Focusing effects on phase front curvature are important in determining the band-width generated in the near vacuum ultraviolet by sum frequency mixing of ultrashort pulses. Noncollinear sum frequency mixing is utilized to produce 6.0 eV fourth harmonic radiation of a passively mode-locked titanium doped sapphire laser operating at a high repetition rate. The spectrum of the generated fourth harmonic radiation is broader than would be predicted if focusing is not taken into account. A method of calculating the bandwidth for sum frequency mixing of short pulses is developed and is experimentally supported.