In the optical lens of optical elements, every point on the object plane in the effective field of view will form a clear image point in the eye image plane during observation. We call the difference between the ideal image and the actual image the image difference, or aberration for short.
The image difference is divided into two categories: monochromatic image difference and color difference.
Monochromatic aberration is divided into:
Spherical aberration: When the paraxial light passes through the spherical mirror, the light passing through the center of the lens and the light passing through the edge of the lens cannot intersect at one point. This aberration is called spherical aberration.
Comet angle: a monochromatic conical beam emitted from the external point of the axis outside the main axis to the optical system. After being refracted by the optical system, if a clear point cannot be formed on the ideal plane, but a comet spot with a bright tail is formed, the aberration is called an aberration.
Astigmatism: Since the point of the luminous object is not on the optical axis of the lens, the beam emitted by it has a great inclination to the optical axis. After the beam is refracted by the lens, it cannot focus on a point. This aberration is called astigmatism.
Image bending: A plane object cannot be truly imaged as a plane, but as a surface, that is, image bending. This will blur the peripheral image of the image, namely the so-called curved image.
Distortion: the bending of a straight line on an object after imaging with a lens. This distortion is called distortion. Chromatic aberration is a serious defect in lens imaging. When polychromatic light is used as light source, monochromatic light will not produce color difference. White light consists of red and orange, yellow, green, blue and purple. The wavelength of various light is different, and the refractive index of each light passing through the lens is also different. In this way, a point in the grid can form a color spot.
Position chromatic aberration: light parallel to the optical axis passes through the lens group and converges at different image points before and after. The focal length of short wave is short, and that of long wave is long.
Enlarge chromatic aberration: after the mixed light emitted by the off-axis light spot passes through the lens group, the mixed light will converge at different heights, so that the image edge can be decomposed into a rainbow, and the different magnifications of different color light presentation modes will be displayed.
Vertigo: The phenomenon that the outer part of the optical axis in the pupil blocks the light.
Stray light: refers to interference light in imaging.
Impact on image quality
The spherical aberration makes the center of the image soft and there is no clearest point.
Astigmatism separates the radial and tangential focus around the image and cannot focus at the same time.
Distortion distorts the image, and the surrounding straight lines become curves.
The position color difference makes the central light spot of the image become a color ring.
The magnification chromatic aberration makes the edge contour of the image scene have color edges.
The vignetting makes the image center brighter than the edge.
Stray light reduces the contrast and saturation of the image, and the backlight is easy to produce fog and illusion.
Reduce spherical error: when using aspheric lens, use composite lens to optimize lens material and lens group structure.
Methods to reduce coma: use composite lens as much as possible and optimize the structure and material of lens group;
Reduce astigmatism: use composite lens, preferably lens material, preferably lens group structure;
Reduce distortion: use aspheric lens, preferably lens structure and lens material;
Reduce the magnification chromatic aberration: give priority to the use of lens materials and composite lenses;
Reduce vignetting: optimize the lens group structure;
Reduce stray light: adopt coating extinction coating to optimize the structure of lens group.
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