dispensing 3 - aspheric lenses

Question 1

What is an aspheric lens?

Answer 1

a lens that is not spherical so it does not have the same radius of curvature over the entire surface so surface curvature is based on a conic section

Question 2

what are the 4 basic types of conic sections of an aspheric lens?

Answer 2

-circle
-ellipse = shape formed by an angled plane through a cone that does not intersect the base of the cone.
-parabola = a curve that is formed by the intersection of a cone with a
plane having one side parallel to the side of the cone.
-hyperbola = a shape formed when a cone is intersected by a plane
that makes a greater angle with the base of the cone than the side of the cone makes with its base.

Question 3

what are p values in dispensing?

Answer 3

the type of asphericity used on a lens surface and knowing this allows you to differentiate the type of conic lenses from each other so you can understand how far the surface is from being spherical

Question 4

what is the equation referring to p values in dispensing aspheric lenses?

Answer 4

y2 = 2𝑟0𝑥+ 𝑝𝑥2

where r0 is the radius of curvature at the vertex of the conic section

Question 5

how can unwanted astigmatism in a spherical lens be neutralised?

Answer 5

by selecting a specific type of aspheric lens

Question 6

what are some broad reasons to use an aspheric lens?

Answer 6

-optically correct lens aberrations
-lens can be made flatter so reducing spectacle aberration and making the lens more attractive
-lens can be made thinner so lighter
-to ensure a good, tight fit in the frame (the steeper the BC the easier it is to dislodge it from the frame)
-to make a lens with progressive optics

Question 7

what does tscherning’s eclipse indicate?

Answer 7

the base curve required to produce a lens with minimal oblique astigmatism

Question 8

on what kind of patients is it necessary to use an aspheric lens? what patients would you maybe recommend it to?

Answer 8

-high plus powers (recommended for patients with power above +3.00)

-you may recommend it for high minus patients >-3.00
-should be recommended for all cylinder powers above ~2.00 D, even
when the spherical component of the prescription is low.
-in anisometropic patients with difference >2.00D as aspherics are normally flatter, thinner, and closer to the eyes and reduce magnification differences

Question 9

how do aspheric lenses actually work in terms of where they start to differ from spherical lenses?

Answer 9

in the middle the aspheric lens starts off like a spherical lens so vertices are spherical but at a certain distance from the oc the lens gradually changes its curvature at a rate calculated to offset peripheral aberrations

Question 10

why is it difficult to get both good cosmetics and good optics in spherical lenses?

Answer 10

because high plus power = steeper base curve making the lens look worse, you can choose a flatter BC but then that means in the periphery, the sphere power will be off and there will be unwanted cylinder

Question 11

what is the definition of aspheric lenses?

Answer 11

lenses that have one or more surface which is not spherical

Question 12

what are the three types of construction of aspherics?

Answer 12

-spherically based lenses
-aspheric lenses
-atoric lenses

Question 13

what are spherically based lenses for simple spheres and for spherocylinders?

Answer 13

o For simple spheres the front surface is spherical, and the back surface is also
spherical
o For spherocylinders the front surface is spherical, and the back surface is toric.

Question 14

what are aspheric lenses for simple spheres and for spherocylinders?

Answer 14

o For simple spheres the front surface is aspheric, and the back surface is spherical.
o For spherocylinders the front surface is aspheric, the back is toric

Question 15

what are atoric lenses?

Answer 15

front surface is spherical and back surface is typically atoric

Question 16

how do you have to put prism into an aspheric lens and why?

Answer 16

incorperated prism by prism must be ground in prism done in the surfacing laboratory so that the correct amount of prism will be found at the centre of the aspheric zone as otherwise, if you just try to decenter an aspheric lens to create prism like you would with a spherical lens, the wearer will no longer be looking through the middle of the aspheric zone

Question 17

whats the easiest way to tell if a lens is aspheric or atoric?

Answer 17

Notice the flatness of the front (and back) curve compared with other lenses of equal powers - not a reliable method though, better to use a lens clock or grid pattern (check PDF)

Question 18

what kind of spectacle wearers will find it easier to adapt to aspherics and atorics?

Answer 18

any wearer where there lenses cause straight lines to appear more curved:
-barrel distortion in minus spherical,
-pincushion distortion in plus spherical)

Question 19

what measurements should you take when fitting aspherics?

Answer 19

-mono PDs
-hight of pupil centre adjusted for pantascopic angle

Question 20

what are the differences between full aspheric and non full aspheric?

Answer 20

-in full aspherics If the eye in not correctly located in the aspheric configuration, this poor fitting can
produce results worse than would be experienced if spherically based lenses were poorly fit because the changes start gradually and increase more rapidly as distance increases from the centre of the lens
-different to non fill aspherics as these are designed with a spherical central area so that If the eye is not properly centred, the consequences are not supposed to be as noticeable.

Question 21

why may a px have difficulty adapting to an aspheric lens design if they have previously been dispensed a spherical lens design?

Answer 21

because they are used to experiencing distortion with spherical lenses. Seeing everything distortion free with aspheric can be unsettling at first because the Px is not used to the new form of image without oblique astigmatism

Question 22

what aberration are aspheric lenses designed to reduce?

Answer 22

oblique astigmatism