressed the original question, I don't know that we could really do much harm at this point with that slight digression...



Best regards,
Darryl

it is possible to correct the static (spectacle magnification) so that the eyes appear equal in size, Darryl you are correct in that it will not fully correct the dynamic (prismatic effect). The beauty of the binocular vision system is that we can measure the ability to tolerate dynamic differences and the eyes can compensate to a certain extent. Our method departs from the entrance or stop distance magnification calculations in that we use ray tracing and can exploit the non-linearity of the peripheral shape and power as opposed to the linear predictions of paraxial equations. So, our lenses make the eyes appear equal in size while reducing the prismatic effect (but not eliminating it).

I could go in to the concept of dynamic aniseikoia and how we measure it, this basic research was instrumental in designing the clinical model for the design. But it is rather long.

[Darryl Meister]

Our method departs from the entrance or stop distance magnification calculations in that we use ray tracing and can exploit the non-linearity of the peripheral shape and power as opposed to the linear predictions of paraxial equations

I understand that you're not relying on paraxial approximations, but ray tracing software will still need to trace a chief ray through some kind of reference point in the visual system, typically either the center of rotation or the entrance pupil, depending upon which you are treating as your stop for the calculations. Unless you are doing something radically different from conventional ocular ray tracing...

Best regards,
Darryl

[uncut]

Right ....but in that case ,PD should be more then earlier ,not Less then earlier

Good observation, IMO............................

[Matthew Wain]

one thing you might consider is altering the magnification of the lower powered lens.

The Formula:

SM(Thick) = (1/1-dF'v) x (1/1-(t/n)F1) or Spectacle Magnification of a Thick (Real) lens = Power Factor x Shape Factor

This allows you to work out the required spectacle magnification when applied to the higher power lens. Unfortunately, at the point in my studying to become a Dispensing optician, I know that for this to work, you need to rearrange the formula to make 't' the subject of the equation, I am just not sure how to do that. This would produce a pair of Isogonal lenses.

A worked example would be:
A lens power +6.00D (F'v), n=1.50, axial thickness (t) = 5mm, Vertex Distance (d) of 11mm on a -4.00 Base Curve lens:
Compensated front surface power of this lens would be +9.68D (using thick lens theory)

SM(Thick) = (1/1-(0.011x6)) x (1/1-((0.005/1.5)x9.68))

SM(Thick) = 1.071 x 1.033

SM(Thick) = 1.106 OR an increased magnification of 10.6%

If this was you higher powered lens, you would be aiming for the same magnification increase in the lower powered lens but this can sometimes produce some pretty horrific results in thickness of the lower powered lens.

I hope this helps.

And if anyone is able to help me figure out how to make 't' the subject of that formula it would be very much appreciated!

[Darryl Meister]

And if anyone is able to help me figure out how to make 't' the subject of that formula it would be very much appreciated!

Using your convention, it would look something like:



where M is the total spectacle magnification (note that t is in meters).

Best regards,
Darryl