Just as the the title says for edging the lens.
I spent about 30 minutes moving the lens around the lensometer trying to find OC and could not for the life of me understand why the cross lines didn’t move.
Durrr.
Just as the the title says for edging the lens.
I spent about 30 minutes moving the lens around the lensometer trying to find OC and could not for the life of me understand why the cross lines didn’t move.
Durrr.
Don, OMD's get zero training in lens fabrication. Plus, Dr. Moore has been here for 10 years...
Hi Doc. All lined bifocals and trifocals are layed out by the seg using the near PD as reference. The surfacing lab would grind the decentration between near and far. Also, keep in mind a plano power has no prismatic effect unless the lab ground it in on purpose or by accident. With your above lens, just use the NPD as reference and layout the lenses seg accordingly along with whatever drop or raise from datum. Upon verification note that the segs measure to the NPD and that you don't have a prismatic error in the distance area more than 1/3 D.
I hope this helps.
Thank you for understanding! We are actually trained to avoid any knowledge of lens fabrication. It makes it much easier to avoid embarrassment and deflect blame when a patient gets mad. It's quite hard to shake the old habits.
Don might be able to hear the gears in my brain grinding all the way up in Cincinnati as I try to parse your instructions. I'm pretty sure when I get a chance to put my hands on it again I will be able to work though it with this advise.
The ancient texts I have refer to using the distance MRP/OC which I obviously cannot find on this plano lens. Will we get better results if we always do a near reference point?
Thanks again.
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Yes Doc, almost always use your near reference point ( the seg) when laying out (blocking) any FT lens. Once again, if your lab surfaced your FT correctly, they put the Distance OC out from the near segs OC 1.5 -2 mm usually to account for convergence at near. Let me give an example;
Frame PD (A measurement + DBL (distance between lenses) minus near PD/2 = decentration of each SEG.
Exp;
frame 52/18 = FPD of 70
Pt's PD = 63/60
seg height 15
Frame B measurement = 34/2 = 17 (datum)
70 - 60 = 10/2 = 5mm17 - 15 = 2mm
So you'd layout the seg 5 in and 2 down. If your lab ground the right amount of decentration (1.5 in ou) your distance PD will be correct also.
Once again, on your plano lens, There is no power so there is no prismatic effect . You can move the lens all around in the lensometer, but the "OC" is not gonna move. With your plano in dist. all you need to worry about is unwanted ground in prism. But the seg does have power, therefore, you need to get your NPD correct.
Check your edger manual. Lined multifocals are nearly always edged using the near PD and seg height, and your edger probably has a setting for exactly this. Our Santinelli, for example, simply has us choose the "BF" setting and bblock the lens up with the seg 5 in and 5 down.
I'm Andrew Hamm and I approve this message.
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