Machine vision lens --- aberration

Aberration means that the lens cannot accurately reproduce the image of the subject in proportion. In layman's terms, aberration refers to poor image clarity or "distortion". In addition to the chromatic aberration between the polychromatic light, the monochromatic aberration of the lens can be divided into five types, which are spherical aberration, coma, astigmatism, curvature of field, and distortion that affect the similarity of the image. . Below, let's take a look at five different types of monochromatic aberrations. Ball difference It is caused by the different light-gathering ability of each point on the lens spherical surface of the lens. The parallel rays from infinity should theoretically converge on the focal point. However, because the converging points of the paraxial rays and the far-axis rays are not consistent, the converging rays do not form a point, but a circle of confusion centered on the optical axis. This aberration is called spherical aberration. The existence of spherical aberration causes the image blur, and as can be seen from the figure below, this blur is related to the size of the aperture. When the aperture is small, since the diaphragm blocks the far-axis light, the diameter of the circle of confusion is small, and the image will be clear. When the aperture is large, the diameter of the circle of confusion is large, and the image will be blurred. It must be noted that the image blur caused by the spherical aberration and the blur in the depth of field are completely different and cannot be confused. Spherical aberration can be eliminated to the maximum extent by means of compound lenses or aspheric mirrors. In a photographic lens, when the number of aperture increases by one stop (the aperture is reduced by one stop), the spherical aberration is reduced by half. When we are shooting, as long as the light conditions permit, we can consider using a smaller aperture to reduce the effect of spherical aberration. The increase in the number of apertures is also limited. After reaching a certain level, it will reach the diffraction limit and cannot be resolved. The use of high refractive index and other additional lenses in the design can reduce the effect of spherical aberration, but this may cause the lens 2/3 and the weight to be too large, and the cost is too high. Coma It is a kind of aberration that occurs when imaging off-axis. A beam of parallel light is emitted to the lens from a point outside the optical axis. After passing through the optical system, it does not form a single point image on the image plane, but forms an asymmetrical diffuse spot, which is shaped like a comet. From the center to the edge, a tail is drawn from thin to thick. The head is bright and clear, and the tail is wide, dim, and fuzzy. The aberration caused by this off-axis beam is called coma. The size of coma is not only related to the aperture, but also related to the field of view. We can also adopt a smaller aperture when shooting to reduce the impact of coma on imaging. Like scattered It is also a kind of off-axis aberration. Unlike coma, astigmatism is only related to the field of view. Due to the asymmetry of the off-axis beam, the converging point of the off-axis meridian beamlet (that is, the diameter direction of the lens) is different from the converging point of the sagittal beamlet (the circular arc direction of the lens). This phenomenon is called For astigmatism. Astigmatism can be understood in contrast to the astigmatism of the eye. Eyes with astigmatism actually have different lens curvatures in the two directions, causing the points to be seen to diffuse into a short line. Astigmatism also greatly reduces the image quality of off-axis imaging. Even if the aperture is opened very small, it is still impossible to obtain a very clear image in the meridian and sagittal directions at the same time. In a wide-angle lens, because the field of view is relatively large, the astigmatism is more obvious. There are two main ways to reduce astigmatism: using a symmetrical structure, and reducing the incident angle of the off-axis field of view light. Field song When shooting objects on a plane perpendicular to the optical axis, the image formed by the lens is not in an image plane, but on a curved surface symmetrical to the optical axis. The defect of this kind of imaging is curvature of field. Field curvature is an aberration that has nothing to do with the aperture. Reducing the aperture does not improve the blur caused by curvature of field. When taking a picture with a lens with field curvature, when the image is adjusted to the center of the screen, the image around the screen is blurred; and when the image is adjusted to the center of the screen, the image at the center of the screen starts to blur again, and cannot be flat. Obtain a clear image in the center and all around on the image plane. Distortion It refers to the deformation of the image formed by the object. Distortion does not affect the clarity of the image, but only affects the similarity between the image and the object. Because of the distortion, a straight line on the object side becomes a curve on the image side, causing image distortion. Distortion can be divided into pincushion distortion and barrel distortion. The fundamental reason for the distortion is that the lateral magnification of the central area of ​​the lens image field is inconsistent with the lateral magnification of the edge area. As shown in the figure below, if the edge magnification is greater than the central magnification, pincushion distortion will occur, otherwise, barrel distortion will occur. Distortion has nothing to do with the F-number of the lens, only the field of view of the lens. Therefore, the distortion of a wide-angle lens is generally greater than that of a standard lens or a telephoto lens. No matter what kind of lens, what kind of distortion, reducing the aperture will not improve the distortion.