Achromat lenses
Achromat doublets are permanently coupled two lenses - focusing and dispersing - made of different types of glass and having different dispersion coefficients. This allows chromatic aberration correction for the light of two specific wavelengths and a substantial improvement for the other wavelengths.
Achromatic lenses can have both positive and negative values of focal lengths. Standard diameters are 1/2 "and 1".
