The interband optical conductivity of Nb was calculated from a self-consistent relativistic band structure. For the optical conductivity, the major relativistic effect is the lowering of the s-like bands relative to the d-like bands, not spin-orbit splitting. k-space searches identified the regions of the Brillouin zone contributing to the principal structures of the optical conductivity. The regions were found to be large volumes of the zone that did not include symmetry points or lines. Along with previous results for Mo, the good agreement between theory and experiment suggests that the effects of the omitted self-energy are small for these two bcc metals.