Low-dispersion Glass
Special optical glass, marketed as ED, UD, LD or SLD, that bends different wavelengths by more nearly equal amounts.
Measured in
count
In depth
Ordinary glass acts slightly like a prism, focusing blue light closer than red and producing colour fringes. Low-dispersion formulations reduce this separation, which matters most in telephoto designs where the effect is magnified. Two or three ED elements are now standard in any serious long lens.
Examples in practice
A 400 mm lens without ED glass shows purple fringing on backlit branches that an ED design removes almost entirely.