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Medical Forum / General / Vision / July 2005Where to get almost-flat polycarbonate bifocals?
I'm looking for polycarbonate bifocals (+1.50 / +2.50) that have an almost flat front surface. A slight curvature would be OK, but the usual curvature is too large. My local shops say they don't have such a thing. Is there a place to get something like this? Thanks - Bob > I'm looking for polycarbonate bifocals (+1.50 / +2.50) that have an > almost flat front surface. A slight curvature would be OK, but the > usual curvature is too large. My local shops say they don't have such > a thing. Is there a place to get something like this? Why would you want that? They will look like mirrors. Is it for a theatrical use? I mean they used to make 'em like that (in the 19th century). w.stacy, o.d. > I'm looking for polycarbonate bifocals (+1.50 / +2.50) that have an > almost flat front surface. A slight curvature would be OK, but the > usual curvature is too large. My local shops say they don't have such > a thing. Is there a place to get something like this? Why do you want a flat front? It would mean a convex back and would cause increased distortions and may bump into your eyelashes. If you are still insistent, you could ask for a +3 base curve and have the lab mount them backwards, ie back surface (which will be about flat) to the front and front surface to the back. Likely will be unwearable. To understand why, here is a brief optics primer: The total power of a lens is the sum of the back and front surfaces. The "curved outwards" convex surface is +. Distortions are minimized with convex front, concave back surfaces, trying for about +4 to +6 front. To get the about +2.00 that you need you would need about +6.00 front and -4 back. The lens, in cross section would look like ( ( You are asking for plano front. That means about +2 back. The lens, in cross section would be |) If you did plano back, then the front would need to be +2. Lens would be (| I think the latter is the best you can hope for. Dr Judy > Thanks - > Bob >The total power of a lens is the sum of the back and front surfaces. The >"curved outwards" convex surface is +. Distortions are minimized with >convex front, concave back surfaces, trying for about +4 to +6 front. You bring up a good point about the possible distortion if it is flatter. Would it be better to use aspheric lenses to minimize this? Is the aspheric part only on the front, or must both sides be aspheric? Do these kinds of lenses require a special lab? >To >get the about +2.00 that you need you would need about +6.00 front and -4 >back. The lens, in cross section would look like > ( ( Yes, this is what they are saying they have to do, using a +6 front. There is a small astigmatism component, +.50, but they said that is not the problem. I asked about using a +2 or +3 front, but they say it can't be made that way. I still don't understand why not. >You are asking for plano front. Actually, it was 'almost-flat'. Originally I was thinking about a +1 front and a +1 back. >If you did plano back, then the front would need to be +2. Lens would be >(| > >I think the latter is the best you can hope for. Thay say it can't be done, that's why my note here. Thanks for the informative reply - Bob >I'm looking for polycarbonate bifocals (+1.50 / +2.50) that have an >almost flat front surface. A slight curvature would be OK, but the [quoted text clipped - 3 lines] >Thanks - >Bob Bob, An aspheric lens would have a front curve of about +4.00. A +2.00 curve (aspheric) is doable, but the vision away from the center of the lens (including the bifocal) will not be as clear compared to a lens that uses "best form" base curves. Hope this helps Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical robopt@execpc.com ~~~~~~~~~~~~~~~~~~ "An expert is a person who has made all the mistakes that can be made in a very narrow field." - Niels Bohr >>I'm looking for polycarbonate bifocals (+1.50 / +2.50) that have an >>almost flat front surface. A slight curvature would be OK, but the [quoted text clipped - 12 lines] > >Hope this helps Yes it does. Sorry for the long delay in responding. Thanks - Bob |
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