Aberrations provide tolerance for defocus
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SAN FRANCISCO — At Refractive Surgery Subspecialty Day at the American Academy of Ophthalmology meeting, an expert shared what he wished he knew about optical aberrations when he started practicing.
Kashif Baig, MD, MBA, FRCSC, said aberrations are all imperfections within the eye that cause light rays to deviate from the desired path.
“We might think that aberrations are bad, but they actually provide certain tolerance for defocus,” he said. “So, it’s not all bad, and we can use aberrations to our advantage.”
Aberrations can be broken down into two categories, chromatic and monochromatic, with monochromatic breaking down into lower-order and higher-order aberrations. Baig said higher-order aberrations make up only about 15% of monochromatic aberrations, but they are difficult to correct with spectacles and make up a large part of patient complaints.
There are several ways to measure aberrations, including wavefront sensors. To help differentiate corneal aberrations from internal aberrations, Baig said to use measurements from topography to subtract corneal aberrations from the internal component. Root mean square, modulation transfer function and point spread function provide objective metrics to determine magnitude of aberration, contrast and image quality, respectively.
There is also an important interplay between spherical aberration, depth of focus and contrast sensitivity. Baig said correcting spherical aberration improves contrast sensitivity but decreases depth of focus. Increasing spherical aberration will expand the depth of focus but also decrease visual quality.
“Some people target a nice compromise,” Baig said. “There is a certain compromise that can be achieved between spherical aberration and contrast.”
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Baig K. Understanding aberrations. Presented at: American Academy of Ophthalmology meeting; Nov. 3-6, 2023; San Francisco.
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