Lensometer "Calibration"

[pezfaerie]

Ok I need input please because our office is split 50/50. When you "calibrate" your lenometer for your eye (zeroing it out) is the Auxiliary prisim ring knob suposed to be vertical or horizantal? I have tried to find the answer online, in manauls, in books and it never really says. So I figured the optical gurus on this site would know the answer.

[Pogu]

I'm no guru, but it seems like as long as the lines are centered on the reticule and things are in focus w/ the drum set to zero you should be good to go. We only play with the prism adjustment if we specifically need to, our two lensometers are constantly being refocused since there are 4 people using them and one wears glasses. Just set the power drum to zero, and focus the eyepiece.

[chip anderson]

When you focus the eyepiece, the black lines are what you focus. If the lensometer doesn't read zero when the black crosshairs (prism circles and crosshairs) are in perfect focus it's the instrument that needs adjusted, not the eyepiece.
Same is true with manual keratometer.
You should not adjust the eyepiece to compensate for the drum being out of adjustment.

Chip

[HarryChiling]

When you focus the eyepiece, the black lines are what you focus. If the lensometer doesn't read zero when the black crosshairs (prism circles and crosshairs) are in perfect focus it's the instrument that needs adjusted, not the eyepiece.
Same is true with manual keratometer.
You should not adjust the eyepiece to compensate for the drum being out of adjustment.

Chip

Good call Chip, I tell the people in our office to place a piece of paper in the lens mount and adjust from the most counterclockwise position towards clockwise untill the lines come into focus and stop at this point. I then have them mark that with a green, white, or red marker. They repeat it 3 times to make sure that they are getting an accurate read. I am undercorrected by about a half a diopter so when they come after me to check lenses opticians are always complaining about how they know when they should round it up to the next 0.25 or down. It's almost always an eyepiece adjustment, I am white, our insurace women is red, and the other optician is green; that way you just walk up and quickly align it and start reading.

Chip, although in some of the schools they are teaching opticians to focus the mires on zero drum reading to adjust for the drum. Not a responsible thing to teach IMO but they're teching them that either way.

[Barry Santini]

But here's the technique I think is best for multiple users of a not-completely-determined-calibration manual lensmeter:

(It is just like what Harry C alluded to above, but ss long as students learn the *why* as well as the traditional technique, I think there is no harm)

What we're really after here is an accurate reading. So, carefully set the power drum so that the *plano* marking is exactly lined-up with the dial indicator (or, for digital readout version, toggle the power precision read-out to 0.01D, and set the reading to *0.00*.)

Then, focus the eyepiece in classic fashion (from extented to retracted, to eliminate user-accomodation) until the *lensometer target* (not the reticle) is in perfect focus. Repeat several times and note where the eyepiece diopter adjustment index lines up. This technique compensates for both your eye and any miscalibration of the instrument. It's much better, in my mind, than "carrying" the power-correction factor around that is found with the traditional technique. The reticle may or may not be in focus, but who cares?

Try it!

Barry

[HarryChiling]

But here's the technique I think is best for multiple users of a not-completely-determined-calibration manual lensmeter:

(It is just like what Harry C alluded to above, but ss long as students learn the *why* as well as the traditional technique, I think there is no harm)

What we're really after here is an accurate reading. So, carefully set the power drum so that the *plano* marking is exactly lined-up with the dial indicator (or, for digital readout version, toggle the power precision read-out to 0.01D, and set the reading to *0.00*.)

Then, focus the eyepiece in classic fashion (from extented to retracted, to eliminate user-accomodation) until the *lensometer target* (not the reticle) is in perfect focus. Repeat several times and note where the eyepiece diopter adjustment index lines up. This technique compensates for both your eye and any miscalibration of the instrument. It's much better, in my mkind, then "carrying" a power-correction factor that might be found with the traditional technique. The reticle may or may not be in focus, but who cares?

Try it!

Barry

Barry did I ever tell you I love you man. :cheers:

[iamnearl]

Sounds as if you may be using a Marco 101. If that's the case, it doesn't really matter where the little knob is as long as the crosshairs are centered up.

[ktltst1]

Great advice thanks!

[Roy R. Ferguson]

Hi All:

This is for the Marco 101. According to my manuals the method also applies to B&L and others.

