  We introduce the notion of defocusing gravitational lens considering a MACHO
located behind a light source with respect to an observer. The consequence of
defocusing effect is a temporal variability of star luminosity which produces a
gap instead of a peak as tell--tale signature in the light curve. General
theory of (de)focusing rays (geodesics) in a gravitational field is presented.
Furthermore, we give estimations of the mass of the lens and the optical depth
connected to such a phenomenon.
