An optimized optical trap is a favorable choice for nanoparticle trapping and micromanipulation of biological tissues. The collimation of laser beam before the objective can have significant influence on the trap by controlling the effective NA of the system. We have shown by both theory and experiment that in the aberration-free condition the filling factor of $W∕D\simeq 0.67$ provides the strongest trap in the lateral directions for a micrometer-size bead. In this condition improvements up to $\simeq 117\mathrm{\%}$ $(\simeq 168\mathrm{\%})$ , compared with the previously suggested ratio of $W∕D\simeq 1$ $(W∕D\simeq 1.25)$ , were achieved in the lateral direction.