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    pd on digital lenses

    On a digital lens, do you measure pd on the front side or back side of the lenses?

    #2
    Curious how much difference this could create and would you return them without having the patient try them?

    It seems the minuscule difference would be within tolerance but I'm willing to be corrected with nothing harsher than a wet noodle please.

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      #3
      It surely depends on the Rx and base curve. Take an 8 bc lens in wrap frame 54 eye when you will find the front pd larger by 3 or 4 mm than when measured from the back. The question arises that normal practice says measure from the front and you will get push back from everyone. If you have a frame with a 4 bc and you put 8 bc lenses in the frame the frame pd will change and often you will end up out of tolerance when completed. There is a solution anyone want to to take a guess.

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        #4
        It doesn't matter, if you do it correctly. Or just do it simply. Line up the 34mm mark on your PD stick with the nasal engraving of the right eye lens, and measure to the nasal engraving of the left eye lens. Or use the zero of your PD stick doing to same procedure and add 34mm to your result. There is little to no wrap to interfere with the measurement between the nasal marks to make more than a hair of difference.

        Edit-added a comma
        Last edited by Kwill212; 03-18-2023, 01:38 PM.

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          #5
          Kwill
          i don’t think that was the question but I could be wrong. You are trying to get a three dimensional reading with a two dimensional measuring device.

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            #6
            You don't think the question was

            "On a digital lens, do you measure pd on the front side or back side of the lenses?"

            What do you think the question was if it wasn't exactly what was stated in the question? You seem to be talking about issues that arise in the surfacing layout of wrap lenses, and not how to verify the correct PD of finished spectacles.

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              #7
              Put a dot 17 mm to right of the nasal dot on both lenses and measure the distance between your result. This is where the eyeball is actually looking.

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                #8
                Originally posted by Lensman11 View Post
                Put a dot 17 mm to right of the nasal dot on both lenses and measure the distance between your result. This is where the eyeball is actually looking.
                Geez it's painful engaining with you. I regret it every time, but it seems like I'm just a glutton for punishment. I guess if we're going to go down this path we might as well go back to the beginning and ask AK47 to state exactly what they mean when they say "digital lens". Then we can go though the next 45 steps of lens making until we get right back to where we were 5 minutes ago.

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                  #9
                  Originally posted by ak47 View Post
                  On a digital lens, do you measure pd on the front side or back side of the lenses?
                  Troublemaker!

                  I think we are talking about an etching on the outer surface (convex side) versus the inner (concave) surface where there is no etching, so the apex of the cornea is looking through the optical center based on the middle of the lens thickness. Or not...

                  The difference of that thickness affecting the pd would be minuscule imo.

                  But I still always put the glasses on the wearer first whether I think they're right or wrong and see if they complain!

                  I'd troubleshoot from there where this this would be one of the last arrows from the quiver I would target as the problem.

                  Today's digital lenses make me assume if my entry is right the lens power and OC's placement is not wrong.


                  Now be nice kwill if I'm wrong.

                  Eventually even wet noodles from guru's can hurt...

                  Comment


                    #10
                    Originally posted by ak47 View Post
                    On a digital lens, do you measure pd on the front side or back side of the lenses?
                    For all lenses, for verification, place the manufacturer's mask on the front of the lens, then on a centering/cutout chart (see below).

                    The final evaluation is with the adjusted/fitted eyeglasses in front of the eyes using a light/corneal reflex.

                    https://www.essilorpro.com/content/d...91_PRO_VAR.pdf

                    Hope this helps,

                    Robert Martellaro
                    Science is a way of trying not to fool yourself. - Richard P. Feynman

                    Experience is the hardest teacher. She gives the test before the lesson.


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                      #11
                      Okay, original poster here, my concern is mainly about wraps in stronger Rx. A couple mm can make all the difference between base in and base out prism. I believe we are supposed to measure digital lenses on the backside, I was really hoping that someone with more knowledge than me could confirm that.

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                        #12
                        I need an education, here. Please help an old guy out.

                        Question #1: What is the practical difference between measuring the distance between two points separated by, say, the thickness of 3 millimeters? I would think measuring two dots on the front of a wrapped frame would be almost exactly the same as measuring the inside.

                        Question #2: Is it even practically possible to measure the linear space between two points on a curve? That would be like a chord (from geometry), right? Wouldn't there be parallax error because the flat ruler would hover over the gap between the ruler and the curved-away-from-you lens? (And bending a flexible ruler just takes you into a different, warped-space universe, so that's a no-no.)

                        Question #3: Isn't such stuff modified, anyway, with the software when the lenses are made (if you do a specifically POW-compensated lens design)? I mean, you give the (admittedly imperfect) pupillometer readings and they know how to manipulate the optical centers? (Hint, they have to move them out about 1 mm per lens with ~15-20 degrees of wrap, for those who like to do their own wrap-designing.)

                        Question #4: In my world, there's no sense in "verifying" measurements when you order a POW lens. The powers won't match. The optical centers won't match. It's a thing I just take on faith.

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                          #13
                          Robert, you're always coming up with something interesting!

                          So, you are introducing a method that works on aligning the pupils' optical axis (I think that's what you find from a corneal reflex) and the lens' front and back surface reflections? (The back reflection would be of variable size depending, I'd imagine.)

                          Comment


                            #14
                            Originally posted by ak47 View Post
                            Okay, original poster here, my concern is mainly about wraps in stronger Rx. A couple mm can make all the difference between base in and base out prism. I believe we are supposed to measure digital lenses on the backside, I was really hoping that someone with more knowledge than me could confirm that.
                            Powers don't matter, although the fitting point and PRP tolerances are not as tight on a low power SV lens.

                            A little more detail from Barry.

                            https://www.optiboard.com/forums/showthread.php/55691-How-do-YOU-measure-PDs-on-wrap-sunglasses
                            Last edited by Robert Martellaro; 03-22-2023, 11:06 AM.
                            Science is a way of trying not to fool yourself. - Richard P. Feynman

                            Experience is the hardest teacher. She gives the test before the lesson.


                            Comment


                              #15
                              Originally posted by drk View Post
                              Robert, you're always coming up with something interesting!

                              So, you are introducing a method that works on aligning the pupils' optical axis (I think that's what you find from a corneal reflex) and the lens' front and back surface reflections? (The back reflection would be of variable size depending, I'd imagine.)
                              The corneal reflex and a reference point on the front of the lens. Also used for measuring IPDs. I used the device below to assist with determining the IPD using a light for the corneal reflex. I would transfer that data to the lens on the fitted frame (before the lab work) to determine IPDs and fitting points. Essentially the verification process in reverse.

                              Hint: when using a centering chart and/or the pd meter shown below, occlude the fellow eye and compensate for parallax error when there are significant differences between the fitting point value and the measurer's IPD.
                              Attached Files
                              Science is a way of trying not to fool yourself. - Richard P. Feynman

                              Experience is the hardest teacher. She gives the test before the lesson.


                              Comment

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