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Coma of a single lens. Each cone of light focuses on different planes along the optical axis. In optics (especially telescopes), the coma (/ ˈ k oʊ m ə /), or comatic aberration, in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components that results in off-axis point sources such as stars appearing distorted ...
As a lens is stopped down from its maximum (widest) aperture, most lens aberrations (spherical aberration, coma and astigmatism) are decreased, but lens diffraction increases. The effect is that for most lenses, the balance between the decreasing aberrations and the increasing diffraction effects of stopping down the lens means that lenses have ...
In most cases, two thin lenses are combined, one of which has just so strong a positive aberration (under-correction, vide supra) as the other a negative; the first must be a positive lens and the second a negative lens; the powers, however: may differ, so that the desired effect of the lens is maintained. It is generally an advantage to secure ...
A perfect lens (top) focuses all incoming rays to a point on the Optical axis. In spherical aberration (Bottom) peripheral rays are focused more tightly than central rays. There are numerous higher-order aberrations, of which only spherical aberration , coma and trefoil are of clinical interest.
The bottom example depicts a real lens with spherical surfaces, which produces spherical aberration: The different rays do not meet after the lens in one focal point. The further the rays are from the optical axis, the closer to the lens they intersect the optical axis (positive spherical aberration). (Drawing is exaggerated.)
In geometric optics, distortion is a deviation from rectilinear projection; a projection in which straight lines in a scene remain straight in an image.It is a form of optical aberration that may be distinguished from other aberrations such as spherical aberration, coma, chromatic aberration, field curvature, and astigmatism in a sense that these impact the image sharpness without changing an ...
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Ironically, the two groups in a Petzval portrait lens are primarily intended to control spherical aberration and coma, but actually make field curvature worse. This tradeoff was desirable because for a long focus lens better correction of spherical aberration and coma permits a faster aperture setting, which is more important than any defect of ...