Week 9: Prescribing for High Ametropia & Magnification Aniseikonia Flashcards
Describe ‘lens form’
The relationship between the front & back curves
Describe Best form lenses
Minimise spherical aberration while still using spherical surfaces
Describe Point focal lenses
Correct for oblique astigmatism
Describe Percival lenses
Mean oblique power (sagittal & tangential power equal and opposite)
Describe Minimum tangential form
Creates minimum tangential error
What is Monochromatic Aberration/Third order/Seidel Aberration?
Blur that occurs when a single wavelength of light passes through a lens/lens system and does not come to a point focus
List some assumptions for Monochromatic Aberration
- Spherical lens produces a point image for a point object
- A line object consists of a series of point objects
- Position of the image is determined by tracing a pencil of light through the lens
- Position is determined by the principles of conjugate foci
- Paraxial rays
What are the types of Seidel Aberrations?
- Spherical aberration
- Coma
- Oblique astigmatism
- Curvature of field
- Distortion
What are some assumptions of Seidel Aberrations?
- All surfaces are spherical
- Consider marginal rays
Describe Spherical Aberration
- Affects images of objects on and off axis
- It represents the difference in the radii of the blur circles
Describe Spherical Aberration
- A rotationally symmetric aberration in which the light rays that pass through the paraxial zone of the pupil focus at a different distance than the rays that pass through the marginal pupil
- Positive when the marginal rays focus ahead of the paraxial rays
- Negative when the paraxial rays focus ahead of the marginal rays
How do you correct spherical aberration?
- Shape of the lens governs the amount of spherical aberrations
- Effect of bending a lens on spherical aberration
Describe Coma
- Applies to rays entering the lens at an angle
- Dependent on lens shape
- Rays from periphery focus closer to axis & produce a larger blurrier spot than paraxial rays
- Can be considered as an oblique spherical aberration or off-axis spherical aberration
How do you correct coma?
- For a single lens, coma can be partially corrected by bending the lens
- Can be corrected by an appropriately placed aperture stop
- Zero coma for a given object distance
- Magnitude of coma in a spectacle lens is theoretically large
Describe Field Curvature
- Petzval surface is a surface free of any astigmatism
- Principally a problem with optical instrumentation (particularly cameras) where image plane is not curved
- Less of a problem with the eye, because the retina is curved
Describe Distortion
- Image produced is sharply defined
- Lies in a single plane (e.g. no curvature)
What are the different distortion and describe them briefly
- Positive Distortion (barrel distortion)
- Peripheral image point is closer to the centre than ideal image
- Anterior aperture stop produces minification of marginal rays - Negative Distortion
- Peripheral image point is further from the centre than ideal - Pincushion distortion
- Posterior aperture stop produces magnification of marginal rays
How does Spherical Lens Result in Oblique Astigmatism?
- Oblique astigmatism is created by an OFF AXIS pencil of light
- Unequal refraction at spherical surface causes the pencil of light to become astigmatic & focus as two line images, called tangential & sagittal images, instead of as a single point
- Two image lines form, perpendicular to each other and separated by the Interval of Sturm
- Distance between the two line foci in oblique astigmatism is called the astigmatic difference
- An oblique pencil will exhibit aberration coma, which is minimised by the pupil
- Oblique astigmatism may be reduced by finding the optimum base curve using Tschernig ellipse graph
Describe Oblique Astigmatism
- Off-axis rays that causes radial and tangential lines in the object plane to focus sharply at different distances in the image space
- Arises from asymmetry in the nature of optical paths followed by rays in the tangential & sagittal planes
Describe Best form spherical lenses
Eliminates oblique astigmatism
Describe aspheric lens
- One which does not have the same radius of curvature throughout the entire surface
What is the purpose of aspheric lens?
