Handout 6 Flashcards
(42 cards)
Aberrations of the eye are taken into account in clinic for
Keratorefractive procedures, corneal grafts, keratoconus
Aberrations of the eye can be measured with
Wavefront sensors
Paraxial ray
A ray which makes a small angle to the optic axis of the system and lies close to the axis throughout the system
Paraxial approximation
Incident rays are close to the optical axis yielding point images for point objects
Is paraxial approximation tru
Not always
Marginal ray
A ray peripheral to the optic axis and is bent more than axial or paraxial rays
When paraxial optics dont apply 2 aberrations will occur called
Monochromatic and chromatic
Monochromatic aberrations aka __. Include : (5)
Seidel Spherical coma oblique ast. Curvature of field Distortion
spherical aberrations, when a pencil of light is refracted by a large-aperture optical system, different zones of the aperture have different focal lengths
Peripheral/marginal rays are focused differently than centra/ paraxial rays which affects sharpness
Large aperture aberrations are mitigated by
The pupil because of its small size only accepts paraxial rays
Spherical aberrations are considered in what lenses
High powered and convex lenses -> correct with aspheric
Spherical aberrations can be negative
Peripheral rays refracted less than central (peripheral hyperopia).
Spherical aberrations can be negative
- Peripheral rays have a shorter focal length/ are refracted more than central rays (peripheral myopia)
- More common than negative
Hen oblique/ off axis rays are refracted by a large aperture optical system at different points than those incident on or near the optical axis produces a
Coma (comet)
Affects the sharpness of image points
-rarely a problem
-significant aberration of the eye itself in keratoconus and can increase after refractive surgery
Factors than can control coma
Aperture size
Lens form
Angle of obliquity
Increase in pupil size leads to
A decrease in image quality owing to increased aberration
-in high powered lenses, it is best to use aspheric lenses
Whats an aspheric lens
Rotationally symmetric surface whose radius of curvature varies with position from lens center -> changes constantly across lens surface
Modifies lens surfaces without changing lens power
Aspheric lenses
Occurs when light passes obliquely through a spherical surface to form an interval of sturm rather than a point focus
Oblique astigmatism
Oblique astigmatism aka ___ forms an __ instead of ___.
Radial astigmatism/marginal astigmatism
Interval of strum rather than a point focus
Relation of oblique astigmatism and pupil/aperture size
Not dependent on each other
All of I together form paranoid all surfaces
Teacup (tangential) and saucer (sagittal) surface. Tan is always steeper
Curvature of field AKA
curvature of image aka power error
-ophthalmic lenses form curved not plane images
A true Petzval surface can only exist when
Oblique astigmatism has been eliminated
