The microlensing effective timescale t _ { eff } = \beta t _ { E } is used frequently in high-magnification ( \beta \ll 1 ) microlensing events , because it is better constrained than either the impact parameter \beta or the Einstein timescale t _ { E } separately .
It also facilitates intuitive understanding of lightcurves prior to determination of a model .
Similar considerations may apply to very low magnification events .
I therefore provide a generalization of this quantity to all events :

t _ { eff } = \beta t _ { E } \sqrt { ( 1 + \beta ^ { 2 } / 2 ) ( 1 + \beta ^ { 2 } / 4 ) } .