# Optical Forums > Ophthalmic Optics >  determining rx with a lens clock

## ammoe2

:D 
Can anyone tell me how to determine a prescription using a lens clock? One of the residents here needs to know for an exam. I though he was joking as I always rely on the lensometer

Thanks in advance

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## Robert Wagner

Good day,

In using a lens clock to determine a power of a lens is very easy.

1. make sure your lens clock is "0'd" meaning on a plain base surface it reads 0.

2. Take a reading on the front surface of the lens.

3. take a reading on the back surface of the lens.

4. Add the two together and you will have the answer.

If you need to find out cylinder with a lens clock you can find the lowest number on the back surface and rotate the lens clock and the the difference is the cylinder power

I hope this helps

Robert 
;)

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## chip anderson

Robert: 

I'm afraid you can't do it very accurately anymore as most lens clocks are set for spherical crown glass index lenses.  We don't use this much anymore as the indices vary so much and aspheric surfaces don't clock well.

Chip

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## Oha

There is a formula to compensate for the correct index of refraction- Im sure Darryl would know it.

And dont forget that thickness too, especially in higher plus prescriptions, can effect the results.

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## Darryl Meister

Most lens clocks are calibrated for a 1.530 reference (or "tooling") index. To convert the surface power measured with a lens clock to the correct, or actual, refractive surface power use:

Actual Power = (Index - 1) / 0.530 * Measured Power

So, if you measured 6.00 D on a polycarbonate (index = 1.586) lens, your actual surface power would be:

Actual Power = (1.586 - 1) / 0.530 * 6.00
Actual Power = 6.63 D

Remember for concave surfaces to use the red or negative numbers on your lens clock. Also remember to hold your lens clock perpendicular to the surface to get an accurate reading. Lens clocks will only give you correct readings on spherical and toric (with cylinder) surfaces. Aspheric and progressive lenses cannot be measured accurately with lens clocks.

Best regards,
Darryl

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## chip anderson

One more small pearl on Geneva lens clocks.  Each mark on the dial is .01 mm depression of the center pin relative to the two other pins.  It can therefore be used as a thickness gauge on contact lenses resting convex down on a flat surface.

Chip

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## Shutterbug

And just a additional note - this procedure is not going to give you an accurate rx, just an estimate.  You're still going to need a lensometer if you want accuracy.  I'm not sure why they would even include this procedure on a test.  It would be more practical to discuss measuring slab-off prism with a lens clock.

:D 

shutterbug

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## DC Optix

Raising this one from the dead...


Need some help.  When using the lens clock in this way, on a toric back surface, how do you determine which curve from the back to use together with the front curve to determine the spherical power?

Thanks in advance!

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## rbaker

Neutralize the unknown lens with trial lenses. Its very accurate and index of refraction is not an issue.

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## chip anderson

First determine which axis you want to be the sphere power, + or - cyl. Then just subtract the back curve on that axis from the front curve. This will leave some error if one doesn't know the index. Or as suggested you can neutralze. If plus you can determine the focal length in each axis then you will know. If minus you have to neutralize the focal length with a plus lens of at least equal or greater power..

Chip

Guess the old lens bench concept just isn't taught anymore.

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## DC Optix

Please forgive the ingnorance :(

How do I determine the axis using ONLY a lens clock?  

The lens clock will be the only tool available and I will have to estimate the Rx using only the surface curvature readings.  Lets say I clock the front and it's +6.25 and find the back to be -6.75 and -7.25.  I'm unsure as to which backside reading I use in combination with the front reading to calculate the spherical power.

Or am I completely wrong on how to calculate it?

Thanks for everyone's help...this just isn't sticking for some reason :hammer:

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## Wes

You will be clocking crown glass lenses for the practical.  The rx should be in minus cyl form. Given the readings you provided use the front reading and the lowest power back reading to get the sph power. Then take the difference between the lowest back reading and the highest back reading. this gives you the cyl value. Total lens power is  -.50 -.50  PM me if you need more assistance.  Wes, SC Licensed Optician

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## chip anderson

Determine axis with lens clock:  You will have two axial planes 90 degrees apart.  if you know the way the lens will sit in the frame, you can get a pretty good but not to the exact degree with a lens clock.  If the lens is unmounted and round you are SOL.   If mounted it can also be determined pretty accurately by just holding the frame/lens at arms lenght and viewing a straight line and tilting the lens a bit.   An experienced optician can usually tell this within a couple of degrees just by looking  Within maybe 5-8 degrees with a lens clock.

