Hi everyone. I'm facing a problem which is new to me with a couple of manual lensmeters. First I adjust the eyepiece reticle in complete focus and then the measuring drum until i get the green target in complete focus and the drum will read zero as expected. The lensmeter will read the correct power until i reach +/-10. I'm using positive and negative spheric lenses from a trial lens set, but when i reach 10 diopters, the drum's reading will be off by +0.12. At 13 diopters it will be off by +0.50. At 16 diopters it will be off by +0.75 and so on. This is the case with a Topcon LM-6ED, but other units are showing the same problem, just with different increments (for example an LM-6E will be off by +1.25 with a +16.00 SPH). I thought it was an issue related to Topcon lensmeters but then i found a Marco 101 with the same problem. I have even found this case some time ago in a japanese lensmeter branded "Carton" which abnormal increments started from +/- 6 diopters.

Usually you would loose the drum's reading ring retaining screws to set it at zero with a focused target to get the lensmeter reading adjusted, but obviously this will not work in this case as the readings are correct in powers below 9.50, 9.75. I've been working with lensmeters for more than 10 years but still i cannot think of a probable cause for such an issue. The guess would be a manufacturing problem but off course those Topcon and Marco models have been around for enough years to be proven great units so it must be something that can be corrected.

Thanks for any insights on this matter!