The cosmological lens equation in a universe with non zero cosmological constant

We derive and solve exactly the Dyer-Roeder equation in a Friedman-Robertson-Walker cosmological model with non zero cosmological constant. To take into account non homogeneous distribution of matter we use the phenomenological clumpiness parameter. We propose also a general form of an approximate solution which is simple enough to be useful in practical applications and sufficiently accurate in the interesting range of redshifts.