Lensmeter calibration problem with high power lens

[grandotitat]

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!

[grandotitat]

Anyone with an idea?

[PRECISIONLAB]

Neutralizing high plus Rx requires back vertex verification. Try flipping the lens around and retake reading. The additional Plus is the result of magnification created by the curvature and thickness of the lens

[Lensman11]

Lensometers always read back vertex power so you should not turn the lens around. The only time you reverse the lens is to read the add power of a lens. The reason that is necessary is because the add on the lens is not against the lens stop so it will read stronger than it was ordered. You must read the distance portion and the add portion turned around the difference is the power of the seg.
Lensometers due to their design will always be inaccurate in the extremes of the range. As the instrument wears it will get worse as the gears on the rack wear unevenly. You can make an adjustment at the eyepiece for several ranges. Use your trial lens at 10 15 and 20 and adjust so the reading is correct at each position and record the adjustment position of the eyepiece for future setting when needed. Second choice is use the trial lens matching the sphere power and zero the instrument for that one time use.
Digital lensometers do not have this issue. I have had many an optician call me to say we sent a lens 0.50 off power and after I tell them to check against a trial lens they always were amazed that lensometers could be wrong.

[grandotitat]

Neutralizing high plus Rx requires back vertex verification. Try flipping the lens around and retake reading. The additional Plus is the result of magnification created by the curvature and thickness of the lens

I will check anyway. It doesn't hurt to see the difference flipping it. This lensmeters are being used is a contact lens lab, and there's been always the dilema if the lens should be read with the concave zone looking upside or downside. I've always seen the lensmeter manuals indicating the concave part should be facing down (towards the objective lens) but optometrists say that the reading should be the same no matter the way it is facing if the lens is good (talking of spheric lenses at least).

Lensometers always read back vertex power so you should not turn the lens around. The only time you reverse the lens is to read the add power of a lens. The reason that is necessary is because the add on the lens is not against the lens stop so it will read stronger than it was ordered. You must read the distance portion and the add portion turned around the difference is the power of the seg.
Lensometers due to their design will always be inaccurate in the extremes of the range. As the instrument wears it will get worse as the gears on the rack wear unevenly. You can make an adjustment at the eyepiece for several ranges. Use your trial lens at 10 15 and 20 and adjust so the reading is correct at each position and record the adjustment position of the eyepiece for future setting when needed. Second choice is use the trial lens matching the sphere power and zero the instrument for that one time use.
Digital lensometers do not have this issue. I have had many an optician call me to say we sent a lens 0.50 off power and after I tell them to check against a trial lens they always were amazed that lensometers could be wrong.

That is a good point. The best idea at this point is to be aware of the difference or get a new lensmeter. Thanks for the insight!

[lensmanmd]

I have had many an optician call me to say we sent a lens 0.50 off power and after I tell them to check against a trial lens they always were amazed that lensometers could be wrong.

[Howard Gorin]

You may have a problem with vertex distance. Frequently blocking pads, felt rings or other items are placed on or around the lens contact to keep contact from damaging the lens. The resulting error is progressive, greater error with greater lens power. The position of the lens contact is adjustable to calibrate the instrument. Wear between the rack and pinion gears is unusual. I have serviced Lensometers, Vertometers, etc and I have never seen that happen. A Lensometer or Vertometer in good repair should be accurate over the entire range +20=-20 diopters