2003ApJ...584..278G

Microlensing surveys derive the microlensing optical depth toward various directions such as the Galactic center from the distribution of observed Einstein radius crossing times. I show that the formula that is being used is invalid for ``exotic'' lensing events. The corrected formula is derived. The ``parallax'' effect (Earth motion) requires no correction. Corrections for the finite sizes of sources and wide binary lenses are small (typically less than 1%), except for blended events. Corrections for intermediate-type binaries such as MACHO LMC-9 can be substantial.