diagram compares the color correction characteristics of fluorite,
ED glass and BK7. Clearly, fluorite has the superior correction characteristics.
The focal length of the three lenses has been optimized at 820 mm
and for the purpose of comparison the focal length of C and F lines
has been made equal. Where the reference scale is set at 0, the vertical
axis represents the wave length and the horizontal line represents
the focal length. Compared to the others, the fluorite shows almost
no color aberration over the visible light spectrum (380nm~770nm)
with a very slight difference in the focal point. Fluorite is especially
good in the shorter wavelength.
Fluorite Advantage for more information