Best Form Lenses L6 Flashcards
What are Best Form Lenses?
= when a lens is mounted such taht the OC of the lens coincides with the visual axis the form of the lens has little significance
- in oblique gaze the lens form becomes more important
- the lens can be centered for one direction of gaze, but the eye will rotate behind the eye and view through off axis visual points
- off axis images are afflicted by various aberrations
What are aberrations?
- transverse chromatic aberration
- distortion
- curvature of field
- oblique astigmatism
- coma
- spherical aberration
- last 5 are monochromatic
What is TCA?
- 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 canse problems
- TCA =P/V
- MAR wont help with TCA, neither would a light tint
- a high n wont help with TCA either as the V value would be lower so would end up with more TCA
What is Distortion?
- affects the shape of the image, but no blurring
- caused by the increase in spherical power with distance from the OC
- you get pincushion with +ve lenses that are high in power - magnifies image as you move from OC
- you get barrel disortion with -ve lenses that are high in power - minifies image as you move away from OC
What is Curvature of Field?
=due to the curvature of the image not matching the curvature of the far point sphere
- increases with the amount of oblique astigmatism
- error in matching of curves is due to the axial power and off axis powes of the lens - as you move away from the OC, image isnt as clear in periphery
What is oblique astigmatism?
=causes a blurring of the image as if it is being viewed through an unwanted sphero cylinder
- may be described as an image px knows is cicular looking oval through correction
- when light reflected away from OC, ray becomes astigmatic so instead of forming one point image as it does through the OC, it forms 2 line foci and a CLC bet the lines
- tangenital ray is along the same axis as the lens and sagittal ray is 90 degrees to this - CLC lies somewhere in bet the 2
What are best form lenses?
= use surface powers to eliminate/minimise certain aberrations
-if the tangenital and sagittal oblique vertex sphere powers match, it will be a perfect lens
What happens with a plano convex lens?
- not best for visual performance
- creates large aberrations when not looking through OC
- lens induces 1D cyl when lookin around 30 degrees away from OC
- make sure you check base curves to minimise aberrations due to distortion
Name the diff types of best form lenses
1) point focal lens
2) minimum tangenital error
3) percival lens
What is a point focal lens?
eg zeiss punctal
- no oblique astigmatic error
- forms a point image whenever px looks away from OC
- lens is incorrect for about 0.25D when around 35 degrees - error is only spherical
What is a minimum tangenital error lens?
- t line remains at 0 throughout ocular rotation - will always receive correct rx around T line
- curve slightly fatter than point focal lens so better cosmetically
- some oblique astigmatic error (v small) only after 20 degrees
- smaller amounts of cyl are tolerated better by px than the equivalent spherical change
- t power same as BVP
- if BVD increases slightly eg specs slip down slightly, it behaves like a point focal lens
- if BVD decreases eg specs pushed up higher to where they were dispensed, it behaves like a percival lens
What is a percival lens?
- t and s lines lie equally either side of retina
- no mean oblique power as deviation of BVP is equal and opp to power of lens
- CLC lies on retina
- large mount of oblique astigmatic error - futher apart the 2 lines, the more error
What are Aspheric lenses?
= an aspheric surface is one which is rotationally symmetrical but not spherical
- when lenses are spherical, the off axis performace can be changed by bending the lens - not so with aspheric lenses
- conic sections
- sag of the lens is smaller than that of a spherical lens (thinner and lighter)
- reduces astigmatic error and distortion for high plus powers
- can be determined by lens measure - will be spherical at centre but becomes astigmatic at edges
History of aspheric lenses
early 1900s = zeiss katral lenses
1960s = convex ellipsoid surfaces
1980s = polynomial surfaces
1981 = hyperboloid surfaces
Aspheric lenses more recently
- take into account rx, facial measurements
- can also now correct low positive powers
- convex oblate ellipsoid surfaces used for negative lenses
- must have correct measurements - take mono PDs and heights
- CANNOT provide prism by decentration - as it important to see through OC for aspheric
- must dipense MAR because curves are flatter as smaller sags so we get more reflections
