Page 3.34. Visual Optics. Chapter 3 Retinal Image Quality. Q1. The origins of spherical aberration (SA), coma and off-axis astigmatism (OAA) respectively are: . (a) all three are longitudinal (b) SA longitudinal, coma transverse, OAA longitudinal
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(a) all three are longitudinal
(b) SA longitudinal, coma transverse, OAA longitudinal
(c) SA longitudinal, coma transverse, OAA transverse
(d) SA transverse, coma transverse, OAA longitudinal
Q2. Decentered ablation is one of the more common causes of visual problems in post-LASIK or PRK patients. One reason for the problems is:
(a) Longitudinal Spherical Aberration
(b) Transverse Spherical Aberration
(d) Off-Axis Astigmatism
Figure 3.68 – The human eye has significant intrinsic oblique astigmatism for larger retinal eccentricities.
Figure *.** – Curvature of field: 3D wavefront profile in the (exit) pupil plane produced by curvature of field for a plane object
Figure 3.57 – refracting surface (devoid of spherical aberration, coma, or oblique astigmatism) with focal length, f, images parallel on-axis light rays at its second focus (F’). Oblique incident rays also refract at the curved surface and focus the same distance away.
Positive meniscus lens produces a concave image of a plane object
Figure 3.58 - The image of a plane object produced by an ophthalmic lens is curved. Here a positive lens produces negative (concave) image curvature for the plane object.
Origin of Curvature of Field
Petzval Condition for lens system:
Spread of T and S foci increases exponentially with
Equiconvex lens produces maximum OA (equal positive power distribution between front and back surfaces)
Equiconvex lens also produces curvature of field
Figure 3.55 – oblique astigmatism produced by an equiconvex positive lens for angles of obliquity and 2 . In dioptric terms, the “astigmatic error” is the difference in image vergence between tangential and sagittal focus. Note that the astigmatic error increases exponentially between and 2.
Far Point Sphere (FPS) = surface of Far Point loci traced out as the eye rotates
Petzval’s surface is flatter (longer radius) than the FPS with a +4.0 D spectacle correction. “Needing” more plus in the periphery to maintain the image on the FPS, the eye becomes “undercorrected.”
As the eye rotates off-axis, the amount of induced off-axis astigmatism increases. This can be corrected by “Corrected Curve” Lenses. However, these lenses cannot correct both OA and curvature of field. Punktal Lenses fully correct OA but undercorrect curvature of field
Figure 3.63 – Left: +4.0 D Punktal lens corrects oblique astigmatism but not curvature of field. The eye becomes undercorrected as it rotates away from the primary position. FPS = Far Point Sphere; PS = Petzval’s surface. Right: Lens Field diagram: power error versus angle of gaze. At 35 the 0.21 D undercorrection could be compensated by accommodation.
(a) produces no image blur
(b) blurs plane objects
(c) blurs point objects
(d) produces transverse displacement of the image
Strongly dependent on paraxial image height
Depends on aperture position in the optical system
Distortion produces NO image blur
Figure 3.69 – Distortion: 3D wavefront profile in the (exit) pupil plane produced by distortion of an object.
Take grid-shaped object and image through a plus lens-aperture stop combination. Resulting image:
With aperture AT the plus lens:
With aperture to right of plus lens:
With aperture to left of plus lens:
With aperture AT the plus lens: chief ray and nodal ray “equivalent”
Could correctly define lateral magnification (m = /) using either and measured along the optic axis, or along the nodal ray (or chief ray) path
(a) pincushion distortion
(b) barrel distortion
(c) no image distortion
(d) transverse displacement of the image
Explain the wavelength-dependence of refractive index
Use this information to describe prism dispersion of white light
Describe and quantify chromatic aberration (LCA and TCA)
Define “Abbe Number” and explain its significance
Explain the principle and function of an achromatic doublet
Why does a prism deviate incident light toward its base?
Angle of deviation vs :
fixed for a given prism, so what determines angle of deviation, d ?
Prism Dispersion is just one example of chromatic aberration
Prism Dispersion “Neutralized” by a 2nd Prism
Prism-Dispersed Colors recombined by a + Lens
Fig 3.78 Page 3.86