The most significant advantage of aspheric lens is that it can correct spherical aberration. Spherical aberration is caused by focusing or aligning light with a spherical surface. Therefore, in other words, all spherical surfaces, whether or not there are any measurement errors or manufacturing errors, will have spherical aberrations. Therefore, they all need a non-spherical or aspherical surface to correct it. By adjusting the conical constant and non-spherical coefficient, any non-spherical lens can be optimized to minimize aberration. Spherical lens is a lens with significant spherical aberration, not a spherical lens with almost no spherical aberration. The spherical aberration in a spherical lens will focus the incident light at many different fixed points, resulting in blurred images. In aspheric lenses, all the different light rays focus on the same fixed point, thus producing images with less blur and better quality.
In order to better understand the difference of focusing performance between aspherical lens and spherical lens, the point or blur of light produced by two equal lenses (f/1 lens) with a diameter of 25 mm and a focal length of 25 mm, parallel and monochrome light (wavelength 587.6 nm) on the comparative axis (object angle of 0 degree) and off-axis (object angle of 0.5 degree and 1.0 degree) were observed. It is known that the spot size of aspheric lens is several orders of magnitude smaller than that of spherical lens.