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    #16
    For new opticians, don't think that an Rx written in plus cylinder form asks for a lens with the toric curve on the plus side - it might have meant that decades ago, when plus cylinder lenses were common, but as has been said above there are no plus cylinder lenses now, however the Rx is written.

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      #17
      Great posts already, but since I see the OP mentioned an existing lens with some of the cylinder on the front... I believe that may be the Hoya ID series progressives (ground as crossed cylinder lenses).

      But, even for those, plus or minus cylinder are just treated as different forms of prescription notation as far as the lab is concerned. The fact that the lens forms the ordered addition by combining cylinder at perpendicular orientations between the front and back surface, is coincidental - for prescribed cylinder, this particular cylindrical component is ground on the back surface of the lens (after accounting for the crossed cylinders that form the addition component), making it closer to a minus cylinder lens despite undoubtedly having some plus cylinder ground on the front surface of the lens (which has nothing to do with the wearer's prescribed cylinder, just the addition).

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        #18
        Originally posted by Prentice Pro 9000 View Post
        Kwill212 as I said it was a slow day

        We have to be curious sometimes, no?

        thank you for the pics! These are very helpful. To be clear, the first one would read as +3.00 +4.00 *180 correct?
        +3.00 +4.00 x 090. The axis (no power meridian) of the cylinder is vertical, axis 090. The front surface of the lens is steeper in the horizontal meridian.

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          #19
          Originally posted by Kwill212 View Post

          +3.00 +4.00 x 090. The axis (no power meridian) of the cylinder is vertical, axis 090. The front surface of the lens is steeper in the horizontal meridian.
          ah yes yes I see now. Thank you!

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            #20
            All that stuff predated me but I learned in school in the '80's that there are two distinct subjects:

            1. Plus cylinder notation, which still exists and is more a convention now, than anything. There are plus cylinder refractors, too. They're easier to use when you do manual retinoscopy, which has virtually disappeared as well. But it's only about writing prescriptions/orders. It has nothing to do with lens design.

            2. Plus cylinder lens form or design, where instead of the toric surface being on the back, it was on the front. As was mentioned it was supposedly difficult to wear a lens with toricity on the front vs. back (vertex?) or at least to flip between the two. In fact, we were told to write "minus cyl form" on all our Rxs so it wasn't made otherwise.

            As to why plus cylinder lens form was popular at all, probably the manufacturers on here could tell us. There had to be some advantage. Fused front surfaced glass bifocals probably necessitated minus cylinder surfacing, but yeah, back surface bifocals like Ultex would have required front surface cylinder. SV could go either way, I guess.
            Last edited by drk; 05-21-2024, 05:57 AM.

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              #21
              Oh, and to be confusing, "eikonic" lenses to minimize aniseikonia were "bitoric" with front and back cylinder. Not that I've ever done that in 35 years.

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                #22
                Originally posted by drk View Post

                As to why plus cylinder lens form was popular at all, probably the manufacturers on here could tell us. There had to be some advantage. Fused front surfaced glass bifocals probably necessitated minus cylinder surfacing, but yeah, back surface bifocals like Ultex would have required front surface cylinder. SV could go either way, I guess.
                Tell me if I'm wrong. My assumption is that +cyl used to be the norm everywhere. Dr.s refracted in it and the lenses were made with it. The industry figured out -cyl manufacturing and since OD's work more closely with opticians it just made since for them to make the switch.

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