Should base curve or front curve be equalized to minimize image size differences? The shape factor formula uses front curve power, but I remember being told to specifiy equal base curves for aniso Rx. Also, is it a bad idea to do one side polycarb and the other side 1.67 so that the lenses can have equal center thickness, but more symmetrical edge thickness? Rx is -300 OD and -600 OS.
It really depends upon the product, but you can generally get away with matching the base curve of the strongest lens (flattest base for minus lenses and steepest base for plus lenses). This may not reduce the magnification difference as much as you'd like, but often provides at least some reduction in aniseikonia as well as a cosmetic benefit. However, aspheric single vision lenses and certain "design by prescription" progressive lenses may have more stringent requirements for base curve selection in order to achieve the intended optical performance. When in doubt, I would consult the manufacturer.Should base curve or front curve be equalized to minimize image size differences?
I haven't tried this, myself, but my biggest concern would probably be over the use of any additional lens treatments, including AR coatings or tints, since it would be difficult to get an exact match. Most 1.67 lens materials have an Abbe value close to that of polycarbonate (e.g., 32 versus 30), and are also at least in the same ballpark in terms of impact resistance and tensile strength. There might also be subtle differences in reflectance, color, abrasion resistance, and so on. Of course, you could always just order the weaker lens with a greater center thickness in the same material if you aren't worried so much about equalizing image sizes.Also, is it a bad idea to do one side polycarb and the other side 1.67 so that the lenses can have equal center thickness, but more symmetrical edge thickness? Rx is -300 OD and -600 OS.
Best regards,
Darryl
Equalizing BC and equalizing CT will both work towards equalizing image size, and I know you know that.Originally Posted by Darryl Meister
To be clear, you are talking about adding center thickness so that the CT of the less minus eye exceeds that of the more minus eye, not equals it.
Just clarifying.
It certainly will in plus lenses, since increasing the center thickness increases magnification by shape. Unfortunately, however, increasing magnification by shape on the weaker minus lens will move the magnification in the wrong direction, since the stronger lens magnifies less (or "minifies" more).equalizing CT will both work towards equalizing image size, and I know you know that
Best regards,
Darryl
Another way to equalize the magnification would be to ply with the vertex distance this makes a huge difference.(try placing a front bevel on one lens and a back bevel or 1/3-2/3 bevel on ther other)
Note:use changes in 1 of the variables to see how much change in magnification results you should find that small changes in vertex distance with tweaks to the bevel placement really make huge changes to the magnification.
SM-Spec Mag
D-Front Base Curve
t-Thickness
h-Vertex Distance
P-Power
SM=Dt/15
-or-
SM=hP/10
I think the equal base curve and center thickness rule really only helps with plus lenses. so adding thickness to the less minus lense won't help much. Aniso (and I assume we are talking about aniseikonia not just anisometropia) myopes tend to be better off in contact lenses were image size should be equal.
it has been along time since I've been to the leaf room(That's for you DrK) but If I remember correctly the typical threshold for aniseikonia is an image size difference of about 4%. A 3D anisometropia is about a 3% difference. Is this patient symptomatic or is the concern about the cosmesis of the lenses? I'm sure some patients are symptomatic at lower levels. Good luck helping this patient.
:cheers: Life is too short to drink cheap beer.
Stopper, are you a Buckeye Grad?
Where else can you find a leaf room? Dr. Polasky would be proud that people are talking about Aniseikonia.
:cheers: Life is too short to drink cheap beer.
K's were fairly equal and she doesn't have problems when wearing contacts, so I just assumed it was an image size issue. We ended up using 2 different indices, equalizing base curve, and specifying bevel position to adjust vertex distance. What surprised me was how confused our lab was after getting the order. Had to call us twice to figure out what we wanted. In any case, patient is doing much better with her glasses. Previous pair was causing her eyes to water and head to swim after 15 minutes.
I actually paid a visit to OSU's leaf room about nine months ago. Interesting place, though it looked as if it hadn't been used in a while.Where else can you find a leaf room?
Best regards,
Darryl
It is mostly used as a teaching tool. It demonstates the effect but you can't quantify the aniseikonia so not very practical for clinical use
:cheers: Life is too short to drink cheap beer.
http://www.nyee.edu/page_deliv.html?page_no=100
Claims to have the last functioning unit in the US.
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Experience is the hardest teacher. She gives the test before the lesson.
OSU had two or three when I was there. Not sure if they are still in use.
:cheers: Life is too short to drink cheap beer.
I'd virtually guarantee that OSU has at least one or two space eikonometers hanging around. It's an odd badge of esoterica that everyone is arguing over.
They had two when I was there last year, although whether or not they were "in use" is another story. Also, the measurements need to be translated into an iseikonic lens configuration (there were tables to use if you didn't do the calculations).
Best regards,
Darryl
They worked when I was there. we used them in Labs and in the Aniseikonia clinic. They were quite proud to have them. They probably think they have the only ones left. Who Knows? Really not a big demand for them
:cheers: Life is too short to drink cheap beer.
Stopper,
PM me so I can find out if I know you!
Wouldn't playing with the vertex (if over 7 diopters or so) affect the effective power of the lens? Wouldn't that be the same as simply dropping the power to minimize the difference in image size in the first place?
AA
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