[en] We report on improved isotope shift measurements of the isotopes ${}^{40,42,44,48}$Ca in the $4\mathrm{s}{}^2{\mathrm{S}}_{1/2}\to <!--\; ???\; -->4\mathrm{p}{}^2{\mathrm{P}}_{3/2}$ (D2) transition using collinear laser spectroscopy. Accurately known isotope shifts in the $4\mathrm{s}{}^2{\mathrm{S}}_{1/2}\to <!--\; ???\; -->4\mathrm{p}{}^2{\mathrm{P}}_{1/2}$(D1) transition were used to calibrate the ion beam energy with an uncertainty of $\mathrm{\Delta <!--\; ??\; -->}U\approx <!--\; ???\; -->\pm <!--\; ??\; -->0.25\mathrm{V}.$ The accuracy in the D2 transition was improved by a factor of 5–10. A King-plot analysis of the two transitions revealed that the field shift factor in the D2 line is about 1.8(13)% larger than in the D1 transition which is ascribed to relativistic contributions of the $4{\mathrm{p}}_{1/2}$ wave function. (paper)