Spherical aberration is caused by a sphere on the surface of the lens. The light emitted from the same point on the optical axis converges through the lens at different points in the image field space, thus the position of the image is moved. It is an inevitable aberration for all lenses that use a spherical lens. Its production is caused by the difference of the incidence Angle between the axis and the Angle of the lens.
When the parallel light is passed by the edge of the mirror (far axis light), its focus position is closer to the lens; The center of the lens, which passes through the center of the lens (near the axis light), is far from the focal point of the lens (the amount of light that goes along the optical axis, called the longitudinal spherical aberration).
Because of this aberration, the spot (Halo) generated by the light passing through the edge of the lens is formed around the image of the paraxial light passing through the central part of the lens, it makes people feel that the image is blurred, and the whole picture is covered with a layer of gauze, and becomes a gray image without fresh sharpness.
The spherical aberration is most obvious when the lens aperture is open or close to the full opening, and the larger the lens, the more obvious the tendency.
In the use of lens, the spherical aberration can be properly eliminated by reducing the aperture.
However, it is important to note that if the aberration of the image is too large, it may be possible to move the focal plane (the focus) by reducing the aberration of the aperture.
It is very difficult to correct spherical aberration of spherical lens. It is usually based on the light of an entry (the distance from the optical axis), then the convex and concave two lenses are used to complete the proper combination. However, as long as the spherical lens is used, a certain degree of spherical aberration cannot be significantly improved.
In order to completely eliminate the spherical aberration of the full-size lens, there is no alternative to using Aspherical lens. The effect of the aspherical lens is to modify the curvature of the surface of the lens to coincide with the focal position of the near-axis light and the far axis.
There are three main ways to make aspherical lenses:
1. Grinding aspherical lenses: grinding directly on the whole piece of glass, the manufacturing process costs relatively high;
2. Moulded aspherical lenses: metal casting technology is used to directly suppress the melted optical glass/optical resin. This manufacturing process costs relatively low;
3. Composite aspherical lenses: a special layer of optical resin is covered on the surface of the glass lens, which is ground to a spherical surface, and then the optical resin is ground to a aspheric surface. The cost of this manufacturing process is between the above two processes.