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What's the RX? What's the material? How are the OCs of previous pairs aligned?
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There are a few things going on here. (My more technically experienced colleagues will have much more comprehensive opinions on the subject, though you will probably get more eyes on it in the "General" section than this one.)
1) I recommend placing the OC much, much closer to the pupil than you're talking about here. The higher the power, the less decentration it takes to really muck things up.
2) The higher B measurement is also a likely culprit. There is more lens farther off axis, and that portion of the lens will show more chromatic aberration.
3) 1.74 materials all have pretty low abbe values, so CA will always be an issue. This is where one of the most important things we do is to manage our patients' expectations. With their higher RX, hyper high index lenses are definitely called for, but CA is an unavoidable side effect. Make sure the patient knows it's going to be there before finalizing the sale and your patient will almost always be fine with it.
My 2 cents... The old glasses of your patient was spheric or aspheric design? Some people don't "digest" aspheric design.
aspheric design you have to set fitting heigh on the center of rotation of the eye, spot the pupil on glasses with the patient's face at 0 patoscopic tilt... so you will have the right drop oc.
Eventually can you ask to your lab a 1.74 lens with spheric design.
I believe 1.74 lenses was all same material: mr-174 abbe 32
Hi Dima.
That would work, however the best form base curve of a spherical surface design and a high refractive index material for a high myope (in this case about -8.00 D) is probably close to +6 or more, resulting in poor cosmesis. In general, as the refractive index increases, so does the best form BC. And once over 8 D, best form breaks down and asphericity must be employed to reduce oblique astigmatism and power error.
http://64.50.176.246/cecourse.php?url=lens_design/
Yes, try to position the Prism Reference Points as close to the pupil heights as possible, reducing the pantoscopic tilt as needed to compensate, resulting in a compromise between acuity and cosmesis.
A frame that positions the PRP (180/datum line) closer to the the pupil heights would be ideal.
Hope this helps,
Robert Martellaro
I learned the word "cosmesis" today.
It's the "look good" part of the ophthalmic optics battle between cosmesis and functionality.
https://www.youtube.com/watch?v=J0RTD7250II
@ Robert Thanks for explanation on aspherics.
Is that the same Merriam Webster that sits on a bench next to the Ohio River in Rosiclare Illinois and complains about caterpillars on her tomato bushes? I know her! I’ll let her know!