EYEPIECE FOCUS:
Focus of the eyepiece is the single most critical adjustment for the accurate use of the Lensmeter.
1. Hold a piece of white paper at an angel between the lens stop and the prism compensating device so as to reflect light into the telescope. Look at the lines of the reticle. They should be sharply in focus.
2. If they are not critically sharp, blur the lines by turning the eyepiece counterclockwise, then turn clockwise slowly until the 1.0 prism diopter circle on the reticle is critically in focus. Do not bracket this focal point by turning the eye lens back and forth. Come to a critically sharp focus with a clockwise rotation and stop.
3. With the power drum set at –2 or –3 diopters (red scale), turn on the instrument light with the on-off switch on the left side of the instrument below the prism compensating device. Rotate the power drum toward zero (turn top of drum toward you), while looking in the eyepiece (telescope), until the target is sharply focused. The drum reading should be exactly zero. If not, repeat Step 2 above and recheck for zero reading of the drum being careful to always turn toward zero by moving the top of the drum toward you. This same precaution should be observed when taking any reading with the Lensmeter.

Roy

[Speed]

Hi All:

This is for the Marco 101. According to my manuals the method also applies to B&L and others.

EYEPIECE FOCUS:
Focus of the eyepiece is the single most critical adjustment for the accurate use of the Lensmeter.
1. Hold a piece of white paper at an angel between the lens stop and the prism compensating device so as to reflect light into the telescope. Look at the lines of the reticle. They should be sharply in focus.
2. If they are not critically sharp, blur the lines by turning the eyepiece counterclockwise, then turn clockwise slowly until the 1.0 prism diopter circle on the reticle is critically in focus. Do not bracket this focal point by turning the eye lens back and forth. Come to a critically sharp focus with a clockwise rotation and stop.
3. With the power drum set at –2 or –3 diopters (red scale), turn on the instrument light with the on-off switch on the left side of the instrument below the prism compensating device. Rotate the power drum toward zero (turn top of drum toward you), while looking in the eyepiece (telescope), until the target is sharply focused. The drum reading should be exactly zero. If not, repeat Step 2 above and recheck for zero reading of the drum being careful to always turn toward zero by moving the top of the drum toward you. This same precaution should be observed when taking any reading with the Lensmeter.

Roy

That's what the book says. Also be sure to check the powers the same way. start well minus of the power and come up to the power, do not work back and forth. If you go past, back up to well - of the desired power. This takes out the play.

[Barry Santini]

That's what the book says. Also be sure to check the powers the same way. start well minus of the power and come up to the power, do not work back and forth. If you go past, back up to well - of the desired power. This takes out the play.

No matter how carefully and finely you turn the eyepiece or the power drum, the precision of an analogue lensmeter's readings will not match the same care applied with an good autolensmeter.

Bottom line: You need both.

B

[pezfaerie]

Wow! What a difference 5 years can make! Surprisingly the argument still lives on (at least you all helped me out, I'll let them continue to argue LOL).

Barry, what is the reasoning for having an autolensmeter? Around here people are a little techno shy when it comes to trusting a computer read out.

[Barry Santini]

Absolute vetting of power can only be done with an auto lensmeter.

the question is: why really care about absolute vetting of power?

B

[HarryChiling]

As nice as an auto lensmeter is local variations are lost and quality of the mires are often missing bits of meta data gleaned from proper use of a manual lensmeter. My personal preference is to have both available, the auto is for quick checks but I often resort to the manual to check the quality of the optics, through the clarity of the mires. I think this is more a preference than a rule.

[pezfaerie]

Absolute vetting of power can only be done with an auto lensmeter.

the question is: why really care about absolute vetting of power?

B

That was going to be my next question. That and when would it ever be necessary?

[Gizzo]

When you focus the eyepiece, the black lines are what you focus. If the lensometer doesn't read zero when the black crosshairs (prism circles and crosshairs) are in perfect focus it's the instrument that needs adjusted, not the eyepiece.
Same is true with manual keratometer.
You should not adjust the eyepiece to compensate for the drum being out of adjustment.

Chip

+1

[Speed]

No matter how carefully and finely you turn the eyepiece or the power drum, the precision of an analogue lensmeter's readings will not match the same care applied with an good autolensmeter.

Bottom line: You need both.

B

And hopfully be lucky enough to spot "waves" or abberations. I discontinued allowing final inspectors to use auto lens meters. Just too hard to spot the waves.