Visual Optics Exam 3

Optical Aberrations

defects in an optical system that degrade the quality of an in-focus image

Spherical Aberration

-occurs for rays parallel to the optical axis

-it is the only monochromatic aberration that affects on-axis rays

-it can also affect off axis rays

-when SA is present, marginal rays focus in a different plane than paraxial rays

Positive (Undercorrected) SA

-marginal rays focus in front (to the left) of the paraxial rays, meaning the marginal zones have more positive power than the central zone

-this typically occurs for positive spherical surfaces

Negative (Overcorrected) SA

-marginal rays focus in back (to the right) of paraxial rays meaning the marginal zones have less positive power than the central zone

-this occurs with positive aspheric surfaces that have a steeper curvature in the center than in the periphery

-negative spherical lenses also exhibit negative SA

Longitudinal SA

-the difference in marginal and paraxial focus position along the optic axis

-usually expressed as the dioptric difference between the two foci

Lateral SA

-the difference in marginal and paraxial ray position at the paraxial focus plane

-for a point source, the lateral SA would be the radius of the patch of marginal rays in the plane of the paraxial focus

Why does accommodation make the SA greater in a schematic eye?

the crystalline lens has spherical surfaces; making the cornea aspheric (surface is either a section of an ellipse or parabola) reduces SA considerably

How can SA of the eye be measured?

by determining the best focus of the eye for various locations within the pupil

What are the experimental values of SA on actual eyes?

-measurements in unaccommodated real eyes show positive SA, but less than the amount predicted from the Gullstrand schematic eye; the smaller SA in real eyes is due to the aspheric surfaces of the cornea and crystalline lens

-accommodation reduces the SA, making it less positive or even negative; this implies that accommodation makes the lens more aspheric (center steeper than periphery)

-there is a lot of variability in SA among individuals, particularly for pupil sizes larger than 3 mm

-the amount of SA can be different when measured along different meridians, thus a measurement across horizontal meridian of the pupil can yield a different amount of SA than a measurement across the vertical meridian

What is SA minimized by in a real eye?

by aspheric surfaces and by the gradient refractive index of the crystalline lens

Coma

an aberration of off-axis rays, present when the marginal rays from an off-axis point do not meet at the same distance from the optic axis (laterally) as the paraxial rays - can be present with spherical aberration

Positive Coma

occurs when the marginal rays meet at a location further from the optic axis than the paraxial rays

Negative Coma

occurs when the marginal rays meet at a location closer to the optic axis than the paraxial rays

When is a coma negative?

when there is a positive SA and the stop is in front of the lens

When is a coma positive?

when there is a positive SA and the stop is behind the lens