spherical aberration correction
Spherical / aspherical details click to enter
The most significant advantage of aspheric lens is that it can correct spherical aberration. Spherical aberration is caused by using a spherical surface to focus or align light. Therefore, in other words, all spherical surfaces, whether there is any measurement error or manufacturing error, will have spherical aberration. Therefore, they will need an aspherical or aspherical surface to correct it. By adjusting the cone constant and aspherical coefficient, any aspherical lens can be optimized to minimize the aberration. For example, refer to Fig. 1, which shows a spherical lens with significant spherical aberration and an aspherical lens with almost no spherical aberration. The spherical aberration in the spherical lens will focus the incident light to many different fixed points to produce a blurred image; In aspheric lens, all different light rays will focus on the same fixed point, Manufacturer of spherical lensso it produces a less blurred and higher quality image.
In order to better understand the difference in focusing performance between aspherical lens and spherical lens, please refer to a quantitative example, in which we will observe two equal lenses (F / 1 lens) with a diameter of 25mm and a focal length of 25mm. The following table compares the spot or blur size generated by parallel, monochromatic light (wavelength 587.6 nm) on the axis (0 ° object angle) and off the axis (0.5 ° and 1.0 ° object angle). The spot size of aspheric lens is several orders of magnitude smaller than that of spherical lens.
Spherical lens with spherical aberration and aspherical lens with almost no spherical aberration
Additional performance benefits
Although there are many different technologies on the market to correct the aberration caused by the spherical surface, these other technologies are far from what the aspherical lens can provide in terms of imaging performance and flexibility. Another widely used technique involves increasing f / #, Manufacturer of spherical lens by "shrinking" the lens. Although this can improve the image quality, it will also reduce the luminous flux in the system. Therefore, there is a trade-off between the two.
On the other hand, when using aspheric lens, its additional aberration correction supports users to maintain good image quality while realizing the system design of high luminous flux (low f / #, high numerical aperture). The image degradation caused by higher luminous flux design can be sustained, because the performance provided by a slightly reduced image quality will still be higher than that provided by the spherical system. Consider a triad lens with focal length of 81.5mm and F / 2 (Fig. 2). The first one consists of three spherical surfaces, and the first surface of the second one is aspheric surface (the rest are spherical surfaces). Both designs have exactly the same glass type, effective focal length, field of view, f / #, and overall system length. The following table quantitatively compares 486.1nm, 587.6nm, and 656.3nm light with modulation transfer function (MTF) @ 20% contrast, parallel and polychromatic on and off-axis. The triad lens with aspheric surface shows higher imaging performance in all field angles. Manufacturer of spherical lensIts high tangential resolution and high sagittal resolution are three times higher than that of the triad lens with only spherical surface.
