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Optical Center and Prism

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    Optical Center and Prism

    I know this has been discussed before-I should understand this by now, but this is confusing...I have an individual whose Rx is OD: -3.75-0.50x045, OS: -4.00-1.00x090! The FPD: 70 (51X19), PPD: 63, " B " size is 44mm.
    Normally, decentration would be 3.5mm in...However, I noticed this persons eyes were ( after checking the frame for excessive wrap, proper face form etc. ) 4mm up from the Datum line and 4mm in towards the nose. I figured since I'm fitting this guy with a mid-index ( Spectralite 1.54 ) it's almost like peeking through a hole in a fence, i.e., this Rx has got to be right over the pupils!... My lab called me and said, " We can't decenter anything over 3mm because you ( meaning me as an Optician ) would be inducing unwarranted prism! Now my OD says, " Well, what difference does it make since the lenses are being surfaced with this Rx; your ( meaning me again ) just telling the lab where his pupils are exactly located ".... Alright so let's see, power in the right should be -4.00 ( since we are 45 degrees away from the 90th, that's 50% of cyl. ) and we are already at 90 degrees in the OS; that remains at -4.00... If we go 4mm up and 4mm in on each eye that's 8mms of movement in each eye... -4.00x.8= 3.20^BU...I believe when figuring vertical imbalance and the directions are the same these figures cancel each other... Ladies and Gentlemen.....your assistance please!

    #2
    Eyecon,

    Actually your labs is not correct about telling you "3 mm".. Sola Spectralite is a aspherical design, which means you can NOT decenter that OC ANY durring surfacing using prism rings!!.. the geometric center of the lens (front apex) should be located EXACTLY over the mechanical center (OC) when we grind in the power.. where you place that optical center, by moving the lens around is up to you.. you the only reason I like getting the measurmennts (frame, oc, PD) is to make sure when I grind this lens on center and you move it before blocking and edging that the lens blank will be big enough to let you do that...
    If your lab is using prism rings when surfacing a aspherical design to gain decentration they are doing it incorrectly! :)
    Because of the nature of design (front curve) you have to cut all aspherical surfaces on center.. if not you'll have axis and power problems galore.. we had a pretty good thread talking about aspherical surfaces a while back on here somewhere that explained in great depth about how the curve design effects the axis and power when at an oblique angle from the apex (geometric center)of an aspherical design :)...
    More or less you grind it on center, move it where ever you need to and then block it to edge..you just have to watch out when picking a frame and make sure you have a large enough lens surface to let you get the decentration and OC placement you need..You just go through the same process just like taking a "stock" finished SV..you dot it up on your lensometer, right?.. then you just "move" the whole lens to move the OC to where you need it to get the proper PD. Same way you do a spectralite :) Be it SV or Bi-focal you grind it on center and move the whole blank.. I think the were more or less confussing you and they are confused BTW I just ran it through my surfacing program and "if" you wanted it 4 above and with that 63 PD you can go all the way upto a 65 ED and it would still cut out..it would be close but it would work... with a 60 ED it ended up needing a 74 mm blank.. and since they are 80 mm blanks then it should be no problem at all.. BTW with the oc being only 4 mm above the datum, you shouldn't raise it up to the center of the pupil.. unless you have ZERO panto tilt :)

    Jeff "sometimes lab rats do dumb things too" Trail

    [This message has been edited by Jeff Trail (edited 09-27-2000).]

    [This message has been edited by Jeff Trail (edited 09-27-2000).]

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      #3
      Dear EyeCon

      If I were you . .
      I'd call your "lab guy" and tell him to take his fat hands off the prism rings, surface on center so that the OC is undisturbed, then decenter the lens to match PD. He should know better. Even an EyeMasters Lab Tech knows better . . . I think!
      If there isn't enough lens that was your responsibility, if your calcs were right there should be no problem.

      Nite nite!

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