Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

OV2021 dispensing > best form lenses > Flashcards

best form lenses Flashcards

1
Q

what do aberrations do ?

A

. they reduce image quality as you move away from the optical centre

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

what are examples of monochromatic aberrations ?

A

. Transverse chromatic aberration
. distortion
. curvature of field
. oblique astigmatism
. coma- found in aperture larger systems (not the eye)
. spherical aberration - found in larger aperture systems ( not the eye)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what is transverse chromatic aberration ?

A

. gives coloured fringes on high contrast targets
. occurs because the refractive index of lens materials decreases as the wavelength of light increases
. TCA of less than 0.1 is unlikely to cause problems
. TCA=P/V

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

what is distortion ?

A

. effects the shape of image

. caused by the increase in spherical power with distance from the OC

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

what is pincushion distortion ?

A

. in a plus lens as the magnification increases away from the centre , it looks like the image is stretched

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

what is barrel distortion ?

A
  • caused by a minus lens

- in a minus lens edges appear smaller due to the minification

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

what is curvature of field ?

A
  • error in the matching of the curves due to the changes in axial power and off axis power of the lens
  • due to the curvature of the image not matching the curvature of the far point sphere
  • sharpest image formed on a curved, not flat plane
  • increases with the amount of oblique astigmatism
  • image in centre is clearer than periphery
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

what is oblique astigmatism ?

A
  • causes a blurring of the image if it being viewed through an unwanted sphero-cylinder
  • this is because light passing through a spherical lens obliquely ( away from the optical centre ) becomes astigmatic
  • instead of uniting at a point, they form two line foci at right angles to each other with centre disc of least confusion
  • plane that contains the optical axis of the surface = tangential ray
  • plane at 90= sagittal plane
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

explain oblique astigmatism diagram ?

A
  • dotted line is an imaginary line which is vertex sphere and is concentric to the eyes rotation
  • we measure the position of tangential and sagittal foci from this position ( dotted line)
  • A2= back vertex of lens
  • distance from vertex sphere to the far point sphere is back vertex focal length of the lens ( A2 to F’)
  • far point sphere is also an imaginary surface concentric with eye centre of rotation
  • tangential foci lies closer to the eye when compared to sagittal ( between these is the disc of least confusion )
  • point object is not focused as point image which results in astigmatic blur
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

what is the aim of best form lenses ?

A
  • use surface powers to eliminate/minimise certain aberrations
  • reduce oblique astigmatism and curvature of field
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

what did lens design move from ?

A
  • lens design moved towards meniscus lenses and away from plano convex and concave designs as the optics are better
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

when is distortion a problem ?

A
  • brain can adapt to distortion
  • it tends to be a problem if there is a significant change in Rx or lens form
  • make px aware of problem
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

what should happen to T and S curves in a perfect lens ?

A
  • if the T and S oblique vertex sphere powers match = perfect lens
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

what happens in a plano convex lens ?

A
  • when eye rotates up to 30degrees, the real effect of lens is +4.25 in S meridian and +5.25 in T meridian
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

what is a point focal lens ?

A
  • one of the best ( best form lenses ) we can give to px
  • the T and S powers are same even when eye rotates about to 40 degrees
  • this means there is no oblique astigmatic error , no added cylindrical effect
  • lens is under powered as eye reaches edges ( e..g. zeiss punktal lens )
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

what is the minimum tangential error lens ?

A
  • curve of this form is flatter than point focal lens
  • T and S don’t match completely when eye rotates to 40 degrees
  • this means there is some oblique astigmatic error present
  • this 0.25DC at 35degrees
  • may be tolerated by same px
  • tangential power is the same as the back vertex power of the lens for all rotations
17
Q

what is an advantage of minimum tangential error lens ?

A
  • it can behave like a point focal lens with increase in BVD
  • it can behave like a percival lens with decrease in BVD
  • so if BVD is where it should be we get the effect of a minimum tangential error lens
  • if the increase in BVD , the lens will behave like point focal
  • if BVD decreases, the lens will behave like a percival lens
18
Q

what is a percival lens ?

A
  • the T and S oblique sphere powers are either side of retina and equi-distance from it
  • this means there is no mean oblique , as the deviation from back vertex power is equal and opposite
  • more obliques astigmatic error, as the lines are further apart when the eyes rotating to 40 degrees
19
Q

how to calculate the oblique astigmatic error ?

A

1- we need to find the distance between the two line foci ( T and S)

. OAE=FT’ - FS’

2- we also can calculate the mean oblique error (MOE)

. MOE=MOP-Fv’

  • the dioptric distance between the circle of least confusion and the far point sphere
  • MOP ( mean oblique power ) = (FT’ +Fs’)/2
20
Q

what is Tscherning’s ellipses ?

A
  • the quadratic equations for aberrations and the lens forms with minimum aberrations can be plotted graphically
  • show what lens we can provide for px which are best form
  • curves used to calculate theses circles are calculated using trigonometric tracing
  • the graphical ellipses assume that lens is mounted 27mm in front of eyes centre of rotation
  • inside of circle is availability of lens and what F2 power is required
  • for all of them , there will be 2 possibilities for power of lens the ostwalt and wollaston curve
  • the wollaston curve are much steeper
  • the ostwalt curve are flatter lens form
  • we get a greater range of powers with minus lens
21
Q

what are the ellipses available in ?

A
  • the ellipses are available for different refractive indices
  • n=1.5
  • n=1.7
  • n=1.9
  • steeper curves required for higher refractive index
22
Q

what are the disadvantages of a wollaston curve

e.g. solar contour lens ?

A
  • wollaston curve lens are not good cosmetically

- expensive

23
Q

when is a minus toric better ?

A
  • optics of a lens are better with a plus toric

- for a minus power are minus toric is better

24
Q

what is an aspheric lens ?

A
  • an aspheric surface is one which is rotationally symmetrical but not spherical
  • when lenses are spherical, the off axis performance can only be changed by bending the lens - not so with aspheric lenses
  • conic sections
25
Q

what happens when a conic sections are rotated about their axis ?

A
  • when conic sections are rotated about their axis they produce conicoids
  • y^2=2r0x - px^2
    . r0= radius of curvature at the vertex
    . p = eccentricity of curve
  • tangential power changes over the surface
26
Q

how are aspheric lenses determined ?

A
  • can be determined by a lens measure-will be spherical at centre but become astigmatic at edges
  • the sag of the lens is smaller than that of a spherical lens-thinner and lighter
  • reduce astigmatic error and also distortion for high plus powers
27
Q

what happens to aspheric lenses in plus and minus lens ?

A
  • plus lens radius of curvature flattens towards the edges

- minus lens steepens towards the edges

28
Q

what is the history of aspheric lens ?

A
  • 1900s - zeiss katral lenses
  • 1960s- convex ellipsoid surfaces
  • 1980s- polynomial surfaces
  • 1981 - hyperboloid surfaces
29
Q

how is the design of aspheric lenses now ?

A
  • take into account prescription , facial measurements
  • can also now correct low positive powers
  • convex oblate ellipsoid surfaces used for negative lenses
  • must have correct measurements
  • do not provide prism by decentration
  • flatter curves = more reflections = provide MAR
30
Q

what are aspheric lens?

A
  • aspheric lenses are flatter in design which allows them to be fitted closer to the eye
  • positive surface stigmatism can be used to neutralise negative oblique astigmatism
  • px moves to edges of lens , astigmatism is reduced
  • good optics
  • good cosmetics

OV2021 dispensing (7 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions