Best Form Lenses L6 Flashcards
What are Best Form Lenses?
= when a lens is mounted such that 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
Name the different aberrations?
- Transverse chromatic aberration
- Distortion
- Curvature of field
- Oblique astigmatism
- Coma
- Spherical aberration
[These are all monochromatic abberrations]
[We are not concerned with coma and spherical aberration as it tends to happen in apertures bigger than the eye]
What is the effect of TCA (Transverse chromatic aberration)?
Why does it occur?
What is our tolerance to 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 cause problems
- TCA =P/V (v value of lens - which is 1/n)
- MAR (anti-reflection coating) won’t 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 the effect of distortion?
Why does it occur?
What type of distortion is seen from positive and negative lenses?
- 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 (so edges are bigger and centre smaller)
- you get barrel disortion with -ve lenses that are high in power - minifies image as you move away from OC (so centre is bigger and edges smaller).
Why is Curvature of Field aberration experienced?
Does curvature of field bear any relationship with another type of aberration?
Why does it occur?
What is the overall effect of 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 powers of the lens - as you move away from the OC ( essentially because in the eye the image is being produced on a curved plane rather than a flat one).
- overall effect of curvature of field is that image in the periphery isn’t as clear as in the centre.
What is the effect of oblique astigmatism?
= It 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 circular looking oval through correction .
- Light passing through a lens away from the OC (so obliquely) becomes astigmatic so instead of forming one point image as it does through the OC, it forms a 2 line foci and a Circle of Least Confusion between the lines -tangential ray is along the same axis as the lens and sagittal ray is 90 degrees to this
- Circle of Least Confusion lies somewhere in between the 2 lines.
What do best form lenses do?
= use surface powers to eliminate/minimise certain aberrations (but this can be to the detriment of the cosmesis)
These lenses don’t aim to minimise TCA nor distortion
What happens with a plano convex lens in terms of oblique astigmatism?
Are plano convex lenses the best in order to minimise oblique astigmatism?
What is something to remember when prescribing this lens?
- creates large aberrations when not looking through OC
- lens induces 1D cyl when looking around 30 degrees away from OC
NO - not the best for visual performance
-make sure you check base curves to minimise aberrations due to distortion
Name the different types of best form lenses
1) point focal lens
2) minimum tangenital error
3) percival lens
What is the concept of a point focal lens?
What is the slight disadvantage of this lens?
What is oblique error?
Give an example of a point focal lens.
- In this lens, the S and T spheres are the same so you aren’t experiencing any oblique astigmatism.
- This lens forms a point image (hence the name) whenever px looks away from OC however the sphere power of the lens as a whole is slightly off
- lens is incorrect for about 0.25D when around 35 degrees (i.e. lens is under power as eye reaches edges) - this error is only spherical thus termed oblique error.
eg zeiss punctal
What is a minimum tangenital error lens?
How does this lens perform cosmetically?
Do you experience as oblique astigmatism with this lens?
Why may you prescribe this lens?
What affects the performance of this lens (is this an advanatge)?
- T line remains at 0 throughout ocular rotation - will always receive correct rx around T line however slightly off on the S line.
- S 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 (Back vertex power) in any degree of ocular rotation in the T plane]
- if BVD increases slightly eg specs slip down slightly, it behaves like a point focal lens -if BVD decreases e.g specs pushed up higher to where they were dispensed, it behaves like a percival lens (This is advantageous).
What is the concept of a percival lens?
What is a disadvantage of this lens - and what affects this?
- The T and S planes lie equally either side of the retina
- This means there is no mean oblique power as deviation of Back Vertex Power is equal and opposite to power of lens
- The Circle of Least Confusion lies on retina
- There is a large amount of oblique astigmatic error - the further apart the 2 lines, the more error
What are Aspheric lenses?
How can you known if someone’s glasses consist of aspheric lenses?
= an aspheric surface is one which is rotationally symmetrical but not spherical
- when lenses are spherical, the off axis performance can be changed by bending the lens - not so with aspheric lenses
- They are made up of conic sections
- The sag of the lens is smaller than that of a spherical lens (thinner and lighter)
- This 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
