A tunable cw Ti:sapphire laser is locked to a self-referenced femtosecond frequency comb generated by an erbium-doped fiber laser thereby creating a single-frequency optical frequency synthesizer. This synthesizer has a tuning range of 760–820 nm with an absolute frequency uncertainty less than 100 kHz. We demonstrate the precision of this cw optical frequency synthesizer by measuring the optical frequency of several transitions on the $D_2$ lines of ${}^{85}\text{R}\text{b}$ and ${}^{87}\text{R}\text{b}$ using a saturated absorption spectrometer.