1.Produce a flatter lens (decreasing magnification) & making it more attractive
- Produce a thinner, lighter weight lens
- To optically correct lens aberrations
Describe Aspheric & Prescribing Prism
- Aspheric lens has a spherical centre with an aspheric periphery
- It is essential that the eye is located at the optical centre of the lens
What are the High Plus Lens Designs and describe briefly
- Regular spherical lenses
- Will suffer from significant peripheral distortions
- Can be corrected through asphericity - Lenticulars
- Lens has a central area with a prescribed power, with a peripheral are of no script
What are the following lens designs for extremely high myopia?
- Minus Lenticular
- Like plus lenticular, but it has a minus optical, and this time a plus carrier to decrease the edge thickness - Myodisc
- Identical to the minus lenticular, but the chamfer in the carrier is flat, hence edge thickness will be the same as that of the aperture
Describe what is spectacle magnification
SM = retinal image size in corrected eye / retinal image in same eye uncorrected
What is aniseikonoa?
- Relative difference in the size and/or shape of the images seen by the right & left eyes
What are the different types of aniseikonoa and describe briefly
- Physiologic
- Occurs naturally - Anomalous Aniseikonia
- Anatomical: caused by the anatomic structure
- Optical: caused by the optics of the eye (inherent optical aniseikonia) or optics of the correcting lens (induced aniseikonia)
Describe Axial ametropia
- Occurs when the axial length of the eye ball is either too short or too long
- May use Knapp’s Law to produce a “normal” image size
Describe Refractive Ametropia
- Length of the eye ball is normal, but the person is ametropic
- May use the strategy of contact lenses as the uncorrected image size will be the same size as the image size for a normal emmetrope
- Introducing spectacle lenses will produce relative magnification differences
What is Relative spectacle magnification (RSM)
RSM = image size for a corrected ametropia eye / image size for a standard emmetropic eye
Describe Knapp’s Law
If ametropia is axial, image size is different from normal eye because axial length is different from normal
Describe distortion
- Occurs because there is different magnification at different areas of the lens periphery in proportion to the distance of those areas from the lens OC
Describe Anomalous Distortions symmetrical
- One eye sees an image symmetrically larger than the other eye
- Meridional aniseikonia has a meridional size difference in a meridian of one eye compared to the other meridian
Describe Meridional Distortions
- If due to a cylindrical lens or meridional magnifying lens, then the image is elongated in the meridian of power
What are the breakdown of binocular fusion?
- Diplopia
- Suppression
What are the distortions of stereoscopic space and describe briefly
- Geometric size effect: slanting of objects when horizontal magnification is produced in one eye
- Induced size effect: opposed slanting induced when vertical magnification is produced in one eye
- Declination: forward/backwards slant induced when cylinders are induced at oblique axes
Who experiences aniseikonia?
- Anisometropes: prevalence of anisometropia > 1D in adults is 5 – 10%
- Patients who have had cataract surgery: 40% of all pseudophaes had aniseikonia related symptoms
How to correct anniseikonia and briefly describe
- Contact lenses
- To achieve normal retinal image sizes, this is the best option for both axial & refractive anisometropes - Spectacle modification
- To equalise retinal image sizes, it is best to use spectacle modification for both axial & refractive anisometropes
- By modifying; base curves, lens thickness, vertex distances
What are the four approaches for correcting anniseikonia
- First Pass Method
- Directionally Correct Method
- Estimating Percent Magnification
- Measure Percent Magnification
Describe First Pass Method briefly
- Short vertex distance
- Small eye size
- Aspheric lens design
- High index lens material
How do you Directionally Correct Magnification Changes?
- Both lenses plus (Anisohyperopia)
- Frame with minimum vertex distance
- Small Eye Size
- Higher Plus Lens
- Lower plus lens - Both lenses minus (Anisomyopia)
- Frame with minimum vertex distance
- Small Eye Size
- Higher Minus Lens
- Minus lens
What is the Estimating Percent Magnification?
1.5% per dioptre of anisometropia for Refractive Anisometropia
How do you measure Percent Magnification?
Measure using a ‘space eikonometer’ or Aniseikonia inspector
