18 - Variables of Lens Design and Base Curves

Question 1

What may we have to do to the power of a lens in a wrap a round frame?

Answer 1

compensating the power so the optical effect of the rx is as wanted

Question 2

What can tilting the lenses cause?

Answer 2

Oblique Astigmatism

Question 3

How does pantoscopic tilt or face form effect the lens power?

Answer 3

spherocylindrical lens with axis of 090/180 changes power when tilted.

Question 4

What are the 4 variables of Lens Design?

Answer 4

1-Vertex Distance
2-Lens Thickness
3-Refractive Index
4-Front and Back lens surface powers.

Question 5

What can happen if an inappropriate base curve is chosen? 2

Answer 5

-reduces the visual quality through the periphery of the lens
-lens aberrations

Question 6

What is Vogel’s Formula’s?

Answer 6

Used to determine approximate base curves for a lens power. (actual base curves for lenses will vary)

Question 7

What is Vogel’s Formula for a Minus Lens?

Answer 7

Base curve= spherical equivalent/2 + 6.00D

Question 8

What is Vogel’s Formula for a Plus Lens?

Answer 8

Base Curve = Spherical equivalent + 6.00D

Question 9

When will lenses be flatter than calculated?

Answer 9

-Plus lenses
-higher index of refraction

Question 10

Practice examples of finding Base Curves of Plus and Minus spherocylindrical prescriptions

Question 11

How do we choose the correct base curve when both are a plus prescription? 2

Answer 11

• the steeper (higher numerical base curve) of the two is the best optical choice
  -cosmetically this may not be followed

Question 12

How do we choose the correct base curve when both are a minus prescription?

Answer 12

-flatter of the two (lower numerical base curve)

Question 13

How do we choose the correct base curve when one lens is a plus and the other is a minus prescription?

Answer 13

• the steeper (higher numerical base curve) of the two is the best optical choice

Question 14

What is an Aspheric Lens? 3

Answer 14

• not spherical
• lens surface changes shape
• changing radius of curvature - varying amounts of surface astigmatism everywhere except the centre.

Question 15

What are 5 uses of an Aspheric Lenses?

Answer 15

-correct lens aberrations
-flatter - reducing magnifcation making it more attractive
-thinner, light-weight lens
-good, tight fit in the frame
-making a lens with progressive optics

Question 16

What are 3 considerations to fitting Aspheric Lenses?

Answer 16

-Use Mono Pd’s
-Measure MRP heights
-Prism cannot be ground into the lens for decentration so moving OC away destroys aspheric optical advantage.

Question 17

What is an Atoric Lens? 2

Answer 17

-A lens optimized in each meridian on a toric lens
-expands the peripheral range of clear vision beyond what is found for either a well-designed spherically based lens or aspheric lens

Question 18

What is happening to the radius of curvature in a Atoric lens?

Answer 18

Radius of curvature is changed at different rates along each meridian to be optimal for the power needed.

Question 19

What is a toric lens?

Answer 19

a lens with 2 curves on the same surface to correct for astigmatism

Question 20

When do we recommend an Atoric lens? 2

Answer 20

• all cylinder powers above -2.00D, even if sph component is low
  -same reasons as an aspheric lens but px needs to know its more expensive

Question 21

When do we reccommend aspheric lens? 5

Answer 21

-Plus/Minus with rx above +/- 3.00D
-Px who experiences Anisometropia
-Care about how their gls look
-Decrease weight for older wearers
-cls wearers to encourage gls wear