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Medical Forum / General / Vision / June 2007Why do opthalmologists use plus cylinder phoropters, while optometrists use minus cylinder phoropters?
Why do opthalmologists use plus cylinder phoropters, while optometrists use minus cylinder phoropters? And why are those glasses they sell in boxes at the store never minus power lenses; they're always plus power lenses? And since a minus lens has a negative diopter, and diopters are the reciprocal of focal length, how can a minus lens exist? That would mean a negative focal length, and distance can't be negative --what would that mean?-- Do they make any lenses w/ a refractive index higher then 1.8? Like an RI of 2.00 or more? What would happen if a lens had an *imaginary* refractive index? Thank you!!!! > Why do opthalmologists use plus cylinder phoropters, while > optometrists use minus cylinder phoropters? Tradition? > And why are those glasses they sell in boxes at the store never minus > power lenses; they're always plus power lenses? Because more people need assistance in reading, thus close-up, thus plus diopters. > And since a minus lens has a negative diopter, and diopters are the > reciprocal of focal length, how can a minus lens exist? That would > mean a negative focal length, and distance can't be negative --what > would that mean?-- It's a geometric thing. Instead of parallel rays being focused to a point a given distance past a positive lens (its focal length), the rays diverge, as if coming from a point some distance *beyond* the lens (its negative focal length.) See: http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/image3.html > Do they make any lenses w/ a refractive index higher then 1.8? Like an > RI of 2.00 or more? I don't think so, but there are people on here far more knowledgeable about available lens options. > What would happen if a lens had an *imaginary* refractive index? Oddly enough, all real (as in 'actual') materials *do* have an imaginary component to their refractive index! It's explained a little, here: "The refractive index of any substance is best described as a complex number, such as 1.34 + i 0.00067. The real part of this number is the "ordinary" refractive index as discussed above, whilst the imaginary part indicates the amount of absorption. If the imaginary part is zero, the substance is non-absorbing." http://www.philiplaven.com/p20.html http://en.wikipedia.org/wiki/Refractive_index Dave > > Why do opthalmologists use plus cylinder phoropters, while > > optometrists use minus cylinder phoropters? [quoted text clipped - 37 lines] > > Dave OK, what if a substance had a refractive index of -1.00+0.00i? I think I heard about this somewhere in a science magazine. A lens made out of such a material would absorb no light, bend light backwards, and, if it was placed in a vacuum, the refractive index of the lens and the vacuum --which is 1.00+0.00i-- would cancel out, making the image be the actual object. Perfect focusing and resolution! > Why do opthalmologists use plus cylinder phoropters, while > optometrists use minus cylinder phoropters? [quoted text clipped - 13 lines] > > Thank you!!!! Hi I have my own doubts as there is any difference of lens that is used by an Optometrist or Opthalmologist.Check on this.The optometrist dispenses lens for Presbyiopia and not for Myoipia or Astigmatism for that matter. > I have my own doubts as there is any difference of lens that is used > by an Optometrist or Opthalmologist. Check on this. The actual lens comes out the same no matter which notation is used. > The optometrist > dispenses lens for Presbyiopia and not for Myoipia or Astigmatism for > that matter. In the US, lenses for presbyopia are sold without prescription. Optometrists prescribe all sorts of lenses, as well as drugs to treat medical conditions. -MT, OD > > Why do opthalmologists use plus cylinder phoropters, while > > optometrists use minus cylinder phoropters? [quoted text clipped - 19 lines] > dispenses lens for Presbyiopia and not for Myoipia or Astigmatism for > that matter. regardless of whether a prescription is written in plus cylinder form or minus cylinder form the final glasses that are dispensed are identical. the two forms of prescription writing (i.e. plus vs. minus cyl) are simply a traditional difference between optometrists and ophthalmologists. and yes, different phoropters are used by optometrists and ophthalmologists in that the JCC lens inside is different. and in the US, optometrists and ophthalmologists measure and write prescriptions for all types of refractive errors be it presbyopia, astigmatism, myopia, or whatever. and they both can treat the primary eye diseases as well. the difference comes in the treatment of the eye involving actual surgical intervention where only ophthalmologists are licensed to perform those therapies. this difference may not apply to where you live. On 6/17/07 8:19 PM, in article 1182136761.560870.56540@n15g2000prd.googlegroups.com, "DarkProtoman" <Protoman2050@gmail.com> wrote: > Why do opthalmologists use plus cylinder phoropters, while > optometrists use minus cylinder phoropters? [quoted text clipped - 13 lines] > > Thank you!!!! It seems that before being concerned about the minutia that distinguish the optical practices of optometrists from those of ophthalmologists, you should first learn some basic optics yourself. Bill  SignatureIraq: About three Virginia Techs a month > Iraq: About three Virginia Techs a month wow salmon egg-- i noticed your signature and i like it. its powerful and helps people put things into perspective doesn't it? so if it's about three virginia techs a month for US casualties, I wonder how it could be expressed for Iraqi casualties. Maybe three world trade centers a month. On 6/18/07 9:34 PM, in article 1182227668.239119.220940@o61g2000hsh.googlegroups.com, "p.clarkii@gmail.com" >> Iraq: About three Virginia Techs a month > [quoted text clipped - 3 lines] > wonder how it could be expressed for Iraqi casualties. Maybe three > world trade centers a month. You know, I don't care that much about Iraqi casualties. At this point I do not consider them my friends. There may be innocents among those casualties, but that is a problem the Iraqis should solve for themselves. Enough American blood and treasure has been unwisely spent to help what seems to be a helpless or antagonistic government. SignatureIraq: Now more than three Virginia Techs a month |
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