Since the rise of the camera industry in the 19th century, the design of lenses has become more and more complicated and has more functions. Lens design engineers inevitably have to deal with "chromatic aberration".
the general optical lens has a lower spherical aberration, but relatively speaking, it will also be accompanied by a larger light scattering, that is, Chromatic aberration, mainly because the wavelengths of different colors of visible light (red, blue, green, etc.) are different, and the refractive index after passing through the optical lens is also different, so it can not converge on the same plane position, especially on the telescope head with long focal length, the red light with longer wavelength and the short wavelength.
Through continuous experiments and tests, engineers have roughly decomposed the color dispersion phenomenon of the image produced by optical glass into "longitudinal chromatic aberration"-concentric color seepage phenomenon and "magnification chromatic aberration" will appear in the central part of the focus-forming different color spots around the focused image, and gradually expanding from the central part to the edge part. The longer the focal length of the lens, the more obvious the chromatic aberration is.
Early research found that natural fluorite has a unique effect of eliminating chromatic aberration, but the crystal of natural fluorite is too small and expensive to be used in the manufacture of lenses. Until the end of 1968, CANON Company of Japan pioneered the technology of synthesizing a large area of artificial fluorite (CaF2 calcium fluoride), and in 1969, it introduced the first-time lens Canon FL-F3 F5.6 and FL-F5 F5.6 with fluorite lenses. In 1973, CANON even introduced the famous FL3 F2.8 fluorite lens.
because the cost of synthesizing fluorite lenses is too high, CANON company later developed another substitute of optical glass mixed with patented oxide, named low dispersion lens "UD-Ultra Dispersion" and more advanced "Super UD" lens, which mixed UD and fluorite lenses and laid a famous "l" mirror legend for Canon in the next 3 years.