# Thread: Position of the Base Curve

1. ## Position of the Base Curve

Hey folks,

Got a quick questions here. When I was working in a lab a few years ago I remember that that base curve of a lens is always on the convex side of a meniscus lens. And I have only worked with minus cylinder form lenses (do any labs even do plus cylinder form lens anymore?) so that was easy enough for me to remember since the convex side was always the spherical side the base curve is situated on.

But now I am reading through Systems for Ophthalmic Dispensing and Optical Formula Tutorial book and they seem to have contradictory definition of where the Base Curve is situated on a plus cylinder lens. Systems states that the base curve is always on the convex side of a lens so that in a plus cylinder lens, the base curve is the flatter curve on the front toric surface. The Optical Formula Tutorial book also defines the base curve of a toric surface as the flatter curve BUT states that in a plus cylinder lens, the base curve is on the concave side (where the spherical surface is situated). Incidentally Systems did mention that a lot of manufactures/sources define the backside spherical surface of a plus cylinder form as the back base curve.

So for a plus cylinder lens, is the base curve situated on the convex or concave side?

2. On a plus cyl lens the lens base curve is the lower curve of the front Toric surface. On a minus cyl lens the lens base curve is the spherical curve on the convex surface. The toric surface on the concave side has a base curve the lowest curve and a cross curve the higher curve they are never referred to as the lens base curve.

3. Originally Posted by azhanli22

So for a plus cylinder lens, is the base curve situated on the convex or concave side?

The back side (concave), using the definition below.

The base curve of the lens is the curve that becomes the basis from which the remaining curves will be calculated.
https://www.optiboard.com/forums/sho...ll=1#post13572

More useful data from Darryl Meister in that thread:

Front curve = Base Curve
Weakest back curve, producing sphere power = Back Base Curve
Strongest back curve, producing cylinder power = Cross Curve

In the absence of cylinder power, the Back Base Curve may also be called the Sphere Curve.

And, in those very rare occasions when the cylinder power is on the front,

Strongest front curve = Cross Curve
Weakest front curve = Toric Base Curve
Hope this helps,

Robert Martellaro

4. "The distance portion curvature on the side that contains the multifocal" is the base curve in a multifocal.

In single vision, in minus cylinder design, it's the front.
In single vision, in plus cylinder design (does anyone do that anywhere, anymore?) it would be the back, I think, but who cares?

5. Originally Posted by drk
"The distance portion curvature on the side that contains the multifocal" is the base curve in a multifocal.
True for segmented multifocals, but not for most PALs. Darry's definition is spot on- I don't see how it can be improved on.

In single vision, in minus cylinder design, it's the front.
In single vision, in plus cylinder design (does anyone do that anywhere, anymore?) it would be the back, I think, but who cares?
Yup, it's been about 20 years since I've seen a backside Ultex (B?). Museum pieces nowadays.

Best regards,

Robert Martellaro

6. That definition is straight from Brooks and Borish's 1980 volume, or something. I was taught it in 1986.

The available PALs?
Silor Super No-Line
Sola VIP (maybe the XL was on the way)
Varilux
AO Truvision (?)

7. Originally Posted by drk
That definition is straight from Brooks and Borish's 1980 volume, or something. I was taught it in 1986.
Written a full 20 years before full backside progressives (base curve front surface, progressive optics/multifocal back surface) from Rodenstock/Multigresiv and Zeiss/Individual became available to the ophthalmic community.

Best regards,

Robert Martellaro

8. Aahh, I remember when Adaptar was released. I was at Sears Optical.

I remember when Varilux Infinity came out. That didn't go over well.

9. Hey y'all,

Thanks for all the replies! It definitely helped with clearing up the confusion. It really seems to me that the more I dig into the nitty gritty part of optics, the more confusing it gets.

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