The dispersion characteristic of conventional optical glass types will vary as a function of the wavelength region being considered. In the 1.0 to 1.5 µm region, the dispersion of traditional crown and flint glass types is found to be nearly the same. This results in a unique condition when attempting a lens design solution for this spectral region. A typical example is described here that will be helpful in understanding this phenomenon.
Bruce H. Walker, "Lens Design for the Near IR … Correction of Primary Chromatic Aberration," Appl. Opt. 34, 8072-8073 (1995)