Base Curves

Question 1

Every lens surface has a ____ Power lens on the front surface of the lens

Answer 1

Plus power

Question 2

Every lens surface has a ____ Power lens on the back surface of the lens

Answer 2

Minus power

Question 3

The formula to determine Power/Base of a lens is called what?

Answer 3

Nominal Lens Formula

Question 4

What is the Nominal Lens Formula?

Answer 4

DL = D1+D2
DL= Total Power
D1= Front surface power(or BC)
D2= Back surface power

Question 5

What is Lens Form?

Answer 5

It is the relationship of the front and back surface curves.

Ex. Front Surface is +8.00 Base and the Back surface is -4.00 Base, the total power is +4.00.

Question 6

What is Vogel’s Rule?

Answer 6

A formula to help measure a Rx’s base curve recommendation.

Question 7

What is the formula for Vogel’s Rule on both plus and minus Rx’s?

Answer 7

Plus Rx: Base Curve = Rx spherical equivalent + 6.00
Minus Rx: Base Curve = 1/2 Rx Spherical equivalent +6.00

Remember - Sqherical equivalent is measured by taking 1/2 of the Cyl. power and adding it to the Sphere power.

Question 8

How do you find an Rx’s spherical equivalent?

Answer 8

Sqherical equivalent is measured by taking 1/2 of the Cyl. power and adding it to the Sphere power.

Question 9

What is a Spherical Lens?

Answer 9

A lens that has the same curvature in all directions.

Question 10

When is it not recommended to change a patients base curve?

Answer 10

When they’re sensitive to Rx changes, material, or design changes.

Question 11

What are the exceptions to not changing a patients base curve?

Answer 11

1. Long Eyelashes: Steeper curve = increase in vertex
2. Anisotropia(major differences in each eyes rx): minimize cosmetic disparity
3. High Plus Patients: Helps to keep the frame shape

Question 12

What is Plate Height?

Answer 12

the height of a lens as measured from a flat plane

Question 13

What are the benefits of a lower base curve(lower plate height)?

Answer 13

1. Flatter(less “bulge”)
2. Thinner center thickness (plus) or edge thickness (minus)
3. lighter in weight
4. less magnification (or minification)
5. Better frame retention in plus power Rx’s

Question 14

What is the downside to Lowering Base Curve (plate height)?

Answer 14

the more you lower the plate height from the optimal base curve you will induce astigmatism

Question 15

Along with Base Curve, what other measurements are important to ensure good vision?

Answer 15

• OC height
• Pupillary Distance
• Seg Height/Fitting Height

Question 16

Why not just use the lowest base curve?

Answer 16

For every base curve you move away from the optimal base curve you induce 0.13 diopters of cylinder.

Question 17

what are the 6 causes of Aberrations?

Answer 17

1. Oblique Astigmatism
2. Power Error
3. Spherical Aberration
4. Coma
5. Distortion
6. Chromatic Aberration