Chip

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## Wes

For what I suspect he is asking, he'll be using loose lenses and will have to give front curve, back curve/s and nominal power within .12 diopter.

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## harry a saake

if its a loose single vision lens, toric, then it has no axis until it is edged into a frame, so the answer would be the two powers and any axis you want to use for the moment from 1 to 180

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## Bob MacNeil

Yes, I agree,  trial lens each meridian, and they can learn the added benefit of finding prescribed prism. 

Bob

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## Wes

The SC practical exam does not require you to provide a made up axis for loose lenses. Is this what you are inquiring about? If so you have your answer. If not, Use Chip's answer for estimating the axis, though I know of no real world situation in which you'd be required to provide axis measurements with only a lens clock.
As stated earlier, if this is for the SC practical, and given the nature of the question, your location, screen name, and the timing of the question, I assume it is; this is the requirement: provide front curve, back curve/s on 5-6 glass finished single vision blanks, and nominal powers on each. No axis.
Does this cover it?


Wes

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## rbaker

In my humble opinion a journeyman optician should be able to neutralize a pair of lenses (including prism power, if any) with a set of trial lenses and achieve at least the accuracy of an autorefractor. Granted, it's not easy and requires practice but once you become proficient you should be able to neutralize to the tolerences of the ANSI standards. 

Please forgive me but I must say "back in my day" we did many out of office clinics and the only kit we carried was a Schiotz tonometer. ophthalmascope, tangent screen, Snellen chart, and a trial frame and lenses. 

I certainly do not suggest anyone throw out all their nice new equipment but being proficient in the original equipment and procedures puts you heads above your colleagues. For example, if you can neutralize a spectacle lens you will find retinoscopy and refraction duck soup.

I would assume that the graduates of the contemporary optical schools are proficient in these skills. If not, shame on you.

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## Fezz

For cripes sake! 

Hold the blasted lenses up to the light, observe, and make an educated (or uneducated) guess! 


Life is WAY too short people!!!




;):cheers::cheers::cheers::D

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## DC Optix

Thanks everyone!  Very helpful...I think I've got it now.  I was making it a lot harder than it actually is.  And yes, Wes, this is for the SC practical.  Although I understand there are changes on the horizon for the format of the test.

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## YrahG

A few simple tips for you.

To determine the back surface meridian to measure, measure the thickest meridian and the thinnest meridian that are 90 degrees apart this generally corresponds with the major and minor power meridians.Always write it in (-) cyl, consistency.The spherical equivalent is a constant, so any 2 meridians 90 degrees apart are going to give you a spherical equivalent that is the same, which means the spherical equiv can be used to make sure that your measures are accurate.

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## Darryl Meister

> When using the lens clock in this way, on a toric back surface, how do you determine which curve from the back to use together with the front curve to determine the spherical power?


The short answer to your question is to 1) place the lens clock against the back surface of the lens ensuring that it perpendicular to the surface, 2) gently rotate the lens clock at least 180 degrees, and 3) note the lowest reading.

The lowest reading corresponds to the sphere power of the lens in minus-cylinder form. Add this value to the power of the front surface in order to detemine the sphere power of the lens. You may also have to compensate for the refractive index of the lens material.

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## DC Optix

Thank you very much, Darryl!  :cheers:

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## samthomCPOA

This method is used with meridian crosses on written tests.  Very basic, how to determine if axis is 90 or 180: The first cross represents the front surface of the lens. These are plus numbers and should be the same.  The second cross represents the back surface.  These are minus numbers. Add the numbers together and put them on the third cross keeping the 90 degree measurement at 90 and the 180 degree measurement at 180.  The highest plus meridian tells you your axis.   

For example, the front surface is +8.00 at 90 and 180.  
The 90 degree back surface is -3.00, 
the 180 back surface is -4.00.  
Your 90 degree sum is +5.00 (+8.00 plus -3.00 = +5.00)  
Your 180 degree sum is +4.00 (+8.00 plus -4.00 = +4.00.)  
The highest power is recorded first and therefore determines the axis.  +5.00 is the highest plus so this lens rx is +5.00 -1.00 x 90.

If the numbers were reversed and the highest plus power was on the 180 line, your axis would be 180.

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## DC Optix

Now THAT is a resurrection from the dead!  LOL

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## loose-screw

I am studying for my ABO exam in May and that is where the optical cross comes into effect
one reading at 180 meridian and one at the 90 meridian on the back side and using the front curve to subtract the difference

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