Help!!!! Let me begin by saying that all equipment is properly calibrated and operating well. The equipment used in the process is as follows: National Optronics 3B Blockers, Mr. Blue Blocker, 7E and 7EX edgers, MEI, Marco 101 lensometer and Tomey auto lensometers.
We have been experiencing wide pds. on high minus lenses (all materials that are -4.00 and up) which call for 3 or more mm. of decentration. No problems with plus lenses. We have been able to compensate for this problem by adding more decentration when blocking with the 3B's or taking off from the Pd when blocking with Mr. Blue. All jobs are edged on geometric center. We do edge all 1.74 on optical center and there is never a problem when blocked on center. For instance a -6.00
-2.00 which calls for -6 will actually be blocked at -7.5 in and the pds are good. If blocked at -6 in it would definitely be wide. Does this have anything to do with prism and can the prism formula be applied? Is it parallax? Is this caused by the light bending?
Any advice would be much appreciated.
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