What is the difference between a spherical lens and an aspheric lens?

The so-called spherical surface and aspherical surface mainly refer to the lens geometry of lenses (various cameras, microscopes, etc.), glasses (including contact lenses), that is, spherical lenses and aspherical lenses. The difference in the geometric shape of the two determines the difference in the refraction direction of the parallel incident light, which affects the quality of its imaging effect.

Spherical lens, its lens is spherical radian, and its cross-section is also arc-shaped. When light rays of different wavelengths are incident on different positions on the rear lens with parallel optical axes, they cannot be focused into one point on the film plane (the plane perpendicular to the lens center and the lens focus line and passing through the focus), resulting in aberration Problems that affect image quality, such as loss of clarity and distortion. Generally, ordinary lenses are composed of spherical lenses.

In order to solve this imaging problem, it is possible to correct the aberration by adding lenses in the lens body, but this may cause adverse effects and further weaken the image quality, because the additional lenses, in addition to increasing the reflection of light in the lens body In addition to causing the flare phenomenon, it will also increase the size and weight of the lens.

Aspheric lens, the lens does not have a spherical curvature, but the edge of the lens is “shaved off” a little, and its cross-section is flat. When the light is incident on the aspheric mirror surface, the light can be focused on one point, that is, the film plane, to eliminate various aberrations. For example, the flare phenomenon will be more serious when using a spherical lens with a large aperture than a narrow aperture, but if an aspheric lens is added, the flare situation can be greatly reduced; another example is that the image appears distorted (pillow-shaped or barrel-shaped), due to the internal lens The light rays are not properly refracted. Taking the zoom lens as an example, it is usually barrel-shaped deformation when zooming to a long focal length, and pincushion deformation when zooming to a long focal length. If an aspheric lens is used, this aberration can be improved.

The use of aspheric lens technology is most beneficial to the production of lenses with large apertures, high-magnification zoom, and extreme wide-angle and telephoto lenses. The image quality is improved due to the reduction of aberrations, and the size of the lens body is also reduced. At present, many lens manufacturers in the market have stated that some of their focal length lenses use aspheric lenses, and even portable zoom cameras (such as 28 to 90mm, 38 to 105mm, etc.) all use aspheric lenses to improve image quality.

The difficulty in making an aspheric mirror lies in the design of its geometric dimensions and the precise control of its geometric dimensions. Japan is the most advanced technology in this area. Currently, there are two main ways to process aspheric lenses: one is to use high-precision grinding technology (manual or mechanical) to reprocess spherical lenses; the other is to use high-precision molds for compression molding or injection molding to directly produce aspheric lenses.

Post time: Nov-30-2022