Spherical aberration correction
The most significant benefit of a non-spherical lens is that it can be corrected for spherical aberrations. Spherical aberration is caused by using the surface of the sphere to focus or focus on the light. Therefore, in other words, all of the spherical surface, no matter whether there is any measurement error and manufacture error, will appear spherical aberration, as a result, they will need a not spherical or aspherical Lenses surfaces, carries on the correction. By adjusting the constant of the cone and non-spherical coefficients, any non-spherical lens can be optimized to minimize the image difference. For example, see figure 1, which shows a spherical lens with a significant spherical aberration, and a non-spherical lens with almost no spherical difference. The spherical difference in the spherical lens will allow the incoming light to focus at many different points, creating a blurred image. In a non-spherical lens, all the different light rays will focus on the same spot, resulting in less blurred and more quality images.
In order to better understand the aspheric lens and spherical lens in terms of focus performance difference, please refer to a quantitative model, in which we can observe two 25 mm diameter equal to the focal length of 25 mm lens (f / 1 lens). The following table compares on the shaft (0 °Angle) and outside the shaft(0.5 °and 1.0 °Angle) in parallel, monochromatic light (wavelength 587.6 nm) generate the light spot size or fuzzy.Spherical lenses are several orders of magnitude larger than non-spherical lenses.
The benefits of additional performance
Although the market also has many different techniques for correction by spherical aberration resulting from the surface, however, these other technology in the imaging performance and flexibility, are far less than aspheric lens offer. Another widely used technique involves increasing f / # by "reducing" lenses. While this improves the quality of the image, it also reduces the flux in the system, so there is a trade-off between the two.
On the other hand, when using aspheric lens, the additional aberration correction support users in the realization of high flux (low f / #, high numerical aperture) of the system design at the same time, still keep a good image quality. Higher luminous flux design causing image degradation can be sustainable, because a slightly reduced image quality performance will still be provided above the performance of the spherical system can provide. Consider a focal length of 81.5 mm, f / 2 triad lens (figure 2), the first is composed of three spherical surface, the second is one of the first surface of spherical surface (the rest) for spherical surface, the two design have exactly the same type of glass, effective focal length, field, f / #, as well as the overall length of the system. The following table is quantitatively compared with the axis of the modulation transfer function (MTF) at the @ 20% contrast and the parallel, multicolored 486.1 nanometers, 587.6 nm, and 656.3 nm rays. A triad of aspheric surface lens has been used, all on the viewing angle showed higher imaging performance, its high tangential and sagittal high resolution, compared with only the triad of spherical surface lens is three times higher.