Chromatic Aberration CaF2 material - FCD100 Hoya & FPL53 Ohara

As lenses (like in refractors) have different refractrive properties depending the wavelenghts and the fact that the focal length of a lens is depending the refractiveness results in colors not focussing into a one single point behind the lens. This means that colors will shift in pictures (color glow around moon, planets) which we call chromatic abberation.

Over time refractors became better with double lenses (achromatic lens; the lenses compensate for two wavelengths) and triple lenses (apochromatic lens, lenses compensate for three wavelenghts). For the latter one the flint lens is made of ED glass. This is glass with a very low dispersion properties or high refraction indeces. Typically CaF2 is favorite but expensive, heavy and difficult to manufacture. Therefor the use of other fluorite glasses are used. Currently the use of FPL53(Ohara, JP) and FCD100 (Hoya, JP) are common in small f/ratio refractors and closing the gap with CaF2 glass.

The table below shows that the bigger the F/ratio the lower the chromatic aberration.

Looking into the data, FCD from Hoya has a slightly better refractive indices then FPL53