What would be the logic in ordering a 1st division lens on a 2 base? I am running into issues with another ECP who believes that in all instances, flatter is better and I disagree.
If I'm wrong, please educate me. :shiner:
What would be the logic in ordering a 1st division lens on a 2 base? I am running into issues with another ECP who believes that in all instances, flatter is better and I disagree.
If I'm wrong, please educate me. :shiner:
Flatter is better if you want thin lenses. Flatter is best if the posterior curve is going to be very short in radius as in high minus lenses.
Otherwise steeper is better at least to a 6 or 8 on the posterior. See origional works on PUNTAL, Curvova, Corrected Curve, etc.
Origional concept was to try to extend the radius of cornea along visual axis during optical rotation.
Since then the objective seems to have been to mimic the action of aforementioned lenses with thinner cosmesis.
Chip
If you Google:
tillyer AND "corrected curve"
and
punktal AND "corrected curve"
you will find far more information than you need.
.
I guess I didn't phrase the question properly. For arguments' sake the Rx is Pl -.75 x 90. Why would the Optician specify a 2 base? If flatter is better then why do we even bother with corrected curves?
I put this decision in the same category with how low can you logically fit a progressive, even a short corridor design before it's little more than SV with some stuff at the bottom.
Judy:
If you Google dis stuff you gonna find flatter ain't better. Just thinner. You wouldn't specify the base and Rx you specified if you were trying for the best vision. You would if you you had a rather flat frame and were trying to conform to it.
You do know that today it's all about selling the most gimmicly product at the highest price, not about the best vision.
Chip
History of lenses:
BI-convex..........Bi concave
Plano-convex.....Plano-concave
Meniscus (base 5)
Corrected curves
and now in 2008 you should not go back 150 years to the older versions.............however we call progressives with all their distortions the latest in technology.
You pull the Rx out of stock. It will be a 4 BC in most brands. I hope the lab isn't surfacing this lens.
Anyway, if you talk to the lab guys they will tell you that offices that specify BC have the most problems w/ remakes.
Even if flatter is better (which it isn't), a 2 BC is wrong for this Rx. Here's why. A flatter curve won't make it any thinner. The limiting factor on this Rx is the stability. You can only make it so thin before it won't stay in the frame. If you ordered a 5 BC and a 2 BC surfaced from the lab they would be the same thickness. It will be as thick as it has to be for it to stay in the frame.
The reason for aspherics is to "correct" some of the aberations toward the edge so it can be make flatter, and therefore thinner. If you are ordering flatter without specifying aspheric, then vision is compromised.
Your suspicions are correct Judy, the optician specifying the flat BC is uninformed.
"best form" in this case will be what fits the frame the best, probably a 4-5 BC. The textbook answer is probably a 6 BC, but the frames are a little flatter now days.
If I sound a little too arrogant in my knowledge. It's because I learned it all on Optiboard!:D
Thanks guys. I posed the question because it's something I seem to be running into a lot more these days. I suppose it's the idea that if aspherics are flatter without compromising optics then flatter anything must work as well. I'm also disappointed when it's a theory espoused by another licensed Optician with as much time in the profession as I have, but apparantly less time in a learning environment.
MarcE, yes the lenses were surfaced by request on a 2 base and the company billed accordingly. :hammer:The patient was unhappy with the correction and we have reordered a stock lens. :shiner:
Its like trying to pick up a turd by the clean end.
You can design a series of aspheric lenses with the design criteria of minimum marginal aberration.
You can design a series of aspheric lenses with the design criteria of minimum edge thickness.
But you can't design a lens series to maximize both aberration and thinness. Todays better aspherics do provide a degree of both characteristics but optimize neither.
Thanks Dick. That analogy is going to stay with me for a long time. :D
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Harry:
Flatter is almost always better for an axial error like spherical aberration correction, but of course, at the expense of off-axis errors (coma, astigmatism, lateral color, and geometric distortion)
Barry
I don't know, it seems that even Dolly Parton is beginning to think flatter is better.
Chip
With the average pupil size being what it is spherical aberration would be insignifficant compared to the otehr aberrations you mentioned. I spent some time a while ago making up my Rx in a bunch of different curves to see what bothered me the most, and I would have to say I noticed anything over a 2.00 change in base curve and the blur wasn't an issue so much as compared to the curvature of field. I am sure that this sensitivity towards one distortion over another may be more patient specific, but when I went too far off from corrected curve the world looked like I was viewing through a fun house mirror.
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On base curves and aspherics I would also like to add that almost 30-40 years ago the ANSI standard for off axis power was removed from the standards so today you may order an aspheric lens and think it is correcting for the use of flatter curves, but their is no standard in place that says they have to so some may only be provideing better cosmetic appeal. Just thought I would mention that and see what others think.
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Barry,
Yep distortion is what bothered me the most. At the time I was working in a small lab where the boss would surface with me and would always tell me flatter is thinner and everyone likes thin lenses. We both made up a few sets of glasses in varying base curves in spherical blanks I even remember they were the Sola CR-39 blanks in the fuschia colored box, the bases were 8, 6, 4, 2.50, and 0.50 I wear a -4.00 sph equiv at the time and he was a +2.00. Anyway he was very similar in results to me I was able to get down to the 2.50 with minimal notice of aberrations and then the 0.50 was horrible, to him he was able to get into the 6.00 and claimed he was fine with the 4.00, but you could tell he didn;t liek them and ended up admiting to it in a week, but the 2.50 he said was horrible. I noticed that the farther from the center it did tend to blur a bit but wasn't that bad of an effect and I could deal, but the 0.50 felt like I was in a fish bowl.
Anyway orthoscopy is an important consideration to me, but I have also foudn that I need to be pretty far off of corrected curve to experience enough distortion to really bother me, I would have to assume that each individual will be more or less susceptable to varying degrees of distortion, but for me it was really the biggest nuisance.
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Harry,
Interesting thing about distortion...
There are two kinds: Rectilinear (geometric) and angular
The error we typically see with in conventional ophthalmic lenses (and terrestrial binoculars and telescopes) is rectilinear
The error we allow in astronomy binoculars and telescope eyepieces is angular.
It's a dillema: Correct one type, and the other goes to hell.
When you see progessive "grid" patterns that appear *less* distorted, they are correcting geometric at the expense of angular.
Typical distortion from simple, spherical *wrap* lenses is of the angular type
FWIW
Barry
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