 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Medical Forum / General / Vision / September 2006Progressives
Hi, One thing that puzzles me. Why can't progressive lenses be made in such a way that each level of correction extends horizontally from edge to edge rather that correction only in a centralized column with blurry peripheral vision. Yes, the lenses would be thicker at the bottom, but that could easily be disguised by techniques such as beveling and locating the extra glass or plastic behind the frame so as not to be visible to others. Bob On 9/8/06 7:03 AM, in article sKeMg.351$yD7.323@newsfe11.lga, "riserman" > Hi, > [quoted text clipped - 8 lines] > > Bob Progressive lenses intrinsically have large aberration. Lens design is always a tradeoff between various desirable and undesirable characteristics. This is especially true of a single shaped piece of glass or plastic. If you were willing to wear and pay for a complex lens consisting of cemented elements, then the designer would be able to improve performance overf what you get now. Bill -- Ferme le Bush The reason is that it's impossible to make multi focal lenses without aberrations unless you have a sudden change in power somewhere on the lens. A sudden change in powers necessarily means a visible line. They do have lenses with unabberated peripheries, but they all have lines wherever the power changes, and these are called bifocals and trifocals. w.stacy, o.d. > Hi, > [quoted text clipped - 8 lines] > > Bob Almost exactly my point! Imagine if you will, a lens made up of ten thin, horizontal segments in a progression from 1.25 to 3.25 diopters, sort of a tenfocal. Then extend the number of progressions from ten to one hundred thinner segments, then to an infinite number of infinitesimal segments while maintaining the same overall vertical distance. Would this not be a lens without discontinuities that you could make using one piece of glass or plastic. Would it not be a smoothly progressive lens without discontinuities or peripheral blur? I would think this could be done with computer assisted design. Am I overlooking something? Bob > The reason is that it's impossible to make multi focal lenses without > aberrations unless you have a sudden change in power somewhere on the [quoted text clipped - 16 lines] >> >> Bob The something you are overlooking is the horizontal prismatic differences between each increment of power. The only way to "smooth" the little prisms out is to polish them out, which results in horrible optics. The newer free form progressives attempt to do it by surfacing point by point across the lens surface, but they still have a fair amount of peripheral blur due to the compromises that must be made in the interest of smoothing out all those little prism differentials. w.stacy, o.d. > Almost exactly my point! Imagine if you will, a lens made up of ten > thin, horizontal segments in a progression from 1.25 to 3.25 diopters, [quoted text clipped - 34 lines] >>> >>> Bob William, In my example the number of "prisms," as you call them, would be increased to infinity thereby making the discontinuities infinitesimal and not visible in any way. The easiest way to do this would be with a computer. That's what I was suggesting. Who makes the newer free form progressives you wrote about? Perhaps they are not perfect because some constraint was applied in their design for aesthetic reasons. I would love to discuss all of this with an optical engineer. Do you have manufacturers' names? Much appreciated, Bob > The something you are overlooking is the horizontal prismatic > differences between each increment of power. The only way to "smooth" [quoted text clipped - 44 lines] >>>> >>>> Bob Essilor makes the Varilux version, Hoya has one, Rodenstock maybe, and there are others. These are indeed computer plotted and generated surfaces, which are apparently somewhat better than standard designs, but there are some unfortunate physical realities of optics that make what you're asking for impossible. You are sort of describing Fresnel optics taken to the limit. The problem is theoretically you end up with a large (infinite) number of clear strips and and equally large number of blur boundaries in between each clear strip. The blur strips win and the lens is foggy. (Sorry I can't do a better job of 'splaining this) w.stacy, o.d. > William, > [quoted text clipped - 61 lines] >>>>> >>>>> Bob > You are sort of describing Fresnel optics taken to the limit. The problem > is theoretically you end up with a large (infinite) number of clear strips > and and equally large number of blur boundaries in between each clear > strip. The blur strips win and the lens is foggy. (Sorry I can't do a > better job of 'splaining this) It isn't easy to 'splain. Another way to look at it - Increasing curvature toward the bottom induces cylinder, and cylinder is blurry. The cylinder is neutralized in the center, creating a "corridor" approximating increased spherical values all the way to the bottom. But it can't be corrected all the way across. Only the central "corridor" can match the prescribed value and the residual cylinder is crowded toward the edges in different and proprietary designs, in an effort to increase the width of the corridor. Not much better. -MT The problem revolves around the fact that the eye moves and the lens does not when it comes to the lateral distortion in a progressive. Maybe we could have a computer designed lens that moves with the eye -- wait, I think it has been done -- it is called a contact lens. In the infinite strip example, the aperture of the eye would be larger than any single power strip in the lens, hence a range of powers would enter the eye simultaneously. This type of spectacle lens would have some of the same potential acuity issues as a multifocal soft lens, only magnified because of the distance between the lens and the eye. Dr. ABS ArsenalFire schreef: > The problem revolves around the fact that the eye moves and the lens > does not when it comes to the lateral distortion in a progressive. > Maybe we could have a computer designed lens that moves with the eye -- > wait, I think it has been done -- it is called a contact lens. With the contactlens you use ALL the different powers simultaneously. In glasses you have to use a certain angle in viewing through the glasses for the wanted power (separately). (as in progressives). > In the infinite strip example, the aperture of the eye would be larger > than any single power strip in the lens, hence a range of powers would > enter the eye simultaneously. You have to use a certain "height" in the glass in needed power to let it be useful. One off the reasons progressives with a short height in addition increase mend (for small glasses meant) sometimes gives more viewing problems as they do in progressives with a longer height in increasing addition. This type of spectacle lens would have > some of the same potential acuity issues as a multifocal soft lens, > only magnified because of the distance between the lens and the eye. Wrong. Jan (normally Dutch spoken) On 9/8/06 3:31 PM, in article fbmMg.23900$kO3.6999@newssvr12.news.prodigy.com, "William Stacy" <wstacy@obase.net> wrote: > You are sort of describing Fresnel optics taken to the limit. The > problem is theoretically you end up with a large (infinite) number of > clear strips and and equally large number of blur boundaries in between > each clear strip. The blur strips win and the lens is foggy. (Sorry I > can't do a better job of 'splaining this) In the limit, as a student of calculus would know, you are back to curved surfaces that do not differ from what you get if you never worried about using steps in the first place. That includes all the aberrations you were trying to circumvent. Bill -- Ferme le Bush On 9/8/06 1:08 PM, in article 05kMg.682$344.32@newsfe12.lga, "riserman" > Almost exactly my point! Imagine if you will, a lens made up of ten > thin, horizontal segments in a progression from 1.25 to 3.25 diopters, [quoted text clipped - 9 lines] > I would think this could be done with computer assisted design. Am I > overlooking something? It would not. The process you describe introduces aberrations. bill -- Ferme le Bush >Hi, > [quoted text clipped - 8 lines] > >Bob Bob, If the index of refraction increased instead of the curvature, then there should be a substantial increase in the field of vision. I wonder if left handed materials or negative index of refraction lenses could be used for this application, probably in combination with regular materials. Regards, Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "An expert is a person who has made all the mistakes that can be made in a very narrow field." - Niels Bohr >If the index of refraction increased instead of the curvature, then there should >be a substantial increase in the field of vision. I wonder if left handed >materials or negative index of refraction lenses could be used for this >application, probably in combination with regular materials. > > I don't think the field of vision would be significantly increased by increasing index of refractio, or am I missing something? And "negative" index of refraction implies its existence in the real world. I'm unaware of any such material. w.stacy, o.d. >"negative" index of refraction implies its existence in the real >world. I'm unaware of any such material. http://www.physorg.com/news68814637.html http://www.sciencedaily.com/releases/2003/12/031202065957.htm http://www.sciencedaily.com/releases/2005/09/050907095715.htm Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "An expert is a person who has made all the mistakes that can be made in a very narrow field." - Niels Bohr Ok well that's interesting stuff, but I doubt I'd ever want to even look at microwave radiation, let alone focus it while looking at it! Far out... w.stacy, o.d. > > [quoted text clipped - 17 lines] > - Niels Bohr > >Ok well that's interesting stuff, but I doubt I'd ever want to even look >at microwave radiation, let alone focus it while looking at it! Far out... Yup, who would've thought. It looks like there has been some success at near infared. http://www.ameslab.gov/final/News/2006rel/metamaterials.htm Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa, Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman On 9/12/06 11:44 AM, in article geDNg.299$Ij.275@newssvr14.news.prodigy.com, > I don't think the field of vision would be significantly increased by > increasing index of refractio, or am I missing something? > > And "negative" index of refraction implies its existence in the real > world. I'm unaware of any such material. I agree with your conclusion. Negative index can be achieved with anomalous dispersion. This happens in materials due to a strong absorption. Much of chromatic aberration in glass arises from a strong absorption in the ultraviolet. The anomalous dispersion is at shorter wavelength than the peak absorption. X-rays generally see an index slightly less than one, because they ARE on the far side of the UV absorption. Microwave lenses are also possible. Without going into detail, it is the concave microwave lens that will focus a parallel microwave beam toward a small spot. Bill -- Fermez le Bush On 9/12/06 11:19 AM, in article l5sdg2tpb7cif15a423k3or2mjlcegsat2@4ax.com, > If the index of refraction increased instead of the curvature, then there > should > be a substantial increase in the field of vision. I wonder if left handed > materials or negative index of refraction lenses could be used for this > application, probably in combination with regular materials. I doubt it. To first order, it is the pathlength through the lens times the index of refraction (optical path) that is the heart of Galilean optics as expressed using Fermat's principle. What I think might stand a chance to improve field of view is if two different aspheric surfaces could be use for shaping the lens. Bill -- Fermez le Bush |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.