best form lenses

Question 1

what do aberrations do ?

Answer 1

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

Question 2

what are examples of monochromatic aberrations ?

Answer 2

. 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)

Question 3

what is transverse chromatic aberration ?

Answer 3

. 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

Question 4

what is distortion ?

Answer 4

. effects the shape of image

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

Question 5

what is pincushion distortion ?

Answer 5

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

Question 6

what is barrel distortion ?

Answer 6

• caused by a minus lens

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

Question 7

what is curvature of field ?

Answer 7

• 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

Question 8

what is oblique astigmatism ?

Answer 8

• 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

Question 9

explain oblique astigmatism diagram ?

Answer 9

• 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

Question 10

what is the aim of best form lenses ?

Answer 10

• use surface powers to eliminate/minimise certain aberrations
• reduce oblique astigmatism and curvature of field

Question 11

what did lens design move from ?

Answer 11

• lens design moved towards meniscus lenses and away from plano convex and concave designs as the optics are better

Question 12

when is distortion a problem ?

Answer 12

• 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

Question 13

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

Answer 13

• if the T and S oblique vertex sphere powers match = perfect lens

Question 14

what happens in a plano convex lens ?

Answer 14

• when eye rotates up to 30degrees, the real effect of lens is +4.25 in S meridian and +5.25 in T meridian

Question 15

what is a point focal lens ?

Answer 15

• 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 )

Question 16

what is the minimum tangential error lens ?

Answer 16

• 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

Question 17

what is an advantage of minimum tangential error lens ?

Answer 17

• 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

Question 18

what is a percival lens ?

Answer 18

• 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

Question 19

how to calculate the oblique astigmatic error ?

Answer 19

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

Question 20

what is Tscherning’s ellipses ?

Answer 20

• 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

Question 21

what are the ellipses available in ?

Answer 21

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

Question 22

what are the disadvantages of a wollaston curve

e.g. solar contour lens ?

Answer 22

• wollaston curve lens are not good cosmetically

- expensive

Question 23

when is a minus toric better ?

Answer 23

• optics of a lens are better with a plus toric

- for a minus power are minus toric is better

Question 24

what is an aspheric lens ?

Answer 24

• 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

Question 25

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

Answer 25

- 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

Question 26

how are aspheric lenses determined ?

Answer 26

- 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

Question 27

what happens to aspheric lenses in plus and minus lens ?

Answer 27

- plus lens radius of curvature flattens towards the edges | - minus lens steepens towards the edges

Question 28

what is the history of aspheric lens ?

Answer 28

- 1900s - zeiss katral lenses - 1960s- convex ellipsoid surfaces - 1980s- polynomial surfaces - 1981 - hyperboloid surfaces

Question 29

how is the design of aspheric lenses now ?

Answer 29

- 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

Question 30

what are aspheric lens?

Answer 30

- 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