Wide Pds. With High Minus Lenses

[David@LUXE]

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.

[PRECISIONLAB]

Since plus power lenses varify and optical center blocked jobs also varify you can eliminate equipment issues. The number that stands out is the fact that jobs are consistantly off 1.5mm which is usually a standard inset value. Check your LMS and make sure correct values are being entered for PD's. Siince plus lenses are not affected perhaps the system is not compensating inset. If the distance PD's are entered in the near pd field the 1.5 is added to achieve a distance value. Layout a test job ..high minus with narrow pd which will calculate -6.0 decentration and varify by manually calculating decentration via measured box method.

[David@LUXE]

Not the LMS. Depending on power and/or amount of decentration compensation could be .5, 1.0, 1.5 or 2.0 additional millimeters. If left uncompensated jobs could be off anywhere from .5 to 2.0 millimeters in each eye. If the LMS was insetting 1.5 mm than the lower powers would be off also. Will check this out first thing tomorrow. I have some help with this tomorrow and will post results. Hard to believe this thread has had nearly 200 views and only one response. Thanks for you help Precisionlab

[Uncle Fester]

Hard to believe this thread has had nearly 200 views and only one response. Thanks for you help Precisionlab

Your headline makes all of us look while you really need a headline that says something like "National Optronics Edger problem".

Then those of us who do not deal with edging would ignore it.

This is also a problem that very few Optiboard posters would be able to help with imo.

Maybe you can ask a moderator to change the headline?

Kind regards and I hope Precisionlab answer works!

[Jubilee]

If this is happening with every combination of blocker and edger then it leaves only a few places to check.

My first thought is the lensmeter. The higher powers would be more sensitive to slight amounts of prism or pin issues.

Second is the LMS. Perhaps certain skus have had misinformation or a recent update had some issues.

Any reason why you aren't blocking OC for all? Are you sure it is not only with one specific blocker or edger?

Are,you double checking the two lensmeters with each other to verify accuracy? I recall working with a few auto lensmeters that had trouble with higher powers and higher index, though most of that was with power not prism.

And yes since this is prism you can manipulate Prentice rule to figure if tthe amount of prism is consistent.

[icmor]

After going through a similar situation, with a different brand of equipment, we found that it was indeed the pins on the lensometer were the issue. Spot a mid to high power lens in the lensometer; flip it 180 degrees, and spot again. If it's spotting in two different places, it's the pins.

[Jubilee]

Now talking with a few of the lab guys at the VA, they have had issues with the 3B blockers and their display inducing error on the high minus lenses. The thickness of the len edges would displace the image by small amounts.

[ml43]

Now talking with a few of the lab guys at the VA, they have had issues with the 3B blockers and their display inducing error on the high minus lenses. The thickness of the len edges would displace the image by small amounts.

easily verifiable by blocking using a pad with no center, then measuring the decentration from the back of the lens. especially if it's more than 1mm :)

[TokaiAU]

I had the same issue with my Mr.blue combo. Mostly on high minus with smaller PD. I now use stickers and leap pad both sides of the lens and now have no issues. It turns out that the chuck pressure was pushing the blocks to the center of the lens while clamping. It drove me mad for months because the Cyl was always correct, just the PD was off.

I hope the Aussie terms translate.

Cheers

Leighton