# Optical Forums > Ophthalmic Optics >  Checking base curve

## eyechicky

How do you properly/precisely check the base curve on a FT bifocal, progressive or aspheric lens using a lens clock. I have gotten mixed reviews and want to be accurate in my technique and results. 

I have found that on a FT lens, I read with the pins horizontal just above the seg line.

On a progressive lens I have taken the higher of the vertical and horizontal measurements I have gotten. I use the same method for aspheric lenses.

Your insight is much appreciated.

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## Darryl Meister

Unfortunately, it is impossible to measure accurately the base curve of any non-spherical lens (including aspherics and progressives) using a conventional lens clock. The curvature of the surface of such a lens changes across the width of the lens clock pins, which are about 19 mm apart. Consequently, the lens clock (calibrated to show measurements from a spherical surface) is "fooled" into thinking the surface is a _spherical_ surface of value X.XX D instead of an _aspheric_ surface of value Y.YY D. The more aspheric the design is, the more the lens clock will be off.

When using the lens clock, it is important to ensure that your lens clock is perpendicular to the lens surface. You may also want to take both horizontal and vertical measurements, and then average them out, in case the lens is warped at all. Finally, since the lens clock is calibrated for a reference or _tooling_ index of 1.530, you will need to either compare the measurement to the 1.530-based value from the manufacturer (called the _true curve_) or convert the value over to the actual power of the surface using the equation:

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

Best regards,
Darryl

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## Aarlan

You can get a decent reading using a small aperture sag gauge (20mm), depending on the level of aspericity.  You'll be off by a bit, but it is far more accurate than a lens clock.

AA

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## Ed_Optician

Now we are splitting hairs  regarding base curve. Thankfully ANSI tolerance is +/- .75 D from specification.  Every brand of lens uses a slightly different series of base curves.  Ideally we should spend more time worrying about the back curve rather than the front curve so that we maintain a constant vertex distance for the pair of glasses not just matching the old front curve

Ed

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## tyler575

> Unfortunately, it is impossible to measure accurately the base curve of any non-spherical lens (including aspherics and progressives) using a conventional lens clock. The curvature of the surface of such a lens changes across the width of the lens clock pins, which are about 19 mm apart. Consequently, the lens clock (calibrated to show measurements from a spherical surface) is "fooled" into thinking the surface is a _spherical_ surface of value X.XX D instead of an _aspheric_ surface of value Y.YY D. The more aspheric the design is, the more the lens clock will be off.
> 
> When using the lens clock, it is important to ensure that your lens clock is perpendicular to the lens surface. You may also want to take both horizontal and vertical measurements, and then average them out, in case the lens is warped at all. Finally, since the lens clock is calibrated for a reference or _tooling_ index of 1.530, you will need to either compare the measurement to the 1.530-based value from the manufacturer (called the _true curve_) or convert the value over to the actual power of the surface using the equation:
> 
> *Actual Power = (Actual Index - 1) / 0.530 * Measured Power*
> 
> Best regards,
> Darryl


Darryl ,

please can you expand on this for todays progressives ... My Dr insists that you can measure a base curve for a progressive lens ( Definity and Physio ) with a lens clock . I have been taught that this is not possible for the reason you state above . Please can you confirm that this is still the case .

Many Thanks
Jayne

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## Darryl Meister

> My Dr insists that you can measure a base curve for a progressive lens ( Definity and Physio ) with a lens clock . I have been taught that this is not possible for the reason you state above


It isn't possible to measure the base curve _accurately_, although you could probably at least get in the ballpark. So, you couldn't use the measurement to do lens calculations, but the measurement would probably suffice for something like picking a new base curve for the patient, which are typically only available in 1- or 2-diopter increments, anyway.

The more aspheric or soft the lens design, the less accurate your measurement will likely be. (Definity, for instance, is a very soft design.) When taking the measurement, you'll want to hold the lens clock horizontally across the distance zone as high on the lens as practical.

Best regards,
Darryl

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## tyler575

Thank you so much for your reply !
Jayne

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