Considering the wave number dependence of the waist size in laser resonator, the exact intensity solution of the pulse through achromat is obtained. The effect of group velocity dispersion (GVD) on the focus was discussed. The influence of lens dispersion up to second order on the spatial and temporal intensity distribution of the femtosecond Gaussian beam was analyzed. The influence of input beam size on the maximum intensity in the focus of singlet lens and achromat was compared.