Base Curves

Question 1

What does the base curve influence?

Answer 1

it is one of the spectacle lens parameters that most directly influences the patient’s vision as well as the appearance of the lens.

Question 2

What does single vision Rx usually have?

Answer 2

A spherical front curve which is the base curve. Technically it is possible to have a plus cylinder form spherocylinder that hs two curves on the front where the weaker or flatter of the two curves is the base curve.

Question 3

What happens if the wrong base curve is chosen?

Answer 3

the patient will lose clarity as they move their eyes through the lens periphery due to increased lens aberrations.

Question 4

What are aberrations commonly controlled with?

Answer 4

base curve selection that are:

spherical aberration, marginal or oblique astigmatism and distortion.

Question 5

How does spherical aberrations happen?

Answer 5

when parallel light rays enter a large area of spherical lens surface.

Question 6

What happens to the peripheral rays due to the curvature of the lens?

Answer 6

It will focus sooner than the central rays and cause a slight blurring of the image. In general this aberration is not a significant problem because the small aperture of the pupil is effective in keeping out of these aberrant peripheral rays.

Question 7

When does a marginal or oblique astigmatism occur?

Answer 7

when a small bundle of light strikes a spherical lens surface from an angle.

Question 8

With the situation of the marginal or oblique astigmatism, what does the rays in the opposite meridians tend to do?

Answer 8

to focus at different points.

Question 9

What is the distance between the two points in marginal or oblique astigmatism?

Answer 9

The distance between the two points of focus equals the degree of astigmatism created.

Question 10

What led manufactures to develop with the situation of marginal or oblique astigmatism?

Answer 10

to develop of corrected curve lens series.

Question 11

When does distortion occur?

Answer 11

as the light rays move from the center of the lens toward the periphery.

Question 12

As the rays approach the edge of the lenses in distortion, what increases?

Answer 12

increases magnification causes the image to be distorted.

Question 13

In the case of strong plus lenses, what is distortion seen as?

Answer 13

an inward curvature known as the pincushion effect.

Question 14

In the case of a strong minus lens, what is the distortion seen as?

Answer 14

an outward bulging known as the barrel distortion.

Question 15

What is the Corrected Curve Theory?

Answer 15

Formulas concentrate on correcting errors experience when looking more than 30 degrees away from the enter.

power| base curve:
-8.00D to -12.00D | plano
………

Question 16

What are the base curves range in minus lenses?

Answer 16

from plano to 6.00D

Question 17

What are the base curves range in plus minus lenses?

Answer 17

from 6.00D to 10.00D.

Question 18

What does minus side curves range from?

Answer 18

-4.00D and -7.00D deliever the clearest vision.

Question 19

As the minus powers increase, the selection of a base curve should?

Answer 19

flatten.

Question 20

As the plus power increases the selection of a base curve should?

Answer 20

steepen.

When changing from a lower index of refraction it may be necessary to slightly reverse these rules.

Question 21

What do you do when replacing one lens of an existing pair of eyeglasses?

Answer 21

when ordering an identically powered second pair of eyeglasses the base curve should be specified.

Question 22

What does the ANSI standards recommend?

Answer 22

the lens supplied is within +/- 0.75D.
Ex:
if you are duplicating a +2.00 base curve lens and it measures +2.50. it is within the 0.75D ANSI tolerance allowed. But, if it measured +3.00D, it would be more than the ANSI standards allow and would not be within the tolerance.

Question 23

A different base curve may be necessary if?

Answer 23

there is a change in the Rx.
of lens material.
an aspheric design.

Question 24

In the simplest form, how do you determine the lens power?

Answer 24

F1+F2=Ftotal

Question 25

How are base curves measured with?

Answer 25

a lens clock, aka a Geneva lens clock or lens measure.

Question 26

What are the three pins that the lens clock have?

Answer 26

that when pressed against the lens and will read the dioptric curvature of a lens surface.

Question 27

What are the two outside pins in the lens clock?

Answer 27

are stationary while the center pin moves up depending on the curvature of the lens surface.

Question 28

What is the face of the lens clock have?

Answer 28

has plus numbers in black and minus numbers in red. Most lens clocks are calibrated to measure lenses with a 1.523 index of refraction.