|Exotic lensing corrections to the microlensing optical depth|
Microlensing surveys derive the microlensing optical depth towards various directions such as the Gal actic Centre from the distribution of observed Einstein radius crossing time. We show that the formula which 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 size of the source 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.