  We report on saturated absorption spectroscopy measurements of the $(4s^2)
^{1}S_0\rightarrow(4s4p) ^{1}P_1$ transition for the four most abundant
even-mass isotopes in calcium. By referencing the laser locked to an ultralow
expansion cavity and carefully investigating systematic errors, isotope shifts
are determined with a precision below 100 kHz, improving previously reported
values by a factor of about five. A King plot analysis employing literature
values of the 729-nm transition in Ca ions shows excellent linearity. The field
and mass shift parameters are determined from King plots with other
transitions.
