best form lenses Flashcards
what do aberrations do ?
. they reduce image quality as you move away from the optical centre
what are examples of monochromatic aberrations ?
. 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)
what is transverse chromatic aberration ?
. 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
what is distortion ?
. effects the shape of image
. caused by the increase in spherical power with distance from the OC
what is pincushion distortion ?
. in a plus lens as the magnification increases away from the centre , it looks like the image is stretched
what is barrel distortion ?
- caused by a minus lens
- in a minus lens edges appear smaller due to the minification
what is curvature of field ?
- 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
what is oblique astigmatism ?
- 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
explain oblique astigmatism diagram ?
- 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
what is the aim of best form lenses ?
- use surface powers to eliminate/minimise certain aberrations
- reduce oblique astigmatism and curvature of field
what did lens design move from ?
- lens design moved towards meniscus lenses and away from plano convex and concave designs as the optics are better
when is distortion a problem ?
- 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
what should happen to T and S curves in a perfect lens ?
- if the T and S oblique vertex sphere powers match = perfect lens
what happens in a plano convex lens ?
- when eye rotates up to 30degrees, the real effect of lens is +4.25 in S meridian and +5.25 in T meridian
what is a point focal lens ?
- 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 )
what is the minimum tangential error lens ?
- 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
what is an advantage of minimum tangential error lens ?
- 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
what is a percival lens ?
- 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
how to calculate the oblique astigmatic error ?
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
what is Tscherning’s ellipses ?
- 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
what are the ellipses available in ?
- the ellipses are available for different refractive indices
- n=1.5
- n=1.7
- n=1.9
- steeper curves required for higher refractive index
what are the disadvantages of a wollaston curve
e.g. solar contour lens ?
- wollaston curve lens are not good cosmetically
- expensive
when is a minus toric better ?
- optics of a lens are better with a plus toric
- for a minus power are minus toric is better
what is an aspheric lens ?
- 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
what happens when a conic sections are rotated about their axis ?
- 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
how are aspheric lenses determined ?
- 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
what happens to aspheric lenses in plus and minus lens ?
- plus lens radius of curvature flattens towards the edges
- minus lens steepens towards the edges
what is the history of aspheric lens ?
- 1900s - zeiss katral lenses
- 1960s- convex ellipsoid surfaces
- 1980s- polynomial surfaces
- 1981 - hyperboloid surfaces
how is the design of aspheric lenses now ?
- 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
what are aspheric lens?
- 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
