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RX:
+4.25 -0.25 x096 1DN 2.5OUT
+4.00 -0.50 x115 1UP 2.0OUT
So I go to make this job and naturally went for my aspheric lenses... then I had to pause and think. If aspheric lenses don't decenter, how does that effect ground in prism? Is this the exception or is the thinness of aspheric lenses nullified because of the prism here? Not sure what to expect from this.
Thanks in advance for any input!
Quince,
Always surface for prescribed prism, assuring that the center of the aspheric surface, or pole, is aligned with the visual axis of the eye. Failure to do so will introduce significant oblique astigmatism and/or power error.
On might also anticipate the eye deviating towards the prism apex, adjusting the decentration so that the pole stays aligned with the eye. This is also an issue with multifocals, where the segment or fitting cross will become misaligned with the eye.
See figure 15.
http://www.apcthai.com/webboard/uplo..._by_design.pdf
Hope this helps,
Robert Martellaro
Is it time to say: always surface prism, unless you have your own finishing lab with stock lenses?
My 2cents - flat base surfaced 1.67 SPHERICAL (not aspheric), like a 4 or 5 base and the patient will see well, be happy and so will you. Simple. Old school.Quote:
Originally Posted by Quince
Just to be clear- I'm surfacing this job either way. I'm was just wondering how effective aspheric lenses are for thinness when there is prism introduced to the equation.
I did a readout with Visual Lab and it actually shows a slightly thinner center with more of a back curve (nice to have of the patient has eyelashes) with a spherical lens.
My instinct is to go with aspheric, but my programming is indicating that I'm wrong.
optimensch- So far it looks like this will be a 6 base poly, but I'm just wondering your reason for backing spherical? What difference do you notice? Thanks!
I believe a flatter spherical base beats a steeper aspherical base in this case - and you can also leave the oc height closer to the boxing center, less need to decenter up to pupil in the case of spherical. Better outcome esthetically, of course it depends on the frame choice, and how high above the midline the pupils are. In a 1.67 it would be easy to get a 4 or 5 base in this case, and I simply do not think with so much prism in play that aspheric is the correct choice. You got bases flying all over the place, up down and all around, keep it simple and esthetic. Its my gut feeling, but of course knowing what the patient had before would be key to making the final decision! I would be doing a 4 base (or 4.5 if available) spherical 1.67, and this job will be at its best. I'd wager a toasted montreal bagel with philly cream cheese.Quote:
Originally Posted by Quince
Robert- I haven't had my coffee yet, but I have bookmarked your link. As always, very informative post.
That sounds delish! But talk about being all over the place! Fortunately Maine is right between (at least vertically). :wink: I'll stop with the terrible humor now...
I looked at both options on the same base and was happy to have a slightly steeper curve going spherical, so I definitely see what you mean by being able to put this kind of Rx on a flatter curve when spherical. I also like simplicity- it will make me more confident in editing the generator thickness on a spherical lens. Thanks again for sharing your experience with this sort of job.
Mispositioning an aspheric lens is one way to generate oblique astigmatism, another is to use non-best form base curves. In this case, a +4.00 1.67 spherical lens on a +4 base curve will have over 1 D of OA 30˚ off-axis. That might be tolerated for SVNO, but probably not for distance.Quote:
Originally Posted by optimensch
Moreover, the addition of 6.5∆ total will generate a moderate amount of chromatic aberration due to the use of very dispersive materials, adding another layer of blur off-axis, and will introduce blur on-axis as well. Aspheric lenses reduce magnification, weight, and thickness due to the use of flatter BCs, without optical compromise, when optimally designed and positioned in front of the eyes.
BTW, the best form BC for a +4 D 1.67 refractive index spherical surface design lens is about +10, and about +8 for lower index lenses like Trivex. That's why high index lenses are usually aspheric...they require more curvature for less oblique astigmatism, unless it's compensated for by the use of aspheric surfaces.
Mo Jalie is always worth reading. One book that others have recommended that I don't have is Ophthalmic Lenses and Dispensing 3rd edition -Mo Jalie. I do have System for Ophthalmic Dispensing 2nd and 3rd edition -Brooks/Borish. Strongly recommended.Quote:
Originally Posted by Quince
Best regards,
Robert Martellaro
When you say 1 D of oblique astigmatism 30degrees off-axis - do you mean that 30degrees of gaze away from straight ahead (on left or right gaze for example) that there is this 1 D of induced astigmatism ? From experience, if I deliver that prescription on a 4 (ok, maybe a 5 base, but max 5) vs a 10 base with its "correct curve", I just know my 5 base wins. The 10 base will look horrible, not sit well in the frame and increase lens thickness - no?Quote:
Originally Posted by Robert Martellaro
I imagine that some unwanted astigmatism 30 degrees off to the side would bother a wearer less than bulging 10 base lenses.....at least up north where I am...
Would be interesting to compare lens thickness, magnification and induced astigmatism at say 15 degrees .... between the 5 base and the 10...
Anyhow, you raise an interesting theoretical point, but in practice I believe people would opt for less thickness,magnification and weight, given the choice.
So many interesting points - what i've found is that often with todays "deeper b" frames and the need for more vertical decentration (especially with aspherics which must be near the pupil height), that sphericals (even choosing a 1.6 index rather than a 1.67 in the case at hand) with flattest curve and somewhat less vertical decentration to the pupil, win on esthetics and frankly i have not encountered many problems with this approach. Of course we are talking only SV distance here. So often younger wearers in particular, saddled with +4 or more, just love when their new glasses are so much thinner and less magnifying....Quote:
Originally Posted by Robert Martellaro
What he's saying, mensch, is that if it weren't aspheric, and made on a +4.00, the optics would stink, and you'd have to use a +10.00 for perfect optics. But if you use aspherics, it will be flat like a +4.00 and will have good optics.
Ive done pretty well with my stupid shamir spectrum sv on jobs I was concerned about. Otherwise I would definitely do straight spherical. Better fit frame and 1.60/trivex will feel much better than 1.67. I'd satin polish and roll the edges on that in 1.60 before I put it in 1.67.
Prism loves high abbe.
Actually nowadays I'd use the variable bevel function on my LEX, still perfecting the usage of it though #humblebragQuote:
Originally Posted by Tallboy
Well aren't you special! I wanted to say the same thing on the thread about gradient tints. We don't have problems here because we have to do them all by hand :showoff:Quote:
Originally Posted by Tallboy
Yes. About 15mm away from the primary gaze, an area where the near zone resides on multifocals.Quote:
Originally Posted by optimensch
As mentioned previously, sometimes we can have our cake and eat it to. Sometimes the lens designer or the client must compromise, although most of my clients opt for better vision when given the choice, for example, better vision and thicker lenses, especially when I tell them that thicker doesn't always mean heavier.Quote:
From experience, if I deliver that prescription on a 4 (ok, maybe a 5 base, but max 5) vs a 10 base with its "correct curve", I just know my 5 base wins. The 10 base will look horrible, not sit well in the frame and increase lens thickness - no?
I imagine that some unwanted astigmatism 30 degrees off to the side would bother a wearer less than bulging 10 base lenses.....at least up north where I am...
Would be interesting to compare lens thickness, magnification and induced astigmatism at say 15 degrees .... between the 5 base and the 10...
Anyhow, you raise an interesting theoretical point, but in practice I believe people would opt for less thickness,magnification and weight, given the choice.
WRT vertical OCs, they should be set according to the degree of pantoscoptic tilt, lowered 1mm below the corneal reflex per 2 degrees of tilt, and is true for all surface designs (see the illustration below). This assures that the optical axis of the lens is aligned with the center of rotation of the eye, minimizing power error, especially for aspheric designs and/or high dioptric values. For example, if the geometric center of the frame is 7mm below the pupil, try to dial in 14 degrees of panto. If that's not possible, and the client won't choose a better fitting frame, or in cases involving anisometropia, use a POW optimized freeform design, with a small amount of prism thinning as needed.Quote:
Originally Posted by optimensch
Best regards,
Robert Martellaro
***************
Excellent information and perspective, thanks. I find it so interesting that your experience with young sv hypermetropes is that they prefer thicker lenses with better optics....I've had this discussion a bunch of times and almost invariably thinner and more cosmetic wins out....Of course if we say "garbage optics" vs "hubble space telescope perfection" maybe the needle would tilt the other way...Quote:
Originally Posted by Robert Martellaro