iseikonic lenses

[myck]

help!
r.x.
o.d. -2.50 -.25 x95
o.s. -2.50 -.25 x45

doc. says he needs 25% magnification in one eye, can anyone help.

thanks

[chip anderson]

I feel that the orthoptist have a web page and can help you with all matters isekonic (they could probably help all of us a lot).

Chip

[Robert Martellaro]

Any chance that the doc meant 2.5%? Increasing the minification of the other lens by manipulating lens design and position might be possible. If it's 25% then I would guess that a telescopic lens would be required. A refractive or physical occlusion may be the only alternative.

Robert

[Stopper]

The only way to increase the relative spectacle magnification will be to increase the thickness and the base curve of the lense. This will be a funny looking pair of glasses. Why does this patient need eisikonic lenses anyway? That prescription certainly will not induce aniseikonia unless this is some strange post-operative cataract or LASIK patient with a large difference in axial lenght of his eye. Also I doubt he needs 25% magnification. 2.5% is more likely. There are nomograms that can be ussed to design eisikonic lenses so you don't have to do all the calculations.

Again, I still don't see why he needs eisikonic lenses.

[Stopper]

Iseikonic lenses- Sorry for the spelling error

[Jeff Trail]

An "aniseikonia test" booklet does have a set of 25 pairs in 1% increaments...I have not seen anyone need 25%, but it maybe the right answer..not as we are assuming 2.5% :)
Now for the combination of curve and thickness to get that far of a swing in a like RX OU..sheesh is that going to be an ugly beast..you are going to have to take advantage of every little trick in the book... I am at home and do not have a surface program (I guess I should but can't have it all) Since we are at it and this is a rare thing might as well have a "math class" and show and tell
First is the number following the script info might be fairly helpful it should have an SM of ?.?? ..now depending on the number we know if it is a reduction or a increase of retinal image size, an important part of the process...since it is a myopic RX the SM should be less than 1 (all plus lens is greater than 1) for anyone interested..if you have a factor of more than one someone has made a mistake somewhere in this RX
Now in an iseikonic or eikonic lens, besides using thickness and base curve a few others were left out and that is use of vertex and actual material and surface design (spherical vs. aspherical)..
as far as the problem you have a number of formula's to plug in shape factor/power factor, %SM..than when you start tinkering with change of shape and vertex...
%SM= (SM-1)100 ..since you were told 25% it saved you partial math :) I just wanted to put the formula in here if anyone was interested..
Once you have come to this point the next formula is estimate how much change will occur in each lens if changes are made to the shape and vertex..
^%SM=^D1t/15
^%SM=D1^t/15
^%SM=^hD/10

^%SM=change in % of magnification
^D1= change in front base curve
^t=thickness in mm
^h=change in vertex in mm
(in this case the ^ is shown as a symbol for "change in" not variable or for prism )

Besides all of this a few things to help you a long the road is choose a SMALL round frame if possible with a short vertex..more room to play.
Make use of lens surface design..use an aspherical design where possible this has a direct effect on SM
Find out if it is a "fusion" problem or a "suppresion" problem.

With (if it is 25%) Chip will love this, you might be better off using contacts than specs :) and or a mix of contact and specs

If you want a good place to find options and explainations "Clinical optics, 2nd edition by Fannin and Grosvenor ia a great book..pag. 300 to 325 has to deal with this subject ...

I don't feel like "doing" the math, it is late and my surface program sure is faster at work ... but maybe someone else has a copy of a surface program with a "specialty lens selection" that can crunch the numbers for you if all you are looking for is a set of parameters ..you really need to supply some numbers though like frame size, vertex etc., etc. if you want the exact numbers..

This was probably NOT what you was wanting but I put most of this into the thread for people who have never had to deal with this problem before.. If no one has designed it for you (which is what I think you are looking for I'll try to get to it sometime Fri. at the office...)

Jeff "oh what a tangled math web we weave when dealing with the crazy stuff " Trail

[dfisher]

If anyone can come up with a solution to increase magnification on that Rx by 25% just shout it out! And what symptoms or exam findings could have possibly been revealed to make this dr. come up with this one?

Even in the days of cataract lenses changing the magnification difference by just a few, maybe even a couple, of percentage points was a challenge.

[Jeff Trail]

Actually it wouldn't be to bad to figure out..but what it would look like is another story :) ...given the information supplied where you wanted a "+" magnification not minification than something rignt along the line of a sag 18.75 at 21 mm CT (CR39 aspheric) with a vertex of 6 mm and 3 mm/C/P would be around 21% or 22% (give or take) ..you could tinker a slight bit with the other eye and get it into minification around 3 or so % to make up the difference... if you keep adding base curve and thickness you could get it to 25% but what a BRICK ..:)
Getting someone through the distortion and just physical weight would be tough though..it can be done..would I want to be the guy getting stuck dispensing it? Nope, and dealing with a magnitude of this type even the edging would be crucial.. a mmm or so of vertex can swing it 3 or 4%..I can give you the EXACT numbers if I have to but plugging these in will give you a fairly close target number

Jeff "I like the math but would hate to grind it" Trail