Sperhical Aberration (SA):
A 4th order aberration of any optical system
Placed in the middle of the Zernicke pyramid.
Occurs from unequal bending of central and peripheral parts of any spherical lens causing blurring of the image.
Either positive or negative spherical aberration occurs.
Positive SA resembles a Mexican hat.
Central rays are bent more than peripheral.
Negative SA produces a doughnut shaped blur
Peripheral rays are bent more than central rays through cornea.
A
A Prolate cornea has a negative Q value, and is flatter toward the periphery.
An Oblate cornea has a positive Q value and is steeper toward the periphery.
A Q value of -0.50 means zero spherical aberration.
Normal cornea is -0.26 which gives positive spherical aberration.
Relaxed young crystalline lens à negative spherical aberration.
Parabola q value of -0.52 is perfect surface with no SA.
How do measure Spherical aberration?
Aberrometers, instruments to measure all refractive error (sph, cyl, HOA)
Hartmann schack, Tscherning, ray tracing, Slit skiascopy/double pass
Factors affecting Spherical aberration:
Age, Accomodation, pupil size, corneal shape, lens curvature
More Spherical Aberration an with enlarging pupil
Cornea shape more prolate with more negative spherical aberration
More flatter is more positive spherical aberration (oblate)
Spherical Aberration of the eye is close to zero at age 19
Positive SA increases with age (cornea pos, while lens neg SA changes to pos)
Lens becomes positive over age and through the 20's, 30's years of age.
Therefore, contrast sensitivity declines with age as SA of eye increases (even with no cataract!!